View
6
Download
0
Category
Preview:
Citation preview
.AN IUVESTIGATIOli OF UNDERWATER ACOUSTICAL
P liEU 0 r:TEli A.
.-
A thesis by
BRIAn RAY.
Submitted for the deg~ee of Doctor of Philosophy_ -/
in the University of London.
Physics Department,
Imperial College,
London S. W. 7.
August 1970.
1
2
ABSTRACT
The aspect of underwater acoustics which the writer has chosen
to study concerns the problems of human communication underwater, and
in particular, communication between divers. The complete communicatioi
chain between the mind of the speaker and that of the listener is
discussed. As a first step an acoustic telephone was constructed
and tested. This revealed a lack of fundamental understanding
concerning the formation of words inside a closed cavity (such as a
diving facemask) and the manner in which the human ears function
underwater. A tentative model for the formation of words inside a
mask is proposed.
Following from this general appraisal, a series of experiments
were designed to test binaural hearing underwater and the results
of two independent series of subject tests showed, for the first time,
that a diver could located a sound source within a target error of
20°. This demonstration of binaural ability is considered important
in understanding the mechanism by which a diver interprets a voice
signal under noisy reverberant conditions.
During these directional hearing tests some subjects reported
that a sound source in front of the body appeared louder than one
behind. This was confirmed with the aid of a suLmersible audiometer.
A second series of audiometer tests using a ubmerged 'dry' laboratory
as a calibration facility, provided additional evidence of a sensitivity
differential and demonstrated the usefulness of this type of
-structure in underwater acoustical observations.
CONTENTS
1. Introduction
.1.1 Introduction
1.2 Resume of diving physiology and technology.
2. A review — and a plan for research.
2.1 Early experiments.
2.2 Development of ultrasonic communication.
2.3 Direct audio communication.
2.4 Electromagnetic communication.
2.5 The effect of helium on speech production.
2.6 The formulation of a programme of research.
3. A complete underwater communication system.
3.1 Introduction.
3.2 The microphone.
3.3 The amplifier
3.4 The transducer.
3.5 A receiving hydrophone.
4. Initial experiments with communication equipment.
4.1 Introduction.
4.2 The formation of words underwater
4.3 Noise and range of communication.
4.4 Hunan hearing underwater.
4.5 Discussion.
4.6 The formation of words.
4.7 The range of communication.
4.8 Directional hearing.
3
5. Directional hearing experiments (1)
5.1 Introduction.
5.2 The sound source.
5.3 Free choice experiments.
5.4 Two choice experiments.
5.5 Results of free choice experiments.
5.6 The results of two choice experiments.
5.7 Conclusions.
6. Directional hearing (11)
6.1 Introduction.
6.2 Different source angles around the subject's head.
6.3 The effect of a rubber hood.
6.4 The detection of an obstacle.
6.5 The effect of the subject hearing the source switched on
6.6 The effect of a reference on the free choice tests.
6.7 Subject inverted.
6.8 Subject signalling the confidence of his judgement.
G.9 The effect of different sound sources.
6.10 Conclusions.
7. Audiometry and observations from an underwater laboratory.
7.1 Initial experiments.
7.2 The construction of•an improved audiometer.
7.3 The second series of audiometer tests.
7.4 Results.
7.5 Other observations from the underwater laboratory.
4
8. Discussion.
8.1 Articulation underwater.
8.2 Propagation.
8.3 Human hearing underwater — the threshold.
8.4 Human hearing underwater — directional hearing.
8.5 Applications to communication equipment design.
8.6 Suggestions for further research.
Acknowledgments.
Appendix 1. The signal to noise ratio from an underwater microphone.
Appendix 2. Significance of the mean vector.
Appendix '3. The design and operation of an underwater laboratory.
References.
List of symbols.
Glossary of diving terms.
5
LIST OF PHOTOGRAPHS
Firr. Photon;raph.
3.9 Four different electrodynwnic transducers.
3.13 Two hydrophones used for recording underwater signals
3.14 Direct audio system in use. Malta 1966.
4.1 Three different types of diving facemask.
5.1 Buzzer sound source.
6.2 Photograph taken fran'free choice' film record, showing subject inverted.
7.1 Crude audiometer in use.
7.3 The improved audiometer.
7.4 Inside the underwater laboratory.
8.2 Steriophonic hearing aid.
A.1 The underwater laboratory.
6
7
Chapter 1
INTRODUCTION
"There is a whole world of sound beneath the waves
waiting to be explored" (Alexander Graham Bell)
1.1 Introduction
Once embarking on any study concerning the underwater
environment, one becomes aware that there are an enormous number
of unknown or unexplored problems. Unlike some other branches of
science these problems are often not obscure, they can be as
obvious to a person who puts his head underwater today as they were
to Bell when he first tried it in the middle of the last century.
Within these few pages it is only possible to recount a study of
one small aspect of underwater acoustics. The aspect that was
chosen is possibly one of the most significant if man is to
continue to explore the oceans. This concerns the problems that
men experience in attempting to speak to one another while working
beneath the waves.
Up to the last decade diving was the province of the sportsman,
the salvage contractor and the naval frogman. The first could not
afford the expense of communication facilities, the second managed
without, and the third did not want his presence revealed at all!
In receht years the scientist has ventured underwater both to gain
first hand experience of marine problems and to study man in an alien
environment. The discovery of petroleum resources under the
sea has brought in the engineer and technician. All these people
need to .communicate to perform their allotted tasks. After all the
8
human society has largely evolved through the use of the spoken
word.
This study will examine the physics and technology of
voice communication between men in the underwater environment. In
this context, it will he necessary to examine the mechanjsm of
word formation under the restrictions that underwater brenthing
. apparatus may impose, methods of transmitting the speech signal
through water and the mechanism of human hearing- underwater. Certnin
items of equipment that were not previously available were developed
to aid this research. The results of exp~riments in the open sea
shed new light on the workings of the human ear underwater. A
submerged laboratory was developed to aid this study of the human
aspects of underwater acoustics.
1.2 Resnmp. ()f otvinp," nhvsinlogv and technolog;v
Before discuss ing the prohlems of human cf)mrnunication underwD.ter
it is appropriate to consider briefly the physiology and technl')logy
of diving. In this introduction emphasis. will be placed ori those
aspects which have a direct. bearing on communicRtil')n. For a more
p-eneral introduction the reader is referred to the BoS.AoC. Diving
!.Ianual (1), or for a co::nprehensi"e study to "The physiology ann
medicine f)f diving- and compressed air wnrk" edited by Bennett
and Elliott (2).
9
Physiolorly
The human body is not sensitive, to an absolute pressure
as such; although it will not tolerate a pressure differential.
Consequently, to survive underwater, man in the first instance
need only provide himself with a supply of breathing gas at
the ambient pressure. The fundamental difference between the
submariner and the diver (or someone in an underwater habitat)
is that the former is working in an environment at the same
pressure as the surface, whereas the latter is exposed to the
pressure of the water, roughly one atmosphere for every ten
metres of depth.
One apparent exception to this rule concerns the middle car
cavities. In the normal way these connect with the throat by two
tubes — the eustachian tubes. These are normally closed and only
open occasionally during such acts as swallowing. It is for
this reason that a change in barometric height, such as descend—
ing a steep hill, is accompanied by a sensation in the ears
which is relieved by swallowing. The trainee diver must be
taught to recognise this sensation and to deliberately "Clear
His Lars" by swallowing or more often 'by pinching his nose and
blowing into it while he is descending. (If his diving equipment
renders nose pinching impractical, he will probably wear a nose
clip.)
Failure to clear the ears properly can result in a
temporary loss of hearing while gross failure can rupture the
ear drum (Miles 3)
10
If these precautions are taken then the average diver,
breathing compressed air, would be comfortable to a depth of
50 metres while the experienced professional might dive to 90 metres.
Three factors limit the use of compressed air to these depths.
Nitrogen has a narcotic effect when breathed under pressures of more
than about six atmospheres, the human system is intolerant to a
partial pressure of more than two atmospheres of oxygen and the densit
of air under pressure may be sufficient to impair breathing.
-To overcome these problems, an atmosphere containing a
reduced percentage of oxygen diluted with helium instead of nitrogen
is normally used when diving below 70 metres. The resulting mixture,
known as "heliox”, is often described by the percentage of the main
constituents. For example a 7/93 mixture would 'contain 7% oxygen
and 93% helium and would be suitable for use at depths between
20 metres and 300 metres. Such a mixture could not, of course, be
breathed at surface pressure as the partial pressure of oxygen would
be insufficient to support life. This brings about logistic
difficulties in recording the.human voice in mixtures such as these.
When the body is subjected to pressure, the breathing gases
will tend to dissolve in the tissues. The longer the time spent
under pressure or the higher the pressure, then the greater the
amount of dissolved gas. When the diver surfaces it is necessary to-
prevent a rapid release of this gas which may otherwise give rise
to the symptoms of decompression sickness (the diver's bends). To
11
prevent this possibility the diver must reduce the pressure on
his body slowly. The time spent decompressing may vary from a few
minutes after spending half an hour diving to 30 metres, to a week
after spending a day at 300 metres. Clearly the longer decompression
times cannot be spent in the water and in these cases it is usual
to bring the diver to the surface in a pressure vessel and to
transfer him, under pressure, to a surface decompression chamber.
Before leaving this brief summary of diving physiology,
mention must be made of cold. Present day protective clothing is
only partially effective in preventing hypothermia and the writer
has been forced to abandon many experiments due to the subject or
experimenter becoming excessively chilled. This phenomena is
not confined to cold water and one series of hearing tests had to
, be called off after only 35 minutes in water at 2oo C.
Diving Equipment
Broadly speaking, present day diving can be put into three
classes depending on the equipment used:
a) Standard diving dress
b) Self contained compressed air open circuit breathing
apparatus generally known as aqualung or scuba diving.
.0 Closed or semi-closed circuit breathing apparatus using
a breathing gas of either oxy-nitrogen (in different
proportions to those in air) or heliox.
The standard diving dress has changed little in the last
century.and is still the most popular for dock and harbour diving.
To high pressure
air supply
Non—return exhaust valve
Rubber diaphragm Tilt valve
!:outh bit
The equipment consists of a loose fitting rubber suit sealed
to. the famous large copper helmet. Heavy lead boots allow the diver
to stand and walk on the sea-bed. Compressed air is fed from
the surface through an airline and the pressure inside the diving
dress is automatically maintained at that of the surrounding water -
by allowing excess gas to escape into the sea through a non-return
valve.
The popularity of scuba equipment, particularly among amateur
divers, since the second world war is due to its intrinsic
simplicity and relative safety. The air, which the diver carries in
cylinders on his back, is not supplied continuously, but is available
on demand. The key to the scuba equipment is the demand valve and
a simple one is shown in fig. 1.
Fig 1.1 A simple demand valve.
12
13
On inhalation, the reduction of pressure in the chamber
will cause the flexible rubber diaphragm to deflect inwards and
open the tilt valve allowing a supply of air all the time the
diver is inhaling. The exhaled air is expelled to the sea water
through a non-return valve. Needless to say, such a simple
device is inherently very noisy. On inhalation there is the noise
of compressed air being released and on exhalation the generation
and release of bubbles.
The scuba diver normally wears a close fitting, free flooding,
foam rubber suit. He does.not generally use a helmet but instead •
a mask covering his eyes and nose. He will usually hold a mouthbit
between the teeth which is coupled to the demand valve. If the
demand valve is attached directly to the mouthbit with just a single
thin. high pressure air hose, connected to the air supply then it
. is known as a "single-hose demand valve", whereas if it is mounted
behind the diver and connected to the mouthbit by large diameter,
low pressure, inhalation and exhalation hoses, it is known as a
"twin-hose demand valve". An alternative to the mouthbit is a
mouthcup which is designed to help the diver speak underwater. This
takes the form of a rubber cup that fits over the mouth instead of
between the diver's teeth. One further alternative is a large
mask known as a full-face-mask which as the name implies, covers
the whole face.
When diving deep, it is uneconomic to expell the exhaled gas
into the water. One breath of 2 litres at.200 metres depth is the
14
equivalent of 40 litres at surface pressure. One method of
overcoming this is to feed the exhaled gas through a chemical bed
to remove carbon dioxide and into a flexible bag or "counterlung".
After a small amount of oxygen is added to make up for that used
by the human body the gas in the counterlung can be rebreathed.
In principle, no gas need be released into the water in such a
closed circuit system. However, there are practical problems and
such closed circuit equipment is rarely seen outside military circles
where there is a requirement for equipment which does not produce
bubbles and is sufficiently quiet to use in the presence of hostile
acoustic detection equipment.
Some of the problems of a closed circuit system can be
overcome if, instead of supplying pure oxygen to the counterlung,
a mixture of oxygen and inert gas is used. As the inert gas (either
nitrogen of helium are described as "inert" in this context) is not
consumed by the body, there must be an escape of gas to the sea in
order to establish an equilibrium in the counterlung. The relative
economy of gas of this semi-closed system falls between that of a
scuba and a closed circuit set. The design of a semi-closed set
is discussed by Williams (4).
The closed, or semi-closed circuit diver normally wears a
close fitting rubber suit either sealed, or like the scuba diverts
free flooding. Although he could use any of the masks of the scuba
diver, he will normally use a full-face-mask or possibly a lightweight
fibreglass helmet.
15
1.3 The problems of human measurements underwater
Under normal surface conditions measurements on speech
and hearing are often made in some form of anechoic chamber.
The lowest speech frequencies have wavelengths in air of about 2 metres
and the design of a chamber with, say 1 metre of sound absorbing
material, can be effective at removing unwanted reflections. In water
the wavelength of this same frequency would be nearer 10 metres and
the equivalent chamber would require 5 metres of absorber on all
sides including the water surface. Even if the idea is not
mechanically impractical then the thought of an operation with
human subjects without a free air/water surface would cause concern
on safety grounds.
Having abandoned attempts to find an anechoic tank, the
writer considered the use of swimming pools, but these exhibit a strong
standing wave pattern at audio frequencies apart from being rather
noisy. However for some recordings, particularly those involving'
a microphone inside the subject's facemask, a pool is suitable and
has been used for this type of work. Although a fresh-water lake
on the outskirts of London was used for some tests, the water
temperature in this country is normally too low for subjects to
remain stationary for long periods without wearing protective clothing
on the head. As the design of such clothing varies considerably,
with some almost completely impairing hearing both underwater and
on the surface, it was decided that for most of these investigations
the subject would not wear any form of hood.
16
The remaining sites that the writer used were in the
Mediterranean, either off the south - coast of France or off the
coast of Malta. In the latter area the water temperature in
Summer is between 22 and 26oC and the weather relatively reliable.
Furthermore, and most important, there were willing divers
available to act as subjects.
The selection of subjects is difficult. For safety reasons
they must be reasonable divers but furthermore they must be capable
of applying themselves to the task of answering simple questions
underwater. This may seem straightforward until it is realised
that the average amateur diver who is in the seas off Britain
would be, and should be, spending 90% of his time making sure
he stays alive. This same diver may well not be aware that his
ears are capable of functioning underwater and if not warned
beforehand may not even recall the sound of a "thunderflash" detonated
in the vicinity.
Previous workers have found a serious fall-off in the
performance of divers performing simple tests underwater when compared
with the surface. Although this was first assumed to be entirely
inert gas narcosis, it has now been shown to be partly just the
effect of putting a man underwater in the open sea. (Baddeley 5).
However this degradation can be small under almost "ideal" diving
conditions. "Ideal" appears to mean warm, calm, clear water with
a safe landing place.
By combining experimental work with a scientific
expedition, the writer was able to obtain the willing help of
experienced divers. Divers on these expeditions would invariably
be keen undergraduate science students in good health, and the
nature of the expeditions are such that the personnel are diving
on most days and are acclimatised to working underwater.
Occasional use has been made of personnel on similar expeditions
from other establishments.
17
18
Chapter 2.
A REVIEW - AND A PLAN FOR RESEARCH
2.1 Early Experiments
The claim to the first scientific experiments in underwater
communications probably belongs to Alexander Graham Bell (Born 1847)
He is reported (6) to have said that as a boy he had clicked stones
underwater and was startled by the loud report. He sent another
boy up to half a mile across the bay and distinctly heard the
stones with his ear submerged. Although the French physicist,
Paul Langevin, with his colleague Chilovsky and the Canadian
Fessenden, had demonstrated active sonar systems before the First
World War, these were not put into effective anti-submarine use until
the post war era. During the Great War the principal submarine
detection systems made use of the human ear. One such system
involved two spaced hydrophones coupled to a pair of earpieces mounted
on separate trombone slides. The operator placed the trombone
mouthpieces against his ears and localised the sound source by adjusting
the two slides until the sound was apparently coming from a central
position. A graduated scale on the slides enabled the bearing of
the submarine to be ascertained (7).
19
By the outbreak of the second world war a considerable
wealth of information was available on the propagation of sound
and ultrasound through the sea. For long range submarine
communication it was possible to pulse a sonar transmitter with a
morse key. This became known as SST or supersonic telegraphy (8).
Over short ranges the human voice could be transmitted into the
water in the same manner as a public address system. By the end of
the war acoustic torpedos, directional hydrophone arrays and sona-
bouys had all proved themselves under operational conditions (9).
In the field of diving, the end of the second world war saw
most of the diving equipment that is used to date in service. Heliox
mixtures had been used on the salvage of the U.S. submarine Squalis,
the Cousteau-Gagan aqualung had been used to over 60 metres and a
Swede, Arne Zetterstrom had dived to over 150 metres on a hydrogen-
oxygen mixture before losing his life on his way to the surface
(through a human error).
2.2 The Develonment of Ultrasonic Communication
The aftermath of the second world war left most of the navies
in the western world with a standardised ship to submarine ultrasonic
voice communicator (13). A carrier of about 8 KHz is amplitude
modulated and the upper sideband is transmitted. At the receiver
a locally generated 8 KHz tone is used to demodulate the incoming
signal. This is completely analogous to a single-sideband radio
telephony channel.
20
Examination of the attenuation of ultrasound in seawater
(fig 2.1) demonstrates the reason for the choice of such a relatively
low carrier frequency. The right hand side of fig 2.1 is a scale
showing the propagation distance corresponding to an attenuation
of 120 db. This may be regarded as being typical of the gain available
from a good receiver and is therefore an indication of the range
available. This does not take into account the "spreading" loss
which would follow an inverse square law in deep water and an
inverse law in shallow conditions where the spreading is in two
dimensions only. No mention has been made of directional transmitting
or receiving arrays as these are seldom applicable to diving operations.
Finally this simplified picture ignores the bandwidth of the signal.
(To be strict, the choice of the figure 120 db implies that the
bandwidth is of the order of that required for voice communication).
There are two further sources of difficulty in propagation which
will limit the use of an ultrasonic communication link. These are
forward scattering and multiple path propagation (fig 2.2). The sea
is not a uniform media and forward scattering is caused by signals
arriving at the receiver after being refracted by layers of,water of
slightly differing temperature. These signals will add to give a
resulting signal that will fluctuate at random in phase and amplitude.
Berktay and Gazey (11) have shown that this should be negligable for
short range communication (<100 metres). In comparison, the parts
of a signal subject to multiple path propagation will arrive at the
receiver very much later than the direct path signal. Multiple paths
are often far more serious in shallow water and around wrecks and
1. air
0.1 - Attenuation
dbimetre
0.01
Multiple paths
sea surface
Receiv- / er
........., '...„.--...1- Receiv-
, ...• -------.4 er ..r ..-.. ~'-•.i .... / or" -. , ‘.,4 _,.- /
..- ///
,..., ....- -7' .04 tb /
....... .0. ... .' .../ i -,r . Tra,nsm- o' -- itter 0- ----P .....
, Transm-6,-- itter
Forward scattering
21
Fig 2.1 The attenaution of sound in seawater (after Tucker Et Gazey,10)
0.12
1.2 Range
Itm
- 12
- 120
• 10 30 100 300 1000
frequency
Fig 2.2 Forward scattering and multiple path distortion.
0.001 ,
harbour installations and these are just the areas where divers are
likely to be working most.
One of the earlier successful communications sets to be designed
specifically for divers was that of Gazey and Morris (12). This
employed a 120 KHz carrier which was frequency modulated. The range
was up to about one killometre. As a precaution against multiple
path signals reflected from the surface or the sea bed, cylindrical
transducers were employed. These were arranged to float vertically
in the water and had an omnidirectional response in the horizontal
plane but were limited to 300 about the horizontal in the vertical
plane.
The debate between frequency modulation (F.M.) and amplitude
modulation (A.M.) is still unresolved. An A.M. system will resolve
multiple path signals and pass them to the listener who will perceive
them as echos or reverberation similar to that experienced in an
empty concert hall. In contrast an F.M. system, although better at
discriminating against noise, will resolve multiple path distortion
in a way with which the human ear is not familiar.
The majority of ultrasonic communication equipment is not capable
of transmitting and receiving a signal simultaneously. This will mean
that the divers must use some form of "Procedure" for operating their
send-receive switch. During a conversation it is not possible for
one party to interupt the other. In radio telephony this is known
as "simplex" operation. It has been proposed to replace the send-
receive switch with an electronic circuit which will perform this
22
23
function automatically (12). This is known as a voice operated
switch and has been used for a number of years on loudspeaking
telephones to prevent a "howl" when the microphone and loudspeaker
are operating simultaneously. The problem with such a circuit is
that it will respond to other loud noises and operate falsely.
Underwater, where the sound of the divers breathing apparatus may
be considerably louder than his voice, voice operated switches
are unlikely to prove successful.
There is one ultrasonic communicator which uses a "duplex" system
where a separate transmitter and receiver can operate simultaneously
(14).
2.3.Direct Audio Communicators
These systems transmit amplified human speech through the sea.
There is no carrier or modulation process involved and the unaided
human ear can be used for reception. It follows that "duplex"
operation is possible and both parties can interrupt one another
while speaking. The main limitations lie with the limited
sensitivity of the human ear and with the difficulty of producing a
transmitting transducer with a sufficient bandwidth to cover the
whole of the audio range.
The threshold of underwater hearing has been measured by many
workers, notably Hamilton (15), Wainwright (16), Montague & Strickland
(17) and Brandt & Hollien (13). Fig 2.3 is taken from this last
author.
db ref
0.0002 dyngs cm
Hearing aid
30.
70 Unaided ears
50 .
To diver's air supply
Flexible membrane
Diaphragm & coil
Fig 2.3. Human hearing underwater (From 18 & 16)
24
p 1 ■ . 125 500 2K 8K
frequency Hz
Fig 2.4 Two types of electrodynamic transducer with pressure
compensation.
Iron
Electromagnet diaphragm
25
There have been two attempts to produce an underwater
"hearing aid". The threshold for a diver wearing that designed
by Wainwright (16) is shown on Fig 2.3. The other, designed by
Bauer and Torick (19) is a binaural device and was designed to
produce the interaural delays that a subject would normally experience
in air.
In order to cover the relatively large bandwidth, from 100 Hz
to 5 KHz, most designers have used electrodynamic transducers. The
more common piezoelectric and magnetostrictive devices that are used
for ultrasonic generation, are rejected because it is difficult to
arrange for the moving element to have the required displacement to
radiate sufficient energy at low audio frequencies. Although
variable reluctance electrodynamic transducers have been used for
underwater communication (20), the moving coil arrangement is more
popular (21)(22). (Fig 2.4).
If the internal mechanism of the electrodynamic transducer is
allowed to flood with water, or even if it is filled with oil or a
liquid with similar acoustic impedance, then the transducer can he
considered as a vibrating plate. It can be shown (23) that where
the dimensions of the transducer are small compared with the acoustic
wavelength, then the intensity radiation from this dipole source
will be given by:-
I 4 4 G 2 Tr f a U cos20 3 2 c r
2.1
Where the velocity of the vibrating plate is:-
U U e 2 rr ift 0
On the other hand if the medium has only access to one side
of the plate which is the situation if the transducer mechanism
is air filled, then the transducer will act as a simple source, and
the intensity will be:-
I= /, f2 02
8 c r2
Where 0 is the net flow of fluid from the source.
Now it can be seen from 2.1 and 2.2 that the radiation from
a simple source will fall off at low frequencies with the square of the
frequency, whereas that of the dipole will fall off as the fourth
power of the frequency. It is for this reason that practical designs
of transducers for audio frequencies are invariably air filled and
operate as simple sources.
As the diaphragm of the transducer must be free to move, some
means of pressure compensation is needed to prevent the hydrostatic
pressure of the sea from forcing the diaphragm into the mechanism
of the transducer. The two transducers shown in Fig. 2.4 illustrate
two different methods of pressure compensation. In the first a
rubber membrane at the rear of the transducer can flex and provide
the compensation; in the second the transducer is provided with a
supply of gas at the ambient sea pressure from the diverts breathing
apparatus.
26
2.2
27
2.4 Electromagnetic Communication
Although conventional radio waves in the 15 to 30 KHz region
are used for submarine communication, the aerial requirements
(up to 10 Km in some cases) render this impractical for diver
communication. There are however, two methods which can be used
over short ranges between underwater swimmers. These use either
the magnetic field produced by a small coil or the electric current
field produced by two spaced electrodes. In the magnetic case
the receiver is a similar coil and in the electric case a second pair
of electrodes are used (28).
The range over which an electric or magnetic field decays in a
conducting medium is described by the classical "skin-depth" relation.
In M.K.S. units this can be shown to be (24):-
2 2.3 (.4.) T>AA
Taking the conductivity of sea water to be 0.5 mho/m, then the
distance at which the field has dropped to 1/0 can be found at any
frequency. Now 1/e corresponds to 8.7 db. By using the same criteria
as was used in Fig 2.1, that the 120 db loss between transmitter
and receiver, a table of the range at various frequencies can be
drawn up. As in the acoustic case this ignores spreading loss
Fig 2.5 Range of magnetic & Electric field communication equipment
Frequency Theoretical Range
1 KHz 280 metres
10 KHz 100 metres
100 Miz 28 metres.
28
Although not liable to the acoustical problems of an
audio or ultrasonic system, these electrical communication links
are very prone to interference from electric power installations
and nearby radio transmitters.
2.5 The effect of helium on speech production
If one breaths a gas with a velocity of sound higher than
air, there is a marked change in voice quality. This distortion is
due primarily to the increased velocity of sound in the exhaled gas.
It must, of course, be borne in mind that the composition of the
exhaled gases will differ from those inhaled due to the metabolic
processes of the human body. The human voice can be considered
as the larynx, a low frequency generator very rich in harmonics,
followed by the vocal tract, a series of cavity resonators (25).
The spectrum of a vowel or voiced consonant will show the line
spectra of the larynx within an envelope function produced by the
relatively low Q resonances of the vocal tract. The most obvious
effect of helium is to shift the envelope up in frequency. The
larynx pitch is not appreciably altered. (Fig 2.6)
One instrument that can be used to analyse helium speech is
the "sonagram" or "voice-print". This produces a trace of frequency
against time, the amount of darkening of the paper being related to
the amplitude at that particular frequency. Fig 2.7 is a trace
of the author's voice recording the words "GOAT BAIT" while breathing
air and a 20/80 oxy-helium mixture at atmospheric pressure. It can
be seen that the instrument has not revealed the fine structure of
the larynx overtones but it has given some indication of the position
"AIR" SPEECH
SHIFT
"HELIUM-MIXTURE" SPEECH
SHIFT
SHIFT
VOICE HARMONICS
VOCAL-CORD PITCH: 100 Hz 100-Hz SPACING----1 (FUNDAMENTAL)
Goat bait Goat bait
3.0K 4 ..i.....;;...i.;
2 .51( -I 'i ,. '' - - 1 1, i„,, • , z 1.1,,iifor L!'ll
•!, 1.5I: 1, -
1.0K
50 0
oxyLhelium
2.0K
air
29
Fig 2.6 The effect of helium on the voice.
Pig 2.7 A recording of some helium speech (retouched for reproduction)
30
of the envelope. The horizontal lines that have been drawn on
the trace to show an estimate of the first three peaks of the
envelope function. These are normally called the voice formants.
At present there are three methods used for attempting to
improve or as it is sometimes described - unscramble - helium speech:
a) Frequency shifting
It is relatively easy to shift the spectrum of a signal by
a fixed amount. This is accomplished by modulating the original
signal with a carrier whose frequency lies well outside the audio
band; for example 50 KHz. If we now select the lower sideband and
perform a second modulation process with a different carrier, in
our example this could be 49,900 Hz, we will recover the original
signal but shifted down in frequency by 100 Hz. This technique in
itself would not be a satisfactory solution as 100 Hz shift would
reduce the larynx pitch to almost zero hertz while hardly improving
frequencies around 3 KHz which may require a reduction of over 1000 Hz.
What is required is proportional frequency reduction. The
"Band-shifting" unscrambler attempts to simulate this by breaking
the incoming signal into three or four frequency bands and by shifting
each of these bands by a different amount. The lowest frequency band
containing the larynx pitch is not normally shifted.
b) Time domain shifting
A tape recorder running at slow speed is one method of obtaining
a proportional frequency shift. It is often observed that the
naturalness of helium speech is improved by replaying it on a tape
recorder at a slower speed than that which was used for recording.
31
This method, although simple, would be impossible for any
real time processor. However, it is possible to break the incoming
speech into short sequences, slow these down, throw away the overlaps
that will result, and finally add these sequences to produce an
unscrambled signal in real time. If the length of sequence is
chosen to correspond to one period of the larynx fundamental, then
the reconstituted signal will retain the correct pitch. This
technique is after W.R. Stover (26).
c) Vocoder techniques
The vocoder is an instrument for analysing the synthesising
speech. It breaks the incoming signal into a large number of
channels (typically between 15 and 30) and can distinguish between
the envelope and the fine structure of the pitch overtones shown
in Fig 2.6. The vocoder does not perform a frequency shift in the
normal sense of the word. When used as an unscrambler it is arranged
to detect the envelope of the speech spectrum, to shift this
envelope down the frequency scale proportionally, and to adjust the
levels of the fine structure to correspond to this new envelope.
This concept was originated by R.M. Golden (27).
2.6 The formulation of a programme of research
What is the chain of communication between two underwater
swimmers who are speaking to one another? Fig 2.8 is an attempt to
break this chain that exists between the mind of the first diver
and that of the second, into as large a number of individual links
as it is possible. Those parts that have a closer connection to
physiology or psychology than to physics or engineering have received
1
Ears A
Psychological > Thought "Noise" 1 1
1
Articulation
Speech in Mask 1
Acoustic Noise Demand valve etc.
Microphone
Transmitter
40
Fig 2.8. The communication chain.
Propagation
Noise in Sea
Receiver
If Necessary.
Earphone 1
Acoustic Noise Bubbles, Breathing etc.
Ear
Psychological
• "Noise"
Perception
32
DIVER ONE
DIVER TWO
33
brief mention in chapter 1. All these links must be considered
when designing a communication system. After all, it is of little
use if the world's finest electronics are operated by a switch,
too small to be grasped by the hands of the diver who is trembling
with cold or anxiety:
The writer has set out to examine this chain, to determine
its weakest links and to concentrate his attention on these. With
this in mind the first stage was to design and construct, from
first principles, a communication system and to compare this with
'those designed by other workers. This initial step was to produce
a better understanding of the whole field and some knowledge as
to the areas where present hypotheses are failing.
The outcome of the early pilot experiments was that the writer
considered that there were two links in this chain that were
particularly weak and furthermore were applicable to study with the
resourses available in a physics laboratory. These aspects were
the formation of speech in a confined cavity and the mechanism of
human hearing underwater. The writer chose to examine the
second of these in considerable detail.
After the initial experiments with complete communication
systems, the difficulties of making measurements in open water were
appreciated. The main problems concern the sea surface. When taking
low level acoustic measurements from a boat, wave motion and the
noises generated on the surface would inevitably be fed down the
hydrophone cable. Further, ironical as it may seem, the problems
of communicating with human subjects underwater proved formidable.
34
It was for these reasons that attempts were made to
construct an underwater laboratory to use for acoustic observations.
This is not the place to describe the designs which were not built
through lack of finance. However, attention will he turned to
one successful design which was used to measure the threshold of
human hearing underwater.
33
Chapter 3
A COMPLETE UNDERWATER COMMUNICATIONS
SYSTEM
3.1 Introduction
The simplest method of voice communication would be to
shout loudly into the water and to use the unaided ears for
reception. However, two serious problems arise. Due to the impedance
mismatch between air and water, only a very small fraction of the
sound energy will be radiated into the water. Furthermore, the
restrictions on the face imposed by most types of diving masks would
make it difficult to shout anyway.
The simplest useful diver communication system is that which
has been referred to in section 2.3 as "direct audio". The diver
who is speaking is given a microphone, amplifier and transducer to
transmit his voice in a similar manner to a conventional public
address system. No receiver is required. As a first stage to
examining the communication chain, a direct audio system of this type
was constructed. The design will be discussed in the next few pages.
3.2 The Microphone
It might be suggested that almost any conventional microphone
mounted in the air space of the face mask, would prove suitable.
However, a diving mask is liable to flood at any time and the microphone
must be capable of withstanding not only immersion but also the
hydrostatic pressures involved. Three possible solutions are ,
illustrated in figs 3.1, 3.2, and 3.3. The first is to use a
36
+ 10 volts.
Fig 3.1
Electrodynamic insert
Resin
Air space Flexible membrane
Fig 3.2
Telephone earpiece mechanism
Fig 3.3
—100db -
—110db
Fig 3.5
input
Diaphragm open to sea on both sides
Hollow piezoelectric cylinder
Fig 3.4
non
ring
'Crystal' microphone insert
Response (reL 1 volt / microbar)
500 1K 2K 4K 8K Hz.
111 220
2N3702
2.2K
1K
2N3810
100p±'
37
conventional microphone with a relatively small internal air
volume. This is mounted inside an enclosure, one wall of which
is made from a flexible membrane. The membrane both transmits
the sound and can flex to accommodate the hydrostatic pressure.
The microphone in fig 3.2 is constructed like a telephone earpiece
and the moving part, the diaphragm, need not be protected from the en—
vironment as it is free flooding on both sides. Finally it is
possible to produce a microphone sufficiently rigid to withstand
the hydrostatic pressure. The small sealed piezoelectric ceramic
cylinder shown in fig 3.3 is possibly the simplest example of this
technique. Incidentally, one advantage of the "stiffn microphone
is that the system self resonances are generally well above the
audio range. This will tend to produce a relatively level frequency
response.
The writer constructed several microphones of this latter
type. The construction technique and a typical response curve is
shown in Fig. 3.4. One may recognise the active part as being the
insert from a domestic crystal microphone. These were chosen because
they were readily available, an important point when one considers
the high mortality rate of any equipment used in the sea.
Unfortunately, although the microphones that were constructed
had an excellent frequency response and were mechanically robust,
they were very insensitive. In Appendix 1 it is shown that even with
a well designed amplifier, such as the design in Fig. 3.5, the
electrical noise will correspond to a sound pressure field of around
2 ±64 db with reference to the usual origin of .0002 dynes/cm. The
writer was of the opinion that, under the conditions of use
where the noise level produced by the diverts breathing equipment
is high, this disadvantage is outweighed by the advantages.
Up to now it has been assumed that the microphone would be
placed inside the facemask to nick up airborn vibrations in the
conventional manner. On the other handl there are two alternative
microphone positions that have been used. The microphone can be
pressed against the boney structure of the head (29) or against
the side of the throat (31). Although the former, known as a bone
conduction microphone, is capable of better reproduction than the
throat microphone, both of these are positioned outside the diver's
mask and consequently have a tendancy to pick up water borne sound.
When attempts were made to use these forms of microphone with a
direct audio communication system there was a tendancy to "howl""
because they were receiving the transmitted signal through the
sea water.
3.3 The Amplifier
The amplifier circuit is conventional It employed a
quasi-complementary, output stage capable of a nominal 10 watts
R.M.S. sine wave drive into a 3 ohm load. This form of amplifier
was first described for audio work by Toby and Dinsdale (32).
In detail the design differs from the original in the use of an
output transformer to provide flexibility in the choice of load
impedance, and in the addition of a low-noise preamplifier.
38
39
Power for this amplifier was provided by a 22 volt nickle-cadmium
secondary battery with a capacity of 500 mAH. It is not
proposed to discuss the circuit further as almost any similar design
would have been suitable.
The mechanical design of the amplifier is rather unusual.
The battery and the amplifier are housed in a perspex tube (fig 3.8).
Perspex was chosen, not for its strength, although this design is
adequate for use to 60 metres but because it is transparent. This
enables any leak or ingress of moisture to be detected at an early
stage.
The method of sealing the microphone and transducer cables
and the method of sealing the control spindles should be clear from
the diagram. These are not the only possible methods but are,
in the writer's opinion, the most reliable. However, there is one
fault which this type of seal will not give protection against.
This is known as "hosepiping" and occurs when water leaks through
a break in a cable sheath and travels down the inside of thn cable,
through the sealing glands and into the instrument. (Probably the
best known example of hosepiping was the accident with the habitat
Sealab 111 where helium travelling up the inside of the electrical
power cables caused the project to be abandoned (33). As the cable
lengths are short in this equipment it was decided to accept this
risk to obtain the ease of changing transducers and microphones that
these seals offered.
Battery Amplifier Control knobs
Brass insert
ring
'C' rings
Brass/ spindle
Fig 3.8 The construction of the underwater amplifier
Cables to transducer & microphone
'Perspex' case
40
Detail of cable seal Detail of control seal
Fig 3.10 Response of 'Subaqua 10' transducer (taken from manufacturers data)
40 '
30 ' db.
20 •
10 -
200 500 1K 2K 5K 10K
frequency Hz.
3.4 The Transducer
A pressure equalised moving coil transducer of similar
design to that discussed in 2.3 was chosen. The construction of such
a device is similar to that of a conventional loudspeaker and best
performed with the facilities normally found in a specialist firm.
Consequently all the devices that were used in the course of this
research were either converted from loudspeakers or constructed by
a loudspeaker manufacturer. The photograph (fig 3.9) shows four
different moving coil transducers; one was a converted loudspeaker;
the others, specialist products. One of these latter, the Goodmans
"Subaqua 10" proved the most effective and reliable and unless
otherwise mentioned it can be assumed that this unit was employed
on all the tests recorded here. The response of the "Subaqua 10" is
shown in fig 3.10.
Before passing on it might not be out of place to qualify
the word "reliable" as it has been used in the preceding paragraph.
All the transducers illustrated in fig 3.9 leaked in service, the
worst leaked more often than not. The "Subaqua 10" leaked once and
with help from the manufacturers was rebuilt with improved seals.
It has since proved satisfactory over a number of years.
3.5 A Receiving Hydrophone
In the foregoing paragraphs the components of a simple direct
audio communication system have been discussed. For the monitoring
of subjects using this equipment and for the purpose of recording,
some form of hydrophone is required. Initially a hollow ceramic sphere
41
Fig 3.9.
Four electrodynamic transducers.
Goodman's "Sub—Aqua 10"
Modified loudspeaker
Raytheon "Yack—Yack" University sound underwater
loudspeaker
42
43
with a nominal resonance of 80 KI-Iz and a sensitivity of around
-103 db ref. 1 volt/pbar, below resonance, was sealed onto a length
of coaxial cable. However, although this proved useful in the
laboratory it was found to have a disappointing performance in the
field. The main problem was noise generated by the long length of
cable. This not only propagated as an acoustic signal down the cable
to the hydrophone but also appeared to give rise to an electrical
sig-nal directly. This latter problem was found somewhat surprising.
One might have expected an electrical signal to be generated in a
similar manner to that generated by a condenser microphone if there
had been a potential across the cable. However care had been taken
to see that this situation had not arisen. When a selection of
coaxial cables were tested by connecting them to an amplifier with
an input impedance of 1 megohm, the least satisfactory (unfortunately
a popular brand) generated several millivolts when handled and was
capable of generating a signal of over one volt when cracked like
a whip.
To overcome these problems a preamplifier was constructed
and attached to the lower end of the main cable. This was designed
to amplify the hydrophone signals and to apply them to the cable
from a low impedance source. The piezoelectric sphere was coupled
to the preamplifier by about 20 ems of lightweight cable and
provided with a small float so that it would hang just above the
preamplifier, the latter acting as an anchor.
When used in relatively shallow water (less than 10 metres)
44
with the preamplifier laying on the sea bed, this system proved
very satisfactory in use. Cable noise, both electrical and
mechanical, was completely eliminated. The circuit is reproduced
in fig 3.11 along with the electrical and acoustic performance.
The latter was measured by comparison with a known condenser
microphone in an airborne sound pressure field. When examining
this design it should be recalled that this preamplifier was constructed
during trials in Malta in 1966 and the availability of components
was limited. This was one of the reasons that the method of
waterproofing the preamplifier was to encapsulate the complete
circuit inside a film cassette can, using epoxy resin as shown in
fig 3.13.
On return to the U.K., it was decided to construct a hydrophone
along similar lines to the above but using a design capable of a
better noise figure, utilising low noise cable, and providing some
useful additional features such as headphone- monitoring. One
addition was the provision of a high pass filter. This can be
switched to remove frequencies below 200 Hz and is useful when
listening with headphones to discriminate against sea—state noise.
In the illustration, fig 3.13, this hydrophone and preamplifier
can be seen alongside its predecessor. The somewhat unusual
circuit is reproduced in fig 3.12.
Unfortunately all was not to prove well for this improved
design. When it had its first serious use in connection with
hearing trials in 1968, the piezoelectric sphere leaked sea water.
45
T2.2 1<
-r
-Ft v.
11 /c
Submerged unit
V/ 3707 /N3707
90 IC 0,F 3.9 k
Fig 3.11 Hydrophone pre-amplifier
r 5 K'
2 1-1
99,-r 4-70 f~ f K 910
f Fl
33
Submerged unit
n
try
2 N 317 /
33k
J
I Re?,
4.4 •
.o33,.F
TO 0
Eltic
AN 3704- //I/
2143707
3•7h
77i /
Electrical perfor:iance with 12 m of cable between.pre-amp and surface.
-72db ref 1 volt/microbar. Gain 33db, Input impedance 800X GOpf. Acoustic sensitivity
Fig 3.12 Hydrophone pre-amplifier
/0.2
J 10k•
1/7 3702.
Gain 40db, Optional filter (18db/octave below 20011z) shown switched out.
46
Fig 3.13.
Hydrophones used for recording underwater signals
Clevite CH-13
Used in underwater laboratory.
Spherical hydrophone with
Spherical hydrophone
pre—amplifier built in with pre—amplifier using
film cassette tin. FETE and high—pass filter.
Although this hydrophone has not played a significant part
in the experiments discussed in this thesis, the writer feels that
the design is sufficiently unusual to warrant inclusion.
47
Fig 3.14.
Direct audio system in use. Malta 1966.
48
49
Chapter 4
INITIAL L^_i_PartElE7:TS WITH CO,a1UNICATION ONIPMENT
4.1 Introduction
In the summer of 1966 various types of communication equip—
ment designed for use by divers were transported to the island of
Malta. These field trials were organised as one of the main projects
of an expedition sponsored by the iloyal Geographical Society and
Imperial College Exploration Board. The team consisted of two
postgraduate and five undergraduate students under the leadership
of the author.
The apparatus used in Malta included communication equipment
built to the design described in the previous chapter and two
commercial communication sets. A series of tests were conducted to
probe the workings and shortcomings of these types of devices.
4.2 The formation of words underwater •
The subjects were seated in about 6 metres of water rearing
conventional open circuit aqualung equipment. A cable from a micro—
phone of the type shown in fig 3.4 was led to an amplifier',
loudspeaker and tape recorder on the surface. The subjects' were
asked to- read a test paragraph, the editorial fro:: the Daily
Telegraph was popular, as distinctly as possible. For comparison,
recordings were made with subjects wearing diving equipment on the
shore. The tests were repeated with different types of facemask
and demand valve. These are illustrated in fig 4.1. There was no
disagreement by the subjects as to the merits of the different
50
Fig 4.1.
Three different types of diving 'facemask'
Full—face mask with
Mouth—cup (or mouth—mask)
demand valve attached attached to twin hose
demand valve
Mouth 'bit' (shown on single hose demand valve)
( The mouth bit and mouth—cup would be worn with a conventional
mask covering the eyes and nose.)
51
types of faceaask. All these tests were performed with a single
hose demand valve.
Condition Comment
Intelligible All masks in air
Full face mask underwater Words missed but meaning generally clear
Many sentences had to be repeated to understand them
Some divers could convey simple words but most could achieve nothing.
Mouth mask underwater
A conventional mouth—bit
Although in not such a drastic manner as the face masks, a
change in demand valve did effect the ability of the diver to
articulate. The actual noise produced by the valve did not seem
too important as most subjects chose a very simple, very noisy,
single—stage valve as being the most convenient for voice commun—
ication *. In general, single hose valves were preferred from twin
hose models and the least liked was a twin hose valve that was
notable for a high exhalation resistance.
4.3 Noise and ran!,:e of communication
ever a range of '25 metres the direct audio set was compared
with an G 17.1z upper—side—band A.M. ultrasonic communicator. The
latter contained a send—receive switch and was operated in a similar
way to an ordinary "walkie—talkie". The table compares the effect
of noise on these two systems.
* The valve referred to is the "Normalair" single hose model. It is made almost entirely of plastic and the pressure reduction from cylinder pressure to ambient pressure is accomplished within 3 ma of the diver's mouth:
52
Origin of Noise Description ._ „ 17e,',1,- icr n effect
on Iiirect .udio ?,:asking.., -1inr, effect :
on Carrier System
Sea state, and waves on shore
Continuous, Low frequency Very little None
Diver's demand valve
Wide band (hiss)
Complete masking
Complete masking
Biological (snapping shrimps)
Impulsive '
(crackling) Small Considerable
human movement :bubble noise
Intermittent Easks weak signals
Small
Reverberation Related to the signal
Not obvious Very noticeable
As can be seen, the serious noise sources over which the
designer has some control are the denand valve, movement of the
body in the water and e:thalation bubbles. It would seem unlikely
that there is any way of silencing the conventional demand valve
sufficiently to avoid masking incoming signals. It follows that
for reliable communication both parties must synchronise their
breathing. Ironically this is aided by the bursts of noise received
from the breathing equipment of the other party. It was for this
reason that steps were not taken to prevent breathing noises from
being transmitted. The use of closed, or semi—closed circuit
breathing apparatus (see 1.2) should considerably reduce the self
generated noise. Unfortunately this type of diving equipment was
not available for comparative testing.
The maximum range of he ultrasonic set was limited to
about 25 metres by the signal becoming garbled with multiple
path distortion. Although this is considerably loss than the
manufacturers specification of up to 5 kilometres, it is felt
that this latter figure probably refers to the use of this
equipment in the open ocean, not in shallow rocky conditions.
nen the moving coil transducer that was used in the direct audio
communicator was provided with a good electrical signal from a
surface microphone and amplifier it could be clearly heard at
ranges of over 100 metres. nen used by a diver the range of
reliable communication was reduced to about 30 metres. This
reduction was probably mainly due to the added distortions from
the diver's racamask. Finally, the rane of a commercially produced
direct audio set (Raytheon Yak—Yak) was found to be between
5 and 10 metres. The particular model available gave a relatively
low output and produced a considerable amount of (dstortion.
the device was completely encapsulated, there was no way of
investigating these shortcomings).
4.4 Human hearin-r underwater
One property of multiple path Listortion is that in general
the false signals will arrive at the receiver from directions
that will differ from the direct signal. If the human hearing
faculty is able to perceive the direction of a sound source
underwater then it is possible that this may be of some help in
reducing the effects of multiple paths.
53
34
To test directional hearing wide—band sound, tape
recorded music was the best available signal, was radiated from
a transducer positioned about 5 metres below. the surface. It
was necessary to use a signal with a wide bandwidth as a narrow—
band one, such as a pure tone, would produce a standing wave
pattern. Under these conditions it may be impossible, in
principle, to define the origin of the sound field. The subject
divers were positioned about 10 metres from the Source and at
the same depth. With their eyes closed, the subjects were first
spun round and then asked to point to the direction of the source.
ro protective clothing was worn on the head. The divers were
requested to try and remain motionless while making their
judgement; fig 4.2 shows the result of 40 judgements. The angles
were measured to the nearest 45 degrees, the figure on the left
is a polar plot of the subjects judgements with respect to the
true direction of the source, and that on the right, a plot of
the direction that the subjects were facing. These results were
tested for significance usinz a method that will be described in
detail later. suffice it to say at this stage that these' results
fail to meet a criteria of significance (i.e. there is more
than one chance in 20 that the subjects indicated random directions).
4.5 Discussion
These trials were in tie nature of pilot experiments and were
in consequence, relatively primitive. After allowing for this there
are three main questions that arise. Is there a physical explanation
Direction of source in both cases
A
CD
0
0
1" •
C4. O
p
CD p w.
CD cn cf-
0)
Direction that the subjects indicated
Direction that the subjects were facing
SCATW, *" represents one indication
56
to the problems of forming words underwater? Nhat limits the
range of a direct audio communicator, and finally, what part,
if any, does directional hearing play. in communication between
divers? These rill be considered in order:
4.6 The formation of words
It was seen in section 4.2 that it was easier for a diver
to form words into a large facemask than into a small one, and
most difficult of all when there was no mask over the mouth.
Both Webb (20) and Hainan (30) have also reported that some types
of mask have a distinct advantage for voice communication. Apart
from accounting for this observation it is also necessary to
explain why there is a difference between the same equipment worn
in air and underwater. An interesting demonstration of this latter
point was performed by taking a fully kitted diver and monitoring
his voice as he slowly lowered himself into the water. The most
noticeable drop in intelligibility came, not when the exhaust
ports of his breathing apparatus submerged as might be expected,
but when his head and mask went underwater. As far as is known
there is not a published mod.el to account for the difference
in facemasks or this last observation.
The writer proposes to adopt a model of the vocal tract
proposed by Dunn (25). This suggests that the larynx can be
represented as a sound source feeding a series of cylindrical
tubes which have similar dimensions to the throat and other
S7
cavities in the human vocal tract. This mechanical model can
then be expressed in electrical terms For ease of analysis. The
writer proposes to add one additional cylinder as a termination
to the model tract, to represent the case where a diver is
speaking into a faconask underwater. The reason for assuming that
a mask can be considered as a closed cavity when used underwater,
but not when used in air, is that in this latter case the walls of
the mask can vibrate as a membrane and transmit some sound.
However, underwater the air—rubber—water aC01.12'6iC impedance
mismatch will prevent appreciable tranmnission and the mask will
act as a closed cavity.
After this model, both with and without mask, had been
reduced to simple electrical circuits, these were constructed and
direct electrical measurements made on the effect of the mask.
Dunn (25) has shown that the formation of vowel sounds by the
vocal tract can be raDresented n
Fig 4.3
length
larynx
a transmission line (fig 4.3)
lips
area Ai
The throaty tongue constriction, mouth and lip constriction are
represented by four tubular transmission lines with lengths 11
to 1 and cross—sectional area Al to A
4 respectively. The vocal
chords are assumed to be an acoustic source of constant volume
(generally referred to as a high impedance source in analon-v to
the electrical case). In this model the resonances of the
transmission line will play the same important role as the
resonances of the vocal tract; that is, they will define the voice
fonnants. No nasal cavities are considered because Dunn suggests
that these are normally closed in good English vowels. In diving,
it has been suggested by `;'ebb (20) that as the mouth and nose
are often in different 'masks' at slightly different pressures.,
this will tend to close the ulvula.
Now if dissapative (i.e. resistive) terms arc ignored,
Dunn showed that each of t':e four small sections of his model
could be reduced to the following electrical equivalent:
Fig 4.4
L L 1
A C
L tan(2.1. C = oc)
/De cosec (wl A c /
58
59
The complete vocal tract of fig 4.3 can now be replaced by the
network below. The output current (representing a volume velocity
from the lips) passes through an analogue of the radiation
impedance of free air given by:—
Fig 4.5
cox 2 c
L, = ,§4 3 V A4 TT
Duml suggests that as C2 and C4 both depend on the volumes of small
constrictions they can be ignored. Further as L1 is in series
with a high impedance it too can be ignored. This reduces the
electrical model to the following:—
Fig 4.6
L,+ L3+ 2Lz
tb= L3+ 2L*
C,= C,
C4= C3
60
This is the electrical model of the vocal tract after
Dunn. Before using this with problems concerning facanasks
there is one further simplification that can be made. This is
the assumption that the dimensions of the vocal tract are small
compared with the wavelengths involved and hence to replace the
tangents in fig 4.4 with their arguments and the cosecants with the
reciprocals of their arguments. Although Dunn does not like this
"lumped constant" assumption but prefers to solve the relations
that include trigonometric functions by graphical means, he does
use this approximation latter in his work and produces the first
two fonnants to within 10'10 of the more accurate method.
The writer proposes to use the lumped constant approximation
because it is then possible to construct the model from relatively
few components in the laboratory. A check on the accuracy of these
approximations is provided by comparing the experimental values
with the formant frequencies calculated by Dunn using both the
more accurate "distributed model" and the lumped constant mojel.
The writer has calculated the electrical components .
corresponding to the three Znglish vowels in the words EAT, LOST
and BOOT. These are the sane three vowels that Dunn analyses. The
physical dimensions of the real vocal tract as obtained from X,--ray
photographs are also taken from this reference.
To sum ups the writer has taken a published model of the vocal
tract and deduced the values of electrical components corresponding
to three vowels.
61
Before constructing three electrical circuits, the writer
"rationalised" the component values by multiplying the inductances
and resistances by 100 and dividing the capacitors by the some amount.
This has the advantage of keeping the same frequency scale but
enabling readily available components to be used. Fig 4.7
tabulates the vocal tract dimensions obtained from Dunn (25) and
lists the electrical values appropriate to the lumped constant
model discussed above.
The electrical model or the facemask has been calculated in
the same way and the component values are listed in fig 4.8 along
with the dimensions of the masks.
In the test circuits that were constructed (see fig 4.9) the
laryn:: current generator has been replaced by a Bruel a Ejaer beat
frequency oscillator with a 40 Liillohm series resistance. The
value of the radiation resistance is not over significant (Dunn
chooses to ignore it) and has been aporo:;.inated to a fi::ed 10 ohm
resistance which would be the correct value at about 800 Hz. !lien
the inductive and resistive components corresponding to the
radiation impedance of free air are replaced by the eciuivalent
circuit of the facemask, one should correctly measure -che potential
across the caoacitor C . This would correspond to the acoustic
pressure inside the mask. liol:ever for convenience, the current
passing through C was measured by monitoring the potential across m
a suall series resistance (10 ohm). Strictly speaking we are now
401L(2. .4211 .21511
1 0-0-
0
401L9- .42H .17H
1.4uF
To level recorder.
T3F3PF T°' 1 0 -n-
0
Fig 4.7
Mechanical and electrical values for vocal tract model.
Vowel 1,
Mechanical
lz 13
dimensions
lg
(from Dunn)
A3
Electrical (see Cb
fig La.
equivalents 4.6)
Lb L,
EAT 7. 1.5 4.5 0 7.7 0.9 1.5 1.5 38 4.3 4.2 1.7 .45
LC ST 3. 4. 8. 1. 2.3 .75 10.9 6.3 5.0 61 7.12 .6 .22
BOOT 6. 3. 7. 6.351 1.14 5.0 .52 27 25 4.3 3.0 .76
cms 2 cms pF mII
Fig 4.8
Mechanical and electrical models for the facemask.
L. L,
Fig 4.0. Examples of two of the test circuits (using rationalised values)
62
Mask Volume Length C, L,,,
Full face 1200cc 10cm 8451.1F .045m11
Mouth mask 200cc 5cm 140T1 .07mH
Bit 20cc 2cm 14r .28m11
'EAT' in free air
'EAT' with mouth mask
measuring the velocity at the lips and not the pressure in the
mask. As these will bear a simple relation to one another and
either will indicate resonances, this substitution is of little
importance.
Twelve electrical circuits were constructed. These
correspond to the three vowels in air and into each of three types
of mask. (The mouth bit of a conventional demand valve has been
treated as if it were a very small mask). The response of these
circuits is shown in fig 4.10. The position of the peaks, or
resonances, which represent the voice formants are of greatest
interest. Before looking at the effects of the masks it is worth
comparing the position of the fonnant in free air obtained by the
writer with those calculated by Dunn.
Vowel "acact" distributed Lumped constant writer's constant model, fonnants model, fonnants eN.nerimental calculated by Dunn using calculated by results taken graphical methods Dunn from fig 4.10
1113 325 2300 322 1650 300 2000
LOJT 640 930 625 873 Broad hump 600-800
BOOT 310 850 .305 794 300 800
The agreement is sufficiently close to give confidence in
the method. The only critisn would be that the 2 of the electrical
circuits is somewhat low. As this was determined by the components
available there is no simple wa: of overcoming this.
Perhaps the most noticeable effect of the mask is a
63
1\iouth bit.
_
cr,
fivimP4'"
N.N.4, 444
CJ CD rn
0
rn
0 1,1
0
0 0
CD
0 CD
0
ti
1 °Batt- -'Lost'
11.10•••• =him*
100 200 500 1K
2K. Free air. frequency Hz.
Full face mash.
Mouth cup.
N
65
negative one. The somewhat drastic change from the small inductive
impedance of air to the larger capacitative ones of the facemask
appears to have had remarkably little effect. To take one exampled
at 300 Hz the change is from about 1 ohm inductive to nearly
4 ohms capacitative in the case of the mouth mask. The only effect
of this appears to be a rise of between 50 and 100 Hz in the first
fonnant of ELT and LOOT. However, in all cases the very small
volume associated with the mouth bit has drastically effected the
formants. No further explanation need by sought to the extreme
difficulty in forminc, words in this case.
It must be remembered that this model is in one way very
different from the true method of voice production. This model
does not include the feedback between the mouth and ears. i!or
this reason real subjects would not be expected to proCuce these
vowel sounds as they would try to compensate for the distortions
that they hear.
To conclude, although the electrical model of the vocal
tract would indicate that the formation of vowels into a mouth bit
or directly into the water is difficult it does not explain why
the larger facemasks produce such poor articulation.
If is unfortunately not possible to produce a simple model
for the production of consonant sounds. For this reason the above
analysis must only be considered as representing part of voice
production.
66
4.7 The range of communication
In theory'. one might expect that a communicator
which relied on the unaided ear for reception, would be
limited in range only by the distance over which the signal
falls below the threshold of hearing. In general this is
not so. It is the steady deterioration of the signal to
noise ratio as one moves away from the transmitter which
sets the maximum range. As the range was increased it was
found necessary to remain motionless to hear the signals. Now
the outer ears are closed by the presence of water when diving
and this condition will give rise to an improvement in hearing
by bone conduction (between 15 25 db, Zwislocki 34). Hence
body and equipment movement noises which may be carried through
the human body become important underwater. These sounds,
along with the exhalation bubble noise, rere found to be an
important limitation to the range of cannunication with the
working diver. A particularly Jimpledemonstration of this is
to scratch the back of one's neck. In air this is not a
particularly loud sound, whereas unaerwater it will appear
far louder and will probably mush other hearing.
4.8 Directional hearing
Binaural hearin-4 can be described as the ability to use
both ears to receive acoustic signals and to respond to small
differences between these two signals. Probably the best
known exanple of binaural hearing is directional hearing.
By detenuining the time, or phase, difference between the
arrival of a signal at the two ears it is possible to
calculate a possible direction for the origin of the sound
in the horizontal plane.
Time difference = d cosy
The diagram shows how this can be deduced. In order to
determine which of the two possible directions is the true
one, it is helpful to move the head. Only the true image
will remain stable.
Before proceeding it should be clear from the diagram
that if we expose the head to a steady state sound field with
a wavelength which is less than the interaural distance, there
rill be ambiguities as to the direction of incidence. This
arises because the two ears may measure their time scale from
different cycles in the wavetrain. In air this would correspond
to frequencies higher than about 1600 Hz. It is for this
reason that a sound source that is used for directional hearing
67
68
tests is often a wide-band interupted one, such as a series
of clicks. Most natural sounds, including speech, fall into
this category.
This picture of directional hearing is oversimplified.
There are two other mechanisms which play a part. The first
concerns differences in amplitude. The head will act a shield
and the ear which is closer to the source will receive a louder
sir-,:nal. This becomes increasingly effective at short wavelengths
where the head becomes a proportionally larger barrier. In air
this conveniently becomes useful at frequencies where time or
phase differences are ambiguous. The mechanisms discussed so
far would preclude a person who is deaf in one ear from
directional hearing. It is certainly true that directional
hearing is very much impaired by deafness in one ear and that
person cannot be said to possess binaural hearing. however, the
fact that such a person may have some directional abilities
requires an el%:planation.
Reflector
a
a
b
•
_ , ;_ --- - b -
Microphone
time
time-3
69
In the diaA:rmn, consider the nature of the signal received
by the microphone when a single pulse is emitted from 'AI
and 'B'. The single microphone with a reflector is capable
of differentiating between these two directions. It appears
that the human pinna is capable of acting as such a reflector.
It has been shown by Datteau (35) that reflections from the
pinna are observed as a change in timbre. If such a change
is made artificially, the subject will perceive it as a change
in source direction.
Directional hearing has an important role to play in
everyday life. It enables persons to become aware of danger
such as the approach of a motor vehicle. It has also been
shown to be important in the discrimination of a speech signal
in the presence of noise (36, 37).
Possibly the most well known example of this is the
"cochtail party effect" (Cherry 3S). A person "listening" to
several conversations simultaneously has little difficulty in
isolating an c' following any one his chooses. It is generally
not possihle to do this when listening to a "monophonic"
tape recording of the same conversations. (This can sometimes
be demonstrated with a stereophonic recording and this point
has been used to advertise -CAP, type of equipment). The abilit
to localise the source of wanted signal as a point in space
is not a necessary requirement and this effect can be
demonstrated where no "real" sound ima, es exist*.
70
Early workers in the field of underwater communication
dismissed directional hearing as not possible (Wainright
Interaural time delays would be shortened due to the increased
velocity of sound and the shielding effect of the head will be
quite different. However, if directional hearing is possible
underwater it would provide an explanation for the superiority
of the direct—audio communication system in noisy, reverberant,
shallow water conditions. After all, reception in these
conditions should be analogous to that in the above "cocktail
party".
One might suppose that this hypothesis could be tested
by covering one ear and looking for a drop in intelligibility.
Unfortunately very little is known about the mechanism by
which sound reaches the submerged ear. If this is by some
mechanism similar to bone conduction, a cover would be ineffect—
ive. A further possibility is to test directional hearing by
asking the subjects to indicate the direction of a sound source.
This is the avenue that the writer has chosen to explore. The
results obtained in 10GG (presented in paragraph 4.4) are
inconclusive. This problem rill be examined further.
* During the last war, messages were transmitted to the French Resistance 13: sending a cor:mon voice signal from two radio transmitters. If the enemy "jammed" these tranmissions then each would be jammed separately. To receive the message, two radios were tuned in, one to each transmitter, and the listener sat bet,:Teel: the radios. The effect was that the speech a:)peared to separate from the jamming noise and could be understood. The 1313c have a reconstruction of this in their archives.)
71
Chapter 5
7.3.-fr2E:LE.1=S (1)
5.1 Introduction
The importance of binaural hearing in everyday life
and particularly in speech communication has been discussed
already. lowever it remains to be seen as to whether
binaural hearing is possible underwater. The reaction
obtained from (mestioning divers is all too often that it is
not.
There are two main methods used for testing binaural
hearing on the surface. Both of these are discussed by Nordlund
(10). The first method is to provide the subject with a pair
of headphones and to insert a time delay or amplitude
difference in the sound that is being played through one
earpiece. The subject is asked to localise the sound as a
"phantom ina.ge" inside his head. .il. common test is to
introduce a time delay anL to ask the subject to recentralise
the image by increasing the amplitude of the delayed channel.
In this manner information as to the relative merits' of time
and amplitude differences can be explored. This headphone
method has been reported as being particularly useful in the
diagnosis of brain lesions (Nordlun:: 40).
The alternative is to position the subject in an
anechoic chamber and to ask him to indicate the direction of
a real sound source such as a loudspeaker. The chief problem
72
with this method is the provision of anechoic surroundings
although there is the subsidiary problem in the selection of
a method for the subject to use to indicate his estimate of
source direction. It was proposed to adopt this second
approach underwater as it should yield results that are
directly applicable to the real environment.
Anechoic conditions are virtually impossible to obtain
at audio frequencies underwater. The best that could be asked
for would be a sandy bottom in the open sea, far from the shore.
(Mud would be better as a sound absorber, but would introduce
safety and logistic problems). If visual or photographic
methods were to be used to record the subject's reaction, it
would be essential to use clear water. Finally the water
temperature must be sufficiently warm to allow subjects, who
would not be wearing protective foam rubber around the head,
to remain practically motionless for at least 33 minutes.
To meet these requirements the directional hearing
tests were organised as part of an undergraduate expedition
to L:alta in a similar way to the cgmmunication experiments
described earlier. This took place in 1908 under the
sponsorship of Imperial College Exploration Board.
The decision on the form of the directional hearing
tests was made after some preliminary experiments off the
French 1:editerranean coast earlier that year. Two main methods
were used. In the first the subject diver was suspended in
73
mid—water and asked to point to an audio frequency source.
In the second series of tess the subject was seated on a rigid
box and asked to judge from which of two known positions a
sound vas originating.
5.2 The round L;ource
The requirements for the source were laid down in
section 4.5. The source must be capable of producing a wide—
band, non—continuous sound of sufficient amplitude to be heard
by a diver under normal conditions. Initially the writer
considered using an electrical tone generator coupled to
a transducer. However, such emiipment tenus to be cumbersome
for a diver and lacks the degree of reliability that would
be preferred before embarking on field trials. To overcome
these disadvantages a simple electromechanical unit was
designed. It consisted of a small container, one wall of which
was mace from sheet metal. Inside this air—filled container
a small hammer was arranged to strike the metal which acted
as a diaphro4la. The repetition frequency of the hammer, about
20 Hz, is too low to be radiated efficiently by a device of this
size, but the signal is rich in harmonics and these) along
with resonances of the case, are transmitted through the water.
Figs 5.1 & 5.2 show how these devices were made.
In use the '0' rings are gre sed, the batteries connected
and the lid pushed on. It is necessary to slacken the bleed
screw to do this, otherwise air pressure will force the lid
"(Yrmg.olil
Battery
R„
Magnet Bell mechanism
Perspex case
ti Bleed valve
Reed switch
74
Fig 5.1 Photograph of the 'buzzer' sound source. (with lid and '0' ring removed)
Fig 5.2 Construction of the 'buzzer' sound source.
Metal lid
The metal diaphragm is a lid taken from the type of tin that is commonly used for packing bulk unexposed 35mm film. It was chosen to be a loose fit over a standard diameter sample of -21- inch wall Perspex tube. An open ended Perspex container is constructed from this tube with an '0' ring seal for the metal lid to fit over. Inside the container is mounted an ordinary electric bell mechanism without the gong and with the hammer bent to strike the tin lid, a battery holder with four "high power" size AA (penlight) batteries and the insert from a magnetic reed switch. On the exterior a small circular magnet is mounted opposite the reed switch and a 4BA hole is tapped through the case and counter—sunk on the outside. A SBA cheesehead bolt with a small '0' ring under the head completes the instrument.
75
off again. The first time the instrument is taken underwater,
the hydrostatic pressure will defoni the metal diaphragla to
dish shape. This is of little consequence as the bell hammer
can be re—bent to accommodate this. Two sources have been
made to this design; they have been used to depths of GO feet
and can be heard by a diver, who is listening,ovei- ranges
or up to a hundred metres.
The response of one of these devices, taken from a
tape recording made "on site" is reproduced below, fig 5.3.
The recording was made on a Philips Cassette recorder and the
response on this machine is also shown. The hydrophone used
was a piezoelectric sphere of the type described in 3.5.
For analysis the tapes were re—recorded on a laboratory
machine, formed into loops and replayed through a Druel
ijaer octave filter. The mean level at eacll frequency was
estimated from observations of the meter incorporated in the
filter unit.
5.3 'J'ree choice experimonts
In these experiments the subject, who would be wearing
about eight pounds less weight than for normal diving, was
tied by one ankle to a fixed ballast on the sea bed. This
weight was normally in 8 metres of water on a sandy bottom.
Apart from a facemask with a hinged metal blackout flap and
the absence of any form of rubber hood, the subjects used
conventional aqualung e,:juipment. The sound source was held
76
Fig 5.3. Response of 'buzzer' sound source (Taken from recordings
made at 5 m in the open sea. Corrected for the tape recorder)
0
10db.
50 oo 200 500
frequency Hz.
. 1K 2K
• 5K
Overall response of cassette tape recorder
Sdb
So 100 /00 500 law 2e 5.e 10; Zc'k
77
by an accompanying diver who remained at the same depth as
at the constant distance from the subject (fig 5.4). A
surface swimmer wearing snorkelling equipment, was instructed
to position himself directly above the subject using the
subject's exhaust bubbles as a guide. The surface swimmer
held a "Calypsophot" underwater 35mm camera with a viewfinder
attached which was marked with "cross-hairs".
The method of conducting these tests was for the
accompanying diver to switch the sound source on and for the
subject, in his own time, to point to where he estimated it
was situated. The subject was permitted to move his head and
to rotate his body in his efforts to localise the source. Once
the subject hid chosen, he was instructed to hold his arm out
until the source was switched off. ',;hen the subject had
indicated his choice the cameraman would line his "vertical"
cross-hair on the direction of the source and take a "plan-
view" photograph of the subject. Aftet the exposure the
cameraman would si,gnal for the source to be switched off. In
order to allow for any confusion that may be caused by features
in the area reflecting the sound, the source operator swam to
a new position after every test. AlthoUgh the aim of the
team was to expose 36 frames at a time, the experiments were
often terminated after a shorter run due to the subject
suffering from cold. Under good conditions 36 exposures could
be recorded in about one hour.
-.7
Tape measure
Fig 5.4 Diagram showing the position of the divers in the free choice experiments.
Cameraman on surface
78
Cmneraman uses subject's air bubbles to position himself vertically above subject.
Sound source
Subject anchored to sea—bed about Sm below surface
79
5.4 Two choice experiments
In these experiments a triangle was marked with tapes
on the sea bed at a depth of 30 feet. 11 heavy steel box
provided a firm seat for the subject at the apex of this
triangle and he was allowed to study the layout before
closing the mask black—out. The sound source was positioned
at one of the two distant vertices and the subject had merely
to raise his left or right arm to indicate his judgement of
the source position. It was found convenient to have a third
diver to record the results.
In operation the diver holding the source would
either swim across to the opposite position on the triangle
or swim to the mid—point of the baseline and back again after
each test in order that neither the time intervals nor his
breathing rhythm should give additional information to the
subject. It was normally found possible to take GO judgements
within one hour.
5.5 nesults of Free choice ex,,eriments
The exi)osed film was processed and printed. The angle
the subject had indicated was measured from the photographs
with a protractor, 0 degrees representing the direction of
the source. The temptation to use these angular measurements
as a direct means of evaluating a mean CLirection and standard
deviation was avoided. (,,uch a proceL,s woulc: allow two angles
of 1750 cala — 175° to produce a mean of 0° with a large
80
deviation when in fact the angles are only 10 degrees apart).
The method used was to represent each measured direction as a
unit vector and to evaluate the mean vector for each subject.
The magnitude of this mean vector will be a measure of the
subjects consistancy and the direction, that or the subjects
average choice. The author considers that far more emphasis
should be placed on the magnitude than on the direction of
this vector, as under operational conditions a diver would
ex-.3ect to collect additional information with which to correct
overall angular displacement.
Subject
FiT 5.5
Mean Magnitude
Results
significance Jtandard Deviation
No. of Photos
Vector - Direction
M .L.
J.;:;.1::.*
J.W.
D. R.
R.L.
P.N.
P.Y .
18
14
24
22
33
35
11
21
0.80
0.84
0.75
0.87
0.52
0.23
0.63
0.72 0.01 —112°
0°
— 1° ,
+ 3
— 34°
— 23° o + 3 —< 53°
4'
<0.001
< 0.001
< 0.001
< 0.001
< 0.01
Not. Signif.
0.001
410
55o
46°
53o
36° 74°
500
Mean of standard deviations on significant occasions = 510
* A different sound source was used in these cases (These will be discussed in G.9
The above table shows the results obtained on eight tests
involving six subjects. The first question to ask is whether
these subjects are pointing in a random direction. The
results in column five are an evaluation of the probability
81
of this null hypothesis. (i.e. this is the probability that
the chance addition of this number of random vectors would
give this or a greater mean). Fig 5.6 shows the relationship
between the mean vector, the number of tests and the
significance. The derivation of this relation is considered
in Appendix 2. It can be seen that one test failed to meet
a 50 criteria for significance.
The interpretation put on the direction of the mean
vector is more difficult. From the magnitude of the mean
vector, it is possible to estimate the standard deviation of
the direction. This is shown in column six. Although there
is a tendancy to point to the left and this may be a failing
of riht—handed subject (all the subjects did point with their
right arm), with one exception this cannot be regarded as
significant. The one subject ('.:.L.) who did point to a direction
significantly different from the true one (P<.05), reported
that one ear had failed to clear properly on that occasion.
On a total of 10 of the 178 photographs considered in
fig 5.5, there was an indication of the true direction, of the
source. Either the source itself was visible or a tape running
between the subject's anchor and the source could be seen.
In these cases the angle between the true source position
and the side of the photograph was measured to provide a check
on the overall accuracy of the viewfinder and cameraman. This
angle shows a mean of 1.2° and a standard deviation of 9°.
Fig 5.6 Reliability of the mean vector
35 -
30'
25
Significance
'NI 20 '
Number of
vectors 15 '
5 `
Not significant
• • •
• • • •
82
• •
•
• N. • ▪ •••
• • •• \ • • \
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Magnitude of mean vector
This figure is obtained from the table in Appendix 2. For small values of 'NI there will tend to be an error in the method that has been used for calculating the significance. l'T.owever, as 'N' tends towards unity, the value of a significant mean vector must approach 1.0. This fact has been used to provide some eNtrapolation for small values of 'N'.
10
a
83
Fib 5.7 Results of two choice triangle tests.
Angle Subject
Total % incorrect
111., ?off, PJ PST JSW JW BR
180° 39/9 45/1 84/10 12%
903 40/13 40/17 40/10 40/1 160/41 25%
60° 26/4 41/5
40/12 28/7 40/5 175/33 19%
45o
41/11 41/8 60/20 60/13 40/13 242/65 27%
20°
60/31 45/12 60/19
00/22 45/13 60/25
60/16 60/20 60/16
40/16 119/36
30/0 60/3
759/242 32%
NOTATIN
No. of judTeTients/::o. incorrect.
Fig 5.8 Consistency of subjects between exoeriments.
Free choice experiments rl1W) choice experiments
Subjects in order of mean vector
1:ean vector
Subjects in4.11 order of score on 20 triangle
% incorrect on 20' T'gle
.37 BR 10% JSW .80 JSW 28.5% JW .79 29.5%
.72 tfl7 30% PN .54 P11 36% RL .52 RL 52%
This list only includes subjects who participated in both series of experiments.
The correlation coefficient between the subject's mean vector and the % correct on the 20
j triangle has been evaluated.
Correlation coefficient = 0.99.
False True position position
Gaussian distribution
<—X
\\\ T
4 10.-t-10(1-2>
84
Although this error is large and it would be desirable
to reduce it on future occasions, it is insufficient to
appreciably alter the results in fig 5.5.
5.6 The results of two choice experiments
The total angle suspended at the subject by the two
source positions was recorded. Fig 5.7 shows the score at
different source angles. The position at the smallest angle
was studied in the greatest detail. At this angle of 20 degrees
750 judgements were recorded on thirteen occasions.
It will be assumed that the scatter in estimation of
source direction follows a gaussian distribution about the
true source position. For the subject to indicate the wrong
position in the 20° triangle test, his estimate would need
to be greater than 100 in error in the direction of the
wrong position . From fig 5.7 it can be seen that this
happened on 32 of occasions.
8S
If the shaded area of this distribution corresponds to
3240 then the area marked as X will be 1840 of the total. It
can be shown from standard tables that 18;; would correspond
to an area lying between 0 and .47d'. hence the standard
deviation is 100,447 = 21°.
Finally the consistancy of the subjects between the
two series of tests was examined. Fig 5.8 ranks the subjects
in the order of their score on the two choice experiments
at an angle of 20 degrees with the magnitude of their mean
vector from the multi—choice experiments. The correlation
is good.
5.7 Conclusions
Most of the divers who helped in these experiments
had the impression that they were not, or at the best only
marginally, able to localise a sound source underwater.
This is sLrane, since the overall conclusion from these
results is that directional hearing is possible underwater.
This conclusion is statistically very reliable' from both
independent series of experiments. In order to throw suspicion
on this result it would be necessary to invoke, either the
subject using his eyes or some form of extra—sensory perception.
its for the former, the heavy copper black—out flap attached
to the mask let in almost no light. It could not be
surreptitiously raised without the use of the hands. In all
blacked—out mask experiments underwater it is suggested that
86
the subject might obtain clues as to his whereabouts by the
action of sunlight shining through imperfections in the
black—out. However, the experiments described above were
designed so that subject awareness of his surroundings would
not be a serious interference. ilfter all, if directional
hearing is possible then the subject may be able to obtain
information about his surroundings by this method. If the
experiments were sensitive to this interference, then although
the overall conclusions may still be valid, they would not
provide a satisfactory quantitive indication. Having said
this, the writer, generally considered to be a proficient diver,
was not aware of gaining visual clues as to his surroundings
on the occasions he acted as subject, and no other subject,
reported visual clues.
87
Chapter 6
7.:112.:.ING (11)
G.1 Introduction
In the previous section the two main experiments
designed to test directional hearing have been described. Both
these experiments reach a clear and statistically reliable
conclusion. That is that a diver can perceive the direction
of a sound source. In this chapter the effect of changes in
either the source or subject are considered. This material is
often derived from a statistically poor sample and must be
regarded as producing pointers to hearing ability underwater
and not unequivocal conclusions.
It is proposed to consider these subsidiary experiments
in turn, dealing first with those arising from the forced
choice exPeriments and later those which are derived from the
free choice ones.
6.2 Different source anles around the subject's head
In all tests of this type considered previously the
subject sat at the apex of an equilateral triangle, facing
the opposite side. In the experiments described below, the two
source positions were not symetrically displaced about the
subject. The table below shows some subject—source positions
and the score obtained. The notation used is that of fig 5 .7
where the total number of judgements is placed before the
number of incorrect answers.
ORIENTATION
-4- 45o
o°
TEST SUBJECT sconE
A J.'I. 40/11
D.n. 40/6
88
B
B.R. 40/12
P.N. 40/19
C
R.L. 40/19
11.L. 40/4
In test A the change in time difference between the
arrival of a sound at the two ears from the two source
Positions will be about 33 pee. This is of the same order
to that experienced in the 200 triangle. 72,oth divers achieved
a score which did not differ significantly from that obtained
on the earlier test.
In the second test, B, there should not have been any
time difference to distinguish the two source positions.
Compared with their previous individual abilities (fig 5.7)
both subjects can be said to have done badly.
Test C would also not be expected to produce any time
difference clues to help the divers. The score of is close
to a null hypothesis, but as this was so on previous tests
score with hood 60/8 One subject — — — D.T. I
200 triangle - score without hood 30/8
89
it cannot be regarded as significant. The good score from
M.L. may have been due to this subject using head movements
to localise the source, although it seems unlikely that this
produce a score that was better than any of the triangle
scores by the saline diver. It is felt rather more likely
that some other mechanism may have helped here. One
possibility (to be considered in a later section) is that
there may be a difference in hearing sensitivity between
sounds arriving from the front and back of the head.
6.3 The effect of a rubber hood
The hood that was used for these experiments was a
conventional wet—suit foam neoprene helmet with small holes
about dianeter adjacent to the diver's ears. The tro
tests completed on the same dive and the helmet was
removed in the water so that the subject would not chan:;e His
depth and "clear his cars" between tests. It was necessary
to remove the facanask and breathing mouthpiece in order to
remove the hood. curing this latter stage there was a
tendancy for the rubber of the hood to become lodged over the
face and for safety reasons further subjects were not invited
to try this experiment. It is suggested that these tests be
repeated with a helmet specially designed to be easily
removed underwater.
90
Although there is the suggestion that a hood may
improve directional hearing underwater it is not possible
to draw firm conclusions from one subject. It can however
be said that this result does not conflict with the reports
of Ide (41) and Feinstein (42) that a hood imoroves directional
hearing.
6.4 The detection of an obstacle
A sheet of foam neoprene about 1 metre square was held
by a diver rouhly 2 metres in front of the subject. As an
alternative the obstacle was held to the side. The buzzer sound
source was operated from a point 7 metres in front of the
subject and the diver was asked to indicate the presence of
the obstacle.
:'.exults p.n. 51/4
J.W. 40/16
One subject appeared to be able to indicate the
Presence of the obstacle, the other diver probably did.
0.5 The effect of the subject hearin7 the source switched 'on
During "debriefing" some subjects reported that they
felt two distinct phases to localisation of the source. There
vas an immediate impression of direction the moment the source
was sritcherl on followed by a longer period when the direction
vas less apparent. On the suggestion of one of the expedition
° me:_lbers a series of tests were desiged using the 20
91
triangle. The subject was asked to breath slowly and.
regularly. The sound source was switched on, either while the
subject was breathing (i.e. the sound or the source was masked)
or when he was holding his breath. The assumption was made that
while the divers were breathing they would be unable to hear
the source switched on. In the breath—holding tests the subject
was asked to come to a quick snap decision whereas he was asked
to consider his judgment carefully in the cases where he did
not hear the source switched on. Atypical run involving GO -
juLgements would contain 30 of each type interleaved in groups
of 15.
Si::: subjects were tested and the score is Clown below:
NOT B=7:LING DP7ATHING
P.J. 30/10 30/12
J.W. GO/10 59/26
P.N. 30/10 30/15
M.L. 30/12 30/7
3.1,. 45/5 45/9
30/S 30/3
TOTAL 225/55 224/77
If one assumes the two distributions are binomial then
the standard deviations will be G.5 and 7.1 respectively. On
this basis the two total scores differ by over three standard
deviations and can be regarded as significantly clifferent.
It appears therefore that the diver should abide by his
initial impression of the source direction.
G.G The effect of a reference on the free choice tests
Two subjects were tested with the free choice
experiment in a similar manner to that described in 5.3 with
the excention that they were seated on a firm steel box. The
divers -:ere at liberty to move around on this seat in their
efforts to localise the source. The results of this test
are shown in rows a a b of fig 6.1 below. Although the
absolute accuracy is not noticeable better than the previous
free choice exneriments, the value of the mean vector is
larger. This represents in both cases a doubling of the
relative accuracy. Their "aim" has now a standard deviation
of about 20° which is similar to that derived from the two
choice experiaents discussed in 5.7.
Fig 6.1
:Airthor Free Choice Tests
Subject No. of nhotos
:.:can Vector 1:agnitude Direction significance
Standard Deviation
a E.L. 12 .936 —12° < .001 10°
b B.J. 14 .053 ...z1.50 -4: .001 22°
c 10 .605 + 2° <:.01 56o
d ' L .: , • • 13 .62 —64o <: .05 65°
e P.N. 17 .784 +24° < .001 48°
f i.L. - 13 .61 4- 1o <..05 65o
}:ethos_ used a L b subject seated on steel box buzzer sound source
c ,2z d subject moored inverted buzzer sound source
c a f subject seated on steel box. Low frequency pneumatic sound source used.
92
93
6.7 albject inverted
On the occasion of three free choice tests, instead
of mooring the blindfolded subjects with a rope around their
ankle, the rope was tied around their shoulders. Under these
conditions the diver's body would hang inverted in the water
(photograph fig 6.2). The first subject to try this kept his
head in a relatively conventional orientation with respect
to his body. After three judgements, 'which included one
vertically upwards, the diver signalled that all was not well
and was helped ashore complaining of vertigo. Two further
subjects were far more successful, they held their heads so as
to face the sea bed and moved their bodies to localise the
source in a line with the top of the head.
In these cases the scores were conmarable to these
obtained in the conventional manner. (rows c d of fig 0.1).
It is worth recording that stringent safety measures were taken
for these tests. Only e:merienced divers were invited to try.
air infla ted lifejacket capable of bring diver and. ballast
to the surface was worn and there was a boat moored close by.
0.8 Libject sinallinE7 the confidence in his judgement
During the course of some of the three choice tests
(recorded in fig 5.5) the subjects were asked to use one of
three hand signals when pointing to their estimate of the
source position. These were indicating with a snorkel, a
clenched fist, or an open hand. The signals were to indicate
94
Fig 6.2
PhotoTraph taken from 'free—choice' film record showing
the subject moored inverted
The sound source was situated beyond the top of this page.
(In order to show the perspective, the complete 35mm frame has
been reproduced)
the subjects confidence in his judgment on a scale of three.
Unfortunately it was only on two occasions that the signals
reproduced unambiguously on the photographs.
shown below:
The score is
Subject Choice No. of Mean Vector Direction Judgments
J.W. Total 24 0.84 — lo
A 3 0.85 — I°
B 12 0.34 — 14°
C 9 0.29 4- 16°
M.L. Total 18 0.72 —112°
(one photo not clear)
A
B
1
12 0.75
_1030
— 96°
C 4 0.31 —1310
Choice A was the most confident, C the least.
No trends are apparent and it appears that the diver is a
poor judge of his own abilities.
0.0 The effect o5-: different sound sources
All previous references to sound sources have referred
to one of two devices described in 5.2. These were sometimes
Imowii as "buzzer" sound sources and their response had a wide
bandwidth covering the centre of the audio band. However, it
was considered useful to investigate the effects of a loT:er or
higher frequency source. For simplicity and reliability
non—electronic devices were examined.
95
96
The low frequency source
The efficiency of a sound source is a function of the
relative dimensions of the working carts ccmpared with the
wavelength of sound (This has been expressed in a more
Quantative way in section 2.3 ). Now as the low frecuency end
of the audio spectrum involves wavelengths of between 3 and 15
metres underwater, the designer of a hand—held source is clearly
in difficulties. The solution adopted is to expend a large
amount of power to overcome the inevitable inefficiency of a
mmall source. The acoustic signal is produced by the release
of bursts of compressed air into the water. To release these
bursts a mechanical oscillator was designed which is analogous
to an electronic "relaxation oscillator". It is small, the
diagram (fig 6.3) is close to actual size, and can produce a
chain of pulses with a repetition frecuency between 1 and 20 Hz.
The setting of the tap on the gas cylinder gave a small
measure of frequency control and larger changes could be
produced by inserting a longer or shorter hose between the tap
and the oscillator. The bleed screw was set so as to' provide
stable operation. In use this source appeared to have two main
disadvantages. It consumed a large amount of compressed air
and it had to be held well clear of the operator's head for
comfort.
Two or the tests listed in 6.1 involved the use of the
pneumatic source. Both of these, e a fy were run with the
Tap
in a similar Response of pneumatic source (taken from recordings way to fig 5.3)
I lE 50 21: 100 200 500
g I 5E
Fig 6.3 Low frequency pneumatic sound source.
Brass tube Aren,,,holes
\\\\\\\\"\\\\ \\\\\\\\A
/ //////
\\\\ \\\\\\\\\\\\\I s>m\\\\\ ii\\\\
Stainless steel Bronze BleedBleed spring ball valve
Retaining plate (with holes t.1 allow a free flow of water)
Steel insert
T 10db
Hz.
97
Flexible air hose
1
air cylinder
98
subject sitting on a steel box in the same way as a b.
However, neither showed the improvement over the original free
choice experiments that was demonstrated by a & b. In the
triangle tests there was one subject who used both the buzzer
and the pneumatic source.
GOo Triangle subject P.N. Buzzer 41/5 Pneumatic 40/12
There is therefore a suggestion that this low
frecuency source is more difficult to localise than the mid—
fremiency buzzer.
The high frecuency source.
A diver propulsion unit was modified by removing the
propeller and substituting a length of slotted aluminium.
It was hoped that the rapidly rotating piece of metal would
proalce cavitation. Alen this was tried the only noise that
vas heard was the noise of the electric motor which sounded
somewhat similar to the "buzzer" scAlrce. No frequency response
is available and the device saw little use. Two of the free
choice experiments in fig 5.5 were made with this source. One
is a good score and the other a bad one.
Tapping a gas cylinder to make a sound.
The tapping of a metal object on the side of a diverts
aqualung cylinder is often suggested as an emergency signal .
To examine the effectiveness of this, it was used as a source in
0 some of the 20 triangle tests.
99
core with Score with Subject rt-linder Buzzer
60/16 60/16
60/20
P.N. 45/13 60/25
M.L. 45/12 60/19
P'2,11C TAGE. 'F23.0 NG 34cifo 33'i;
The cylinder used was small (1 litre water capacity)
and was held in the hand and tapped with a heavy diving
knife. This source produced no noticeable difference in score
from the buzzer and can be recommended as a means of signalling.
6.10 Conclusions
The e:meriments described in this section rill act as
pointers to those areas of this field where future exploration
should. be most fruitful. They also provide the background for
advice that can be given to the we-thing diver.
In order to localise a sound effectively, the diver
should try to provide himself with some reference such as the
sea-bed. The sound should be a mid-freouency one such as the
running of machinery or the banging of metal objects. A series
of thunderflashes of other lor freouency sources may not be the
most useful. The diver should turn to face the direction from
which he feels the sound is coming and he should make up his
mind without undue hesitancy. He should not be put off by a
feeling that he is wrong.
Chapter 7
.AUD I CI. ET ra. ID OD 1V2,..T I 01-;:,.; 101-?-01.. All. UlaM1.1.72,1' at. LA1.1 0 T 0 BY
7.1 Initial e:meriments
Liubjects who were involved in the directional
hearing tests in Malta, 1003, reported the impression that
a sound. source in front of the body appeared louder than
one behind. At the time the author viewed these comments
with considerable sceptisism. However, it was felt desirable
to confirm that this differential was indeed an illusion and
to this end a simple audiometer was constructed. This
instrument was designed to produce a constant audio tone,
the level of which could be accurately adjusted.
At the time the only available components were either
those taken as spares for other work or those obtainable fran
the local 1,:altesse radio shops. In the final device, shorn
in fig 7.1, the sound radiator was a moving coil transducer.
A multi—way switch was let into the rear face of the transducer
and sealed with a combination of '0' rings and candle. wax. A
chain of carbon resistors was wired across the switch, contacts,
the values having been chosen to provide an attenuator with
eleven steps of 4 db. The source of electrical power was the
mechanism from an ultrasonic pinger that was to hand, mo;:ified
to produce a pulse train at about 000 Hz. In the photograph
this is the tubular device strapped above the transducer.
In use the audiometer was positioned firmly on the
100
• r F., .ftje 4"; 4 • . • 4.• ..••• _ ' • ' •
101 Fig 7.1
'Crude' audiometer in
use. (Audiometer can
be seen in foreground
Fig 7.3
The 'imprlvedl audio—
meter complete with
transducer.
102
sea bed 5 metres from the subject diver. The level of the
tone was reduced in 4 db steps until the subject indicated
that he could no longer hear it. The level was then increased
until it was just audible to the subject. This is sometimes
described as making one "excursion" around the threshold.
In conventional surface audiometry, a large number of excursions
can be made in a relatively short time, underwater there are
difficulties imposed by both the inhalation and exhalation
noise. Under (Inlet conditions exhaled bubbles can clearly
be heard until they reach the surface.
The techniue adopted was for the subject and
experimenter to synchronise their breathing, take two or three
good breaths and hold their breaths for up to 30 seconds. The
subject could easily require several such periods before
reaching one decision. 1:eedless—to—say the arrival of any
power boat in bayi or even the operation of a compressor
on shore, 100 metres away, was sufficient to curtail the
exerimons. "Iurthenuo-fe it was foune, that many demand valves
tended to release an occasional bubble and it was necessary
to test a number or these in order to select those which were
satisfactory. The subject did not wear a rubber hood but
was provided with a steel box for a seat and an extra
weightbelt to place across his lap.
There is one further way in which these experiments
103
differ from conventional audiometry. In air it is usual
to conduct the experiments in an anechoic chamber. If it
is desired to change the orientation of the source, then the
subject remains in a fixed position andl the soun generator
is moved around the subject's head. Lt first sight, it might
be supnosed that this method was applicable un.erwater.
However, the sea—bed is far from anechoic and if the transducer
were to be moved, different conditions would ensue and the
sound field at the subject would change. The alternative,
which was adopted by the writer, was to turn the subject
round the keep the source stationary.
the duration of one dive it was found possible
to malt:e two excursions wih the subject facing, the source,
two with his back to the source, followed by two more to the
fc.ce an C. two more to the back. The reason that the directions
were interleaved in this way was to reduce possile error due
to slow changes such as learning or cold. It is important
that the subject remains sea-ced for the whole of the experiment
as any chJ.n7;e in depth might proC.uce a difference in any
residual pressure across the ear Crwn.
Five subjects were tested in this manner. On return
to the US., the Crude attenuator was calibrated and the mean
level for each direction of each subject evaluated. (fig 7.2)
104
Fig 7.2 Results of original audiometer tests (August 1068).
Sub— ject
Front back
Level at which sound could not be heard
Level at which sound could just be heard
Level at which sound could not be heard
Level at which sound could just be heard
Mean front/ back
difference
front 36db 26db 28db 26db
JW back 26 22 26 19
front 32 28 32 28 5.1 db.
back 32 22 26 22
front 32 28 36 28
back 32 26 32 26
front 39 D IJ 28 36 28 5. 4 d b .
back 26 22 26 22.
front 32 28 36 22
back 28 26 28 19 PN front 32 26 36 28
4.8 db.
back 28 22 — —
front 28 26 28 26
back 22 19 22 19 ML
1 front 26 22 26 22 4.5 db.
back 22 19 26 19
front 36 32 36 32
back 32 28 32 26 JSW
front 46 36 46 39 5.1 db.
back 39 32 39 32 •
NOTE. Underwater the position of the attenuator was recorded as a number (1 to 12). On return to the U.1. these numbers were substit—uted for the actual attenuation measured inalaboratory at Imperial College. It is these values th[A, are recorded here. :lence a large figure represents a quiet sound and a small one, a relatively loud sound.
-105
7.2 The construction of an improved audiometer
Even allowing for the restrictions of the crude
instrument described above, there a7)peared to be some
difference in threshold between the front and back of the body.
It seemed essential to confirm these findings in a more
satisfactory manner. The main disadvantages of the simple
au(Aometer were the coarseness of the attenuator, the
ill—defined nature of the tone, and the lack of any method
for determining the absolute sound pressure level at the
subject's head.
In the second design, constructed the following year,
a pure tone is generated by a low distortion Wein Bridge
oscillator (similar to a design published by Eullard Ltd,40).
The output from the oscillator is coupled to an 'L' section
attenuator with a range of 60 db in 2 db steps. A buffer
amplifier (a compound emitter follower) between the attenuator
and the transducer insures that the attenuator is correctly
matched under all conditions. These three items were
assembled in a heavy gauge Perspex box with sealed controls
for the oscillator frecuency and attenuator setting. The
transducer, again a moving coil type, was connected to the
amplifier by about four feet of cable in order that it could
be placed on the sea—bed clear of the operator's body. The
whole instrument was powered by dry batteries and switched on
by moving an external magnet adjacent to a reed switch. This
106
instrument (photo fig 7.3) was rather bulky and required
60 lbs of lead weights to hold it on the bottom.
7.3 The second series of audiometer tests
Plans were made to take the improved audiometer to
L:alta with a group of students in the summer of 1960. The
project was organised in a similar way to those of 1036 and
1963. However, unlike the previous two, the main object of
this expedition was not the acoustic experiments in themselves;
rather it was the testing of a submerged laboratory as a base
for acoustic experiments. For a description of this project
and photograph the reader is referred to Appendix 3. Despite
some very severe weather which destroyed nearly half the
ecuipmcnt, a tro—man underwater laboratory was operational,
7 metres below the surface, towards the end of August 1969.
In the experiments that are to be described, the writer
used this laboratory as a control centre and calibration
facility. The era,; - in which this vas arranged is best described
with reference to fig 7.4. The calibration hydrophone vas
a Clevite CH13 which vas coupled to a Druel Kjaer Precision
Ljound Level meter and octave filter bank inside the laboratory
The filter bank was needed to help extract the signal from
the ambient noise when making a measurement of the sound
pressure level produced by the audiometer at the subject's
head. The person sitting in the laboratory was able to observe
both the subject and the audiometer through the mirror smooth
Level meter
Octave filter
Underwater laboratory
Transducer
10 in.
llydrophone
5 metres
7 m.
107
Fig 7.4 Audiometer experiments with the underwater laboratory
Audiometer
Inside the underwater
laboratory.
The B & K sound level
meter can be seen on
the stool in the centre
of the phAograph. The
'entrance' is in the
foreground.
108
water surface in the entrance. Had this structure been
available earlier, the electronic part of the audiometer
would have been installed in the laboratory, leaving only
the subject and transducer in the water. In any future
trials with this ecluipment, this apl)roach will be adopted.
The method of operation was similar to that used
previously. The attenuator was moved in 2 db steps in the
period when the subject was breathing. This precaution was
taken to preclude any possibility of the subject hearing noise
generated by the switch. 1;orm.Aly only the maxima and minima
of the attenuator settings were recorded, however, fig 7.5
shows the conduct of one particular trial in full. The
audiometer was left in the same position on the sea—bed
between runs as there appeared to be no reason for bringing
it ashore. Unfortunc.tely the weather was still to prove
unfrienly and after five days submergeO, the audiometer
was washed oIL'f the rock on which it was stationed in a storm.
This action somehow cut the sheath of the transLucer cable and
rat,er entered the instrument down the insiac of this wire.
The attenuator was found to be open—circuit when the instrument
was recovered.
7.4 itesults
Five subjects were tested with the uncalibrated source
and three with the pure tone audiometer at 1 I'dz. Of these
eight subjects, seven showed a marked improvanent in hearing
Att
enua
tor
Se
ttin
g
A.
• Time
Subject's Response 4- —Yes
0 — No
B.
Fig 7.5 _::ample of one audiometer test with the subjectl(PS), 'A' facing the source, 'B' back to the source. An attenuator setting of 30db produced a sound pressure of 34db (ref .0002 dynes/cm2) at the subject's head o
COdb'
bound level in the sea (from Clevite CH13 hydrophone)
Sound level in the laboratory (from B cti K condenser mic..)
f 63 250 1K 4K 16K
GOdb Sound pressure rel. .0002 dyne/cm2
40db
20db
110 Fig 7.6 Results of audiometer experiments. (including fig 7.2)
Subject Difference front/back a tone
Calibration Conditions
RL (8) , 5.4db Original wide-band audiometer
no calibration
PN (q) 4.8db EL (0 4.5db JY/ (8) 5.1db
removed aoualung from back JS11 3c) 5.1db
PS (12) 5.2db EFT 54db at liaiz
Held aqualung in hands
NS (6) NS (7)
4.7db 2.6db
EI‘‘`f for both runs 56db at 11. 1z
Blacked-out facemask Both tests I
'ithout any facemask 1 same dive.
MB (23) -0.8db myT 66db at lRiz
Tests performed rapidly. Subject ha4 forced choice every breathing pause
% = Mean front threshold (reference .0002 dynes/cm2 )
Figure in brackets after the subject indicates the total number of threshold 'excursions' involved. (i.e. the total number of pairs of
readings.)
Fig 7.3 Analysis of ambient noise in the sea and in the underwater laboratory (life support system switched off)
Frequency Hz.
n
111
sensitivity when the sound source was in front of the body
(see fig 7.0). In the case of the one subject who showed no
significant difference, the test was conducted faster than
previous ones and the subject was asked to reach a decision
in every breathing pause. It may be sinificant that the
absolute value for the threshold is also somewhat higher
in this case. On the one other occasion that the front—to—
back difference was small, the subject had removed his
facemask and performed part of the test with no protection
over his ayes or nose. With the exception of this test the
subjects could generally see the audiometer when it was in
front of than, but they were instructed to fix their gaze
on some object such as a rock and not to study the instrument.
In any case they would not have been able to gain additional
information from the position of the controls as these were
not visible to the subject.
In the case of the three subjects who were tested with
calibrated equipment, the threshold with the source in front
of the body has a mean of 50 db ref .0002 dynes cm-2
., In
fig 7.7 published values for this threshold are compared.
Fiq 7.7 .Q.12m 7lr1soy, of :lublished uni7 erwater threshold
fi71;ures at 1 KIz
Author
Reference No. Threshold
Ide (1044) 41 75 db
Hamilton (1957) 15 52 db
Imthor neference No. Threshold
'jainwright (1050 IG GO db
nontague a Arichland (1067) 17 60 — 75 db
Brandt C. Horner' (1067) 18 70 db
Sivian (1047) 44 45 — 55 db
:.i:11 tit (1068Y 45 51 db
In addition to these tests, the sound pressure field in
the sea and in the umierwater laboratory were analysed using
the octave filter bank. These measurements are summarised
in Fig 7.8. The noise showed a distinct hump around 8 1;11z
and is believed to be almost entirely biological. It is
normally credited to the "snapping LAibjectively it
appeared that this noise increased just after dush when a
consierable number of small creatures between 1 and 3 mm in
length, chose to climb up the inside wall of the laboratory. No
noise could be detected as arising from the laboratory itself
(all life support emainment switched off) and the airborn sound
pressure level inside annears to be entirely the "snaying
shrimps" on the outside.
It may seem surprising that the absolute values for the
thresholds lie below the ambient noise in octave analysis. This
is ”erfectly reasonable as the human car is an e:xellent tool for
detecting a distinctive sound below a broad—band mashing noise.
The writer will return to this topic in a more ouantative way
later.
112
113
';:lien it became clear that there was real difference
between front and bad: sensitivity, some consideration ras
given as to the cause. The first suspect was the compressed
air cylinder that the diver wears on his back. However,
removing this and asking the subject to hold it above his
head did not seem to effect the situation. One other
possibility is that it is due to the presence of the air
cavity of the face mask. Unfortunately it is not easy to
spend a half hour without any protection for theqyes. The
high salinity of the Mediterranean tends to cause an
uncomfortable stinging sensation.
7.5 Other observations from the underwater laboratory
In section 2.4 the possibility of using an electric
conduction field for communication was discussed. During the
design of the underwater laboratory it was thought desirable
for there to be some means of wireless caaninication to the
shore. Because of its inherant simplicity a conduction system
was used. A block diagram of the this etuipment is shown in
fig 7.9.
The transmitter consisted of a conventional preamplifier
followed by a push—pull output stage capable of driving 1 anp
into a 4 ohm load. The microphone was always "live" and the
unit clerived rower from the main laboratory batteries. In
the event of a power failure a relay automatically changed
over to an internal dry battery. The "earth" of the amplifier
Receiver
Loudspeaker Amplifier
Electrodes
Fig 7.9 Electric conduction communicator used in underrater laboratory.
Transmitter
To laboratory 12 v battery
114
Relay coil
Pre—amp Power amp
Relay
>y 12 v
Lamp
,arth to laboratory ///7///, frame
Electrode
115
was connected directly to the steel base rinL; of the.
laboratory and the output was taken to a disc of perforated
aluminium, 30 cms diameter, rhich was suspended 2 metres to
one side of the structure. On shore some 70 metres away,
two electrodes about 7 metres apart were connected to an
amplifier and loudspeaker.
There is little that can be said as regards the
operation of this device apart from that it worked. No
electrical measurements were taken as priority was given
elsewhere. However, there was one surprising observation that
must he recorded. ?wen when the transmitter was switched off,
there was a sound from the receiving loudspeaker resembling
snapping shrimps. '21ether these creatures are emitting an
electric field or whether the explanation lies elsewhere,
would appear to recmire further investigation.
116
Chapter S
DI:SCUL:SION
S.1 _Articulation underwater
Probably the most serious limitations to present
underwater communication systems is the difficulty in forming
words. It might be suposed that the solution has already
been outlined in section 4.7; that is to use a mash which covers-
the mouth and has a relatively large volume. Unfortunately a
large volume represents a possible "dead space" of gas which
will not be completely flushed on each breath. This can lead to
a build-up of carbon dioxide with serious physiological
repercussions. 2_ solution which has been adopted by one
manufacturer is to insert a lightweight oral-nasal mask within
the main mask. This inner mask can be acoustically transparent
while isolating the main volume of gas from the breathing
systan.
The desiTn of non-return valves in breathing eciuipment
presents a similar problem. -,?or reliability and saf 4-, there
are often two, relatively stiff, non-return valves in. the
exhalation path. These are suspected of causing rapid
fluctuations in back pressure as they operate. It was found
easier to speak into a demand valve incorporating a single,
sesitive, exhalation non-return valve.
If electronic methoC:s can be used to aid articulation,
then they may be of immediate application to existing breathing
117
systems. One such possibility is to use acoustic feedback
inside the mask. If a loudspeaker is inserted into the
facemask and driven such that it will tend to cancel the
original sounds in the mask (i.e. ne2:ative feedback), the
acoustic impedance inside the mask will fall. This idea
is not new and has been proposed for noise cancellation near
machinery in industrial premises (:Aephens Dates 43). An
extension of this idea would be to mount the transducer of
a direct audio c=unicator on the facemask in such a way that
the sound in the water is reinforced while that in the mask
is cancelled. If such a system could be produced it would be,
in effect, an impedance converter or active acoustic transformer.
A further possibility, which seems to have strangely
been ir;nored, is the use of "side—tone". In the early days
of teleDhony it was found that a tele)hone was improved if
a fraction of the voice siznal from the microphone was fed
bac:: to the earpiece. This fraction is known as the side—tone.
If a modern telephone were produced without side—tone it
would sound most unnatural and would be difficult to use.
1. person who is speaking listens to his own voice and
his aticulation is influenced by what he hears. If the voice
is altered then the speaker will attempt some measure of
correction. (This was observed after three clivs in the helium
atmosphere of Jealab 2 (47). Althou::;h this is normally to be
118
desired, it may introduce considerable complications if
a helium speech unscramblor is being used. In this latter
case the unscrw:ibler is asked to deal with the end product
o -' a feedback problem involving the human brain. It is
suggested that a more logical approach is to feed a side—
tone back to the diver after the signal has been unscrambled.
In this case the human feedback should improve the final
signal. • If this technicue proved beneficial it may be
worth considering the possibility of electronic correction
for fomant shift due to the facemask impedance at the same
tine that the helium distortion is corrected.
3.2 Propaation
There is a vast amount of information on the distortion
of :articular signals as a result of transmission through
sea vater. lk)wever, there appears to have been almost no
serious attar is to correlate this with a psychoacoustic
approach. For eNample, the writer has_ already sug!sested
that multiple path fistortionsin an annlitue modulation
ultrasonic system are more easy for the human ear to interpret
thal‘ those in an F.H. system. To confirm this one would need
to construct a model capable of reproducing these types of
distortion and 1,est for intelligibility using samples of
speech recorded in the facemask of a submerged diver. If
a versatile model was available it might be possible to
establish the optimum freQuency, type of modulation, and
119
directivity of the transducers, for any particular situation.
In recent years a number of devices using electric
or magnetic fields have appeared on the market. In some
cases, the claimed range is far greater than tho,t predicted
by the "skin—depth" calculation in section 2.4 and there is
a suggestion that it may be possible to transmit electromagnetic
fields beyond this range. natever the answeri there is an
almost complete lack or any published material on this subject.
Until this omission is rectified, it will be difficult to
estimate the potential use for electromagnetic systems.
8.3 Human hes7.rin underwater — t' le threshold
In 1047 Sivian (44) predicted that the threshold of
human hearing underwater would be in the range 43 — 55 db,
plus any attenuation due to a pressure imbalance across the
ear druri. The value obtained from the tests conducted in
lailta and some of the published figures listed in fig 7.7,
correspond closely to this prediction: Other published values
do not. Very little emphasis has been placed on the second
part of Liiviants prediction and all too frecluently one sees
a stata-aent such as "The divers equalised the pressure
across their ears" and the matter is left there.
The average diver will attempt to "clear his ears" after
a decent of about two metres and would find it difficult to
detect a change in depth of less than metre. Consider the
e:,uivalent chani;c in barometric heirht on the surface. As
120
the density of air is some SOO times less than that or water,
the change of r metre in water becomes 400 metres in air. Most
peoDle would detect a change in their hearing when descending
a hill of only a fraction of this hei7ht. The problem may be
even worse with closed circuit breathing sets where the diver
breaths gas from a bag often worn on the chest. This may well
be an appreciable distance below the cars. To sum Up, it
should not be assumed that pressure differentials across the
ears are nep;ligable. However, in reality, it may be that the
best one can do is to use open circuit scuba where the demand
valve is at a similar height to the ears, and to stress to
the subjects that they should tae plenty of time coughing,
suc!:ing, blowing their nose, etc., before starting the tests.
()lice the tests have canlence the subjects should not change
I:A:mover a threshold is measuredun:_er conditions that
are other than absolutely quiet, one must estimate the errors
introclucca by the background noise. Hawkins C: Uevens (-13)
have investiated the masking effect of a broad—band noise
on a pure tone. Fig 8.1 has been taken from this reference.
It shows the change in threshold (in air) for a pure tone
of llalz against the level of a masking noise in an octave band*
(*Havkins a jtevens refer to the noise in a "critical band" which is 03 Hz wic.e at 1 use in this context it is
more convenient to consier an octave band (a))rox. 730 Hz). For a broad—band noise this will correspond to a level 10 di) higher thn for the critical—bnd.)
Change in threshold of pure tone.
30 "
20 -
10 .
Fig 8.1 Masking of pure tones (After Hawkins Stevens, 48)
40 -.
121
/ (e* ,0
1. Y.J /
or / Sc,/ )c.)
04
No / 4C,
ii / • 944' / 'b-.. oz:'' / 0
.0. N / N
4 o / o N N° o
e / c N> / o-,"7•
/ --,,-
/
1)
0 10 20 30 40 Level of masking noise in octave band (11Cllz)
Fin 8.2 Sterionhonic hearing aid
122
Two lines have been superimposed on this diagram, one
corresponding to the condition that the mashing noise is 10 ab
above the observed threshold and the other that it is equal to it.
From the positions where these lines cut the threshold curve,
it should be clear that if one observes a threshold ecfual to
the noise then this is very nearly the true threshold. On the
otherhand if one finds the threshold more than 10 db below
the noise, then one is observing a mashing phenomina and not
a true threshold. In this latter case the result can only
be considered as an upper limit to the true threshold. Due
to the uncertainty of the use of this masking relation, it
would be unreasonable to expect this to provide an exact
method for correcting for anbient noise. This appears to be
an area worthy of further investigation.
Unfortunately, in the case of two of the subjects tested
the threshold appeared to be more than 10 db below the noise
level (recorded in fig 7.8) and hence these fi2;ures are upner
limits to threshold of underwater hearing. however, if it is
assiuned that the noise is omnidirectional, it will not
affect the validity of the Front/back differential measurements
for any o2 the subjects.
8.4 Human un erwater — Cirectiort7.1 hec,ring.
As has been seen before, most of the divers who helped
in these experiments had the impression that they were not,
123
or at the best only marginally, able to localise a sound
source underwater. These results show that this impression
cannot have been crained throu7h lack of ability to localise
sound. Is this possibility due to a lack of confidence in
interpreting binaural information? This is, after all,
not unreasonable as the binaural information reaching the
subject rill be distorted, mainly because the velocity of
sound in water is some five times greater than in air.
Consequently the time difference between the arrival of sounds
at the two ears will be shorter in rater than in air. Possibly
of greater significance, is the fact that the shift in time
difference as the head is moved will not correspond to that
normally ex-)erienced in air, (Thurlow h :tunge 49) and a
cine—film of some of the free choice tests shows that the
subjects made exagerated head movements before comin:: to their
decision underwater.
Is this lac!: of correspondence between head movements
and the movement of the binaural image that is destrdying
confidence? The probla,1 could be compared with that 'of
following a moving target through a pair of binoculars. The
apparent image dis7)lacattent will not correspond to head
movements and some practice would be desirable before using
binoculars for this work. This suggestion would also explain
why the diver's first impression on hearing the source switched
on tended to be more accurate (at least as regards to the
124
appronriate side of the head) than his considered opinion.
This follows if one assumes that the diver obtains his
first impression before he has had time to move his head.
In air it is possible to localise a wide band sound
source to 2 degrees (Fordlund 40) and on a velocity of sound
hypothesis alone this would correspond to 10 degrees underwater.
However, as underwater one would expect little help from the
pinna and inter—aural amplitude differences will differ from
those normally experienced, it would be unreasonable to
expect this overall accuracy. The figure of 20 degrees from
both the two choice experiments and the free choice ones where
the subjects were seated, appears to be the best available
measurement of underwater sound localisation at the time of
writing.
It is interesting to consider the time difference
between the arrival of a sound at the two ears that corresponds
to an angular displacement of 20°. This will be about 30
microseconds for an interaural distance of 15 ems. :jome
measurements of time difference thresholds (in air) by
Klumn a Eady (51) are reproduced below. These figure's suggest
that under ideal conditions it should be possible to localise
a source to within 70 underwater. They also provide an
explanation as to why the subjects found it more di2ficult
to localise a low frequency source.
Threshold o1 time difference (Nlump Eady 51)
Broad band noise 10
,repeated clicks 11
150 — 1700 Hz 9 Pink nois
e425 — 600 Hz 114
90 Hz 75
250 Hz 27 Pure tone 500 Hz 17
1000 Hz 11
Time
in
microseconds
The accuracy obtained by the subjects who were freely
suspended has no counterpart on land. It represents the case
where the diver has no reference apart from the vertical, on
which to base his directional judgenent. During the period in
which he is making his judgement he must presumable use the
inertia of his body as a reference frame. ihen these problems
are born in mind it seems remarkable that the human system is
capable of localising the sound to within 50 degrees.
This latter ability would be important in using an
acoustic beacon to diver navigation under very low visibility
conditions. If on the eight occasions of the free choice
experiments recorded in fig 5.5 the subjects had been asked
to seek the source under zero visibility conditions, six would
have been expected to find it, one would not (apparently due to
ear clearing trouble) and the outcome of the eighth cannot be
reliably predicted.
125
126
Under normal diving conditions where the subject has
some reference clues one would expect localisation to be in
the range 20 to 50 degrees.
At this stage it would be premature to suggest a model
for the mechanism of underwater hearing. Previous workers have
considered that hearing is either tympanic, that is sound
transmitted along the auditory canal in much the same way as
in air, or bone conduction. In the first case the value for
the threshold is explained as arising from an ilm)edance mismatch
at the druii, and in the second there is a similarity between
the thresholds of bond conduction hearing in air and underwater
hearing. The writer is of the opinion that it may not be
meaningful to consider underwater hearing as being either
bone conduction or tympanic. These ideas apply in air where
the acoustic impedance difference between flesh—and—bone and
air is so large (103 — 10") that one can consider a sounu
wave as travelling in one or the othef. flesh has a similar
acoustic impedance to water and it may be more reasonable to
consider that underwater the two hearing organs exist in an
infinite fluid. This ignors the water—skin boundary completely!
The middle ears, sinus cavities and the facemask along with
other air spaces are impedance discontinuities in the
neighbourhood of the receptors and must be taken into account.
It has been shown that bubbles of air in the vicinity of a
transducer can have a dramatic effect on its performance and
127
produce directionality in an otherwise omnidirectional
device. (Hunter 52). It is hoped that further tests with
and without a facemask may shed additional light on this
hypothesis. If it can be substantiated it may provide an
explanation for the difference in threshold between the front
and back of the head.
8.5 Ap7)lications to co:ummication eouirient desir4n
The most important recommend•Aion stemming from this
work is the adoption of an attitude of mind. If an item of
equipment is designed to help two human beings communicate
underwater, then the most important test is whether it can do
just this. Yet one hears of devices that are ca?able of
providing a mmnunication link between two boats , being
built into waterproof cases and soh. asaivei. telephones".
Yar more consieration should be given to the use of
stereophonic receivers. These have been recommended for use
on the surface and it has even been suggested (W) that
troops be provided with stereophonic radio receivers for use
in battle conditions. further, Pollack C. Picket (54) have
shown that a stereophonic telephone can enhance the apparent
signal to noise ratio by between 6 and 12 db. This type of
equipment has seen almost no use underwater because it has
generally been assumed that binaural hearing is very much
degraded.
A stereophonic receiver produced by the writer for use
128
with direct audio systaas is shown in fig 8.2. The two
hydrophones are spaced apart to produce an interaural time
delay similar to that experienced in air. This particular
design was unsuccessful due to acoustic feedback through
the water between the earphones and hydrophones. However
this configuration could be used as a receiver for an
ultrasonic carrier.
A summary of recosmiendations for the design of
underwater communication enuipment follows:-
1. Design breathing set, mask and microphone to be capable of working as one unit.
2. Reduce the acoustic impedance seen by the lips.
3. ._educe exhalation pressure and in particular pressure fluctuations caused by operation of non-return valves.
4. Consider the application of side-tone.
0 • Helium recordings at atmospheric pressure are no guide to the intelligibility of helium speech un-er high pressure. speech recorded in a compression chamber is only a limited guide to the sounds produced inside the facanask of a working diver.
G. ajuima07A must be checked for the encumberance caused to the cold, exhausted)worl:ing diver.
7. Beware of directional transmitters and receivers where the diver is recluired to adopt a particular orientation for cmalunication.
8. Consider the advantages of a "duplex" system (see 2.2)
9. Consider the use of a stereophonic receiver (or the unaided cars).
10. The ear is tolerant to certain types of propagation distortion, experience based on sonar or telemetry may not be applicable.
S.G Sua-gestions for further research.
At the time of writing the author knows of no comparable
study of hulan communication underwater. For this reason it
is, in general, difficult to compare this work with that of other
people. In consequence further research can be along one of two
lines. The first is to treat this work as a series of pilot
experiments and to use the results for the design of more
exacting and critical experiments. This approach is probably
most suitable for the problems of forming words underwater.
The second is to continue with further research at this sa:ne level.
The writer would argue that the main results from the directional
hearing tests are sufficiently reliable and that further
experiments should investigate the directional threshold to a
similar statistical reliability.
some attempt should be made to eliminate pressure
differentials across the diver's ears. Asking the diver to "clear
his ears" is not good enough and this problem requires further
investigation.
129
ACKNOWLEDGE IENTS
The writer is grateful for the help and encouragement
given by his supervisor, Dr R.W.D. Stephens, and for the
provision of facilities in the Physics Department of Imperial
College. A grant from the Science Research Council followed
by generous help from the 3M's Company enabled the writer to
pursue this study.
The fieldwork in Malta was organised as part of three
expeditions which were sponsored by Imperial College Exploration
Board. The author is grateful to the Board and to the many
other organisations who backed these expeditions.
The safety and operational guidelines involved in the use
of the underwater laboratory were formulated after several
informal discussions with the Director and Staff of the Royal
naval Physiological Laboratory and other sections within the
Royal luyy. In lialta help for this project vas given by various
Government departments, the Biology Departs ent of the Royal
University, and Dry Docks.
The experiments involving human subjects would not have been
possible without the considerable degree of cooperation shown
by the personnel, most of whom were members of Imperial College
Underwater Club.
130
131
Appendix 1.
ME SIGNAL TO NOISE RATIO Frtkl! AN UNDERWATER M.ICROPITONE
It can be shown (30) that the thermal noise in an
incremental bandwidth developed across a network containing
resistive and reactive components is given by :-
2 V.(d0 = 4kTR f df
Where R, is the real component of the network impedance at
frecueney f.
Considering the amplifier (fig 3.5). The input circuit will
be 1 megohm in parallel with the cable and transducer capacitance.
The impedance at the input, Z, will be given by :-
1
j (.0 C IZ
R 7 = J cii c = n(1 — j ,,, pLi)
R + .1
3 to C 1 4- to2 R2 C2
The real part of this will be t—
L;ubstituting this in Nyquists
over the audio band (100Hz t 1012.1z)
formula
:— r W
1
I?.
1 u09
1./ C2
(A.1) and integrating
V2 4kTR ± 4 it 11C` t2 df
I00
10
41:TIL 1 tan 1 2 -n- Ref] 2 TT w 100
Jubstituting R = 106
Ohms C = 10 9 uF
1.38 x 10-23 T = 290° A
V -= 0.85 liVo 1 t
t.1
The noise generated by the mnplifier will add to this.
The amplifier shown in fig 3.5 was one of the more successful
developed specially for this type of source. It has a high
input impedance, a gain accurately defined by feedback (20db),
absence of any input capacitor and a good noise figure with
capacitative transducers. Further the use of a field effect
device in the first stage avoids the low frequency noise that
bipolar transist)rs produce when driven from a high impedance
source. The noise figure frun an amplifier of the type of fig 3.5
would be in the order of 2db. This corresponds to a total
noise at the amplifier input of about 1 microvolt.
The sensitivity of the microphones described in 3.2 is
approximately —110db ref 1 volt/ybar. flence the electrical
noise corresponds to a sound pressure field of —10db to the above
reference or +64db with reference to the usual origin of
.0002 dynes/cm .̀
132
Appendix 2.
SIGNE'ICANCE OF THE LEAN VECTOR
In order to assertain the reliability of the mean vector
it is necessary to evaluate the probability that this (or a
greater) value could occur as the chance addition of random
vectors. Only if this probability is less than 1 in 20) ie Ko)
can the results normally be considered significant.
Consider the addition of random unit vectors. The magnitude
of the sum of 'N' such vectors will he given by :-
2 cos 4 )2
+ ( sin 6 12 I
0 cos 9 cls 0A + S sin 0 sin 0n +
+ 02
2 cos 0
cos(0 — 9) + N
Now if 'N' is large and the vectors random) the summation over
the cosine term will vanish) leaving the familiar result that :—
Now if one makes the assumption that the distribution of
the component of the mean vector along any one axis is a normal
one of the type :—
P (x )dx a 1 A dx (Normalised)
Then if two such orthogonal distributions are combined by the
substitutions :—
133
134
r2
x2 + y2 P(r)dr = P(x) P(y) dx dy
l<
dx dy = r de dr dO = 2 Tr
The distribution of radius vector will be of the form :—
P(r)dr 2 r e dr
The mean of this distribution can be evaluated by
r̀ e 1d r
1.253 (from tables 57)
0
r e --Adr
Consider now a vector of length z such that the probability
of this or a longer one is 5.S. This length will be given by :—
r e --Yar 0.05
z z. z . 2.45 •
For the purpose of this calculation it is proposed to use the
ratio of this vector to the mean as this will be a dimensionless
constant which should be applicable to all similar distributions.
The value of this ratio for a probability of (and also 1`; and
0.1(A has been evaluated.
Probability
2.54
1.06 3.04
2.42 3.72
2.07
Let us assume a null hypothesis that the subjects in the
free choice tests arc pointing in random directions. If N
observations are made on a certain subject then one would expect
the mean vector to be in the order . The probability that
.05
.01 .001
it will exceed 1.96 r: is less than and in this case it is
more plausible to discard the null hypothesis and to assume that
the subject is pointing to a real direction.
To make this procedure easy to apply to the experimental
results, the following table has been drawn to show the
probability that any mean vector is the result of the addition
of random vectors. As the assumption has been made that N is
large, these calculations must only be taken as a guide if the
value of N is small.
Ko probability (1.96/50
1.;: probability (2.42 N)
0.l probability (2.97/1-/T)
10 .618 .765
15 .505 .625 .767
20 .437 .542 .665
25 .391 .484 .595
30 .357 .443 .543
35 .331 .410 .502
Fig 5.6 is a graphical representation of this table.
It is suggested in chapter 5 that the magnitude of a—subject's
mean vector is a good guide to his accuracy. If this is so it
should be possible to relate the mean vector to the standard
deviation of the target error. The cosine-1 of the mean vector
should correspond to the mean target deviation. 1NOw for a normal
distribution the mean deviation can be shown to be 0.8 x standard
deviation (50). The values given in fig 5.5 for the standard
135
deviation were obtainer' in this manner.
Finally, to serve as a check, it is useful to compare the
value for the standard deviation obtained by the above method
with that calculated from the root mean square of the angles
involved. To avoid the +175° —115o anomaly that was discussed
in section 5.5, this comparison was done for a subject who
indicated the direction to within 90° of the true direction on
all occasions. Only one subject managed this WSW fig 5.5).
In this case the standard deviation calculated from the mean
vector was 46° and that obtained from the roA.-mean—square of
the angles, 41°. Considering that N was small, 14, this can
be considered as satisfactory agreement.
136
Appendix 3
:2.71) 01'72;i1:2I1ION Or Ai'?
Introduction
In the Autumn of 106G, the writer beemae aware of
the limitations of conducting experiments on divers in tae
oven sea, from the shore or a boat. The most serious problems
were those associated with the sca surface which presents a
very real barrier between the experimenter and his exleriments.
Cue solution, which had been tried to a limited extent in
other countries, was to construct a submerged, air-filled,
laboratory. This could be operated with the air insiCe at
a pressure equal to that of the surrounding water and allow
direct access to the sea through an open entrance in the floor.
The first proposal submitted by the writer was known
as the "Kralcen" project. sifter two years development it
becal,le clear that the necessary financial backing was not
forthccmin-, an: the plans were reluctantly dropped. In the
Autumn of 1908 the author assembled a joint Em)erialt-LInfield
College group and co-ordinated plans for a far simpler and
less expensive laboratory*
(* lathough the author was in overall charge and was the sole designer of the Life -upport and communication systams, the overall design, construction and operation of this facility represents the work or a large team.)
137
Fig A.1
138
The underwater laboratory in use. Malta 1969.
139
The expense and complexity of Eraken was largely
due to two factors: the problem of placing the habitat on the
sea-bed and the support recluirements once it was operational.
The present project, which grew from the pool of knowledge
gained in the design of Eraken, was an attempt to construct
an inflatable dwelling sufficiently light to be handled by
swimmers in the water, yet comnrehensive enough to be
completely independent of the surface with no umbilical cable.
,:iunplies would be brought in by divers and the only boat
available an inflatable dingy.
There have been previous inflatable habitats, notably that
of Edwin A. Link (L:aclnnis,55) and more recently one used by
the Moscow "Dolphins" Club (Barton,56). Although little is
known about the latter, the former, being a particularly deen
dive, re(aired considerable surface support and relied on
pol:er, gas and communication cables to the surface.
The Desin
A two-man size was adopted as this represent4 the
recluirement oj: the smallest safe working team. Alter some tests
which involved students living in a scaled polythene enclosure,
a size of 8 ft by 6 ft by 6 ft high appeared adecluate. Allowing
one foot clearance between the floor and the water surface
raises the overall height to seven feet, and-gives a submerged
bouyancy oL around eight tons. Al ter a survey of some possible
flexible materials, a nylon-neoprene fabric, manufactured by
140
the Avon :lubber Company for the construction of inflatable
boats, was selected.
Although it was always realised that, if the fabric
stress was to be minimised, the final shape of the inflated
house should resaable an upturned water—drop, initially it
was by no means clear how one could achieve this: A family
of theoretical cross—sections was produced by an iterative
process involving the relations linking the curvature of a
membrane and the pressure differential across it. The cross—
section that appeared to offer the most suitable height—to—
width ratio was chosen. The next starre was to construct a
straight sided approximation to this cross—section curve. When
this process was extended to three dimensions a 48 faced (plus
th figure evolved.
The reason for translating what should be a smooth shape
into one with a nuuber of flat faces, was that all the seams
in the fabric could now be cemented along straight wooden
jigs. Previously it had been founL1 that a strai,rht semi could
be made with a strength nearly equal to that of the virgin
material, whereas it was found difficult to produce a satisfact—
ory curved seam. _Before cutting the material and commencing
conLruction, one final requirement had to be fulfilled. Under
the calculated working stress the fabric would stretch by
between 5 and Furthermore, the stress—strain relationship
for this fabric varied markedly with the angle to the weave.
141
It is difficult to foresee any accurate methoa of allowing
for this anisotropic behaviour. The technique adopted in
this design was to reduce each of the flat pannels by an
estimated fraction.
The base frame was a ring of 3 inch steam pipe with
12 attachment points for moorings. The fabric was taken
round the pipe and cemented back on itself. In use, the
floor area was Sft N ilft and the walls leant out to provide
Oft N ilft Gins at worhing height. Estimates indicated that
the maxi :um stress would not exceed 1/5 the breaking stress
of the fabric.
It should be realised that although the structure was
constructed as a series of flat panels, in operation the
forces involved would distort the fabric into a smooth shape.
The degree to which this has happened can be seen from the
accompanying photograph.
ballast in the fora of natural rocks was
Used to nrovi(le an anchorage for the ha'Jitat. The initial
preparation inclui,ed surveying the site and attachin four
wire ropes to the ehosen rock. A series of wires were
attached to the 12 points on the house ring: and taken through
four large shackles. In operation, those shackles lay about
four feet below the ring (see fig A.1).
142
The habitat was lashed as a flat package and manhandled
into the water. At this stage the air that was trapped under
the fabric tended to keep it afloat. After being towed into
position above the ballast, the package was untied and a
vent value opened tc allow it to sink. 'our divers guided
it onto the selected rock. Once the four mooring wires had
been attached to the shackles, a small volume of air was
introduced into the house. At this stage the length of the
mooring wires could be adjusted by hand.
Once all appeared to he in order, the structure was
partially inflated and the final levelling operation started.
This was accomplished by attaching a Tirfor pulling machine
in parallel with each of the mooring wires in turn. The
tension ras taLen up by the Tirfor and it was then a relatively
easy matter to ajust; the length of the cable ar.t release the
machine. A.:ter some practice this whole levelling operation,
involving all four wires, could be accOmnlished by three divers
in just over an hour. Finally the house was fully inflated
with an air line from the surface.
The wooden floor, supported on a fexion frame, was
bolted to the main ring as one complete sub—assembly. A total
of about 20 man—hours were spent by personnel, working inside
the inflm.ed house, constructing all interior fittings. These
included the bunks, table and storage lockers and were made
from wood and DeNion with the aid of a hacksaw, drill,
Fig A.2
143
STIO-;-2
Main Camp
Two vehicles Tent accodation for up to 12 personnel.
Telephone connected to jt. Pauls Bay G.P.O.
40 watt fluorescent floodlight. 0 lead acid acctrlulators. 240 volt converter for fluorescent
light. Kyclrogen, oxygen and nitrogen store. 613(„a lime store (5 cwts). First aid kit.
Landin7 Place
Compressor • 830 cu ft 3300 psi air bank. Normal diving equipment — aqualungs
etc. incl-eing fluorescent
and quartz iodine spot lmnps. Derrich — 5 cwt lifting capacity. Diving ladder. ;Lyon led shank and 5 1p outboard. Under water tug. Various watertirht cases for transport of items to the house.
H011;f_l
Domestic
Two bunks, top bunk converts to table.
Life sup,)ort unit. Fuel cell (on sea, bed).
ervo Vie: 0.L150 oxygen meter. :an rose carbon dioxide meter.
-;rae-fer detector tubes for ;as analysis.
Hot coffee heater. Food store. Low voltage fluorescent light. Porta—shower fresh water sl:ower. Gaprleter for metering o::yr4en
input. on r c inr; valve and cylinder.
Tele:)houe. ConC_uction communicator. First Aid kit.
Jcientific
Clevite CH13 hy:Iro?hone. :gruel a 1.jaer precision sound level meter and octave filter bank. 1 inch condenser microphone. ,Aibmersible arZiometer. :-,Arominor test meter. • 2 Calynsonhot ca eras, one with Clash.
2 -:: 20 watt fluorescent lights for photography.
144
screwdriver and spanners.
The Modular Life larpport system
Once the habitat was inflated and level, the first
item to be installed was the life support module, These
units, of which three were constructed, consisted of a case
17" by 13" high with a sealed lid and a all:2,11 air cylinder
with valves to automatically pressurise the LSM.to ambient
sea pressure. Lead acid cells provided a 24 hour supply of
power and their weight gave the unit an overall negative
bouyancy (lo lb s) and insured that the bo:: had a sufficiently
low centre of p_-ravity to be stable when carried by a swimmer.
Mounted above the batteries, a single integral unit housed the
blower was drawn from the inside of the case so as to prevent
the possibility of a build—up of hydrogen or acidic vapours.
The waste heat from the motor and the work done compressing
the air by the single. stage cen-f_rifu.7a1 blower, produced a
slight temvierature rise and ensured that the air meeting the
absorber was not quite saturated with water vapour. sls is
common European practice, a soda—lime charcoal absorber was
used. This material has the advantage that it is most effective
at the high levels of humidity that were bound to be present.
The exhaust from the absorber was not ducted away, instead it
was released through an orifice at a relatively high velocity.
This "jet" of gas was deflected around the habitat by the
curved roof. This method a)nears highly successful as no
145
pockets of carbon dioxide could be detected.
plan proof 12 volt outlets were provided for lighting
a minature immersion heater (for hot drinks) and communication
equipment. An inlet was available also for coupling to an
external power source. -111 outlets were switched and protected
by circuit breakers and the battery voltage was monitored. The
main illumination was with low voltage fluourescent lights.
,llthough a conventional telephone was used in the
habitat for some of the time, including the use for one call
from the house to London, a "wireless" telephone was available
for use in emergency. This communicator used the ionic
conduction field generated by two spaced electrodes places
beneath the house. Although the communicator was normally
powe:'ce from the it was arranged to switch over
autom:Aically to an internal battery in the event of a power
failure. A pilor light in the communicator circuit indicated
this conition provided some illumination as it was
assumed that the main ligh',ing would have been inoperative
in this situation.
The low pressure oxygen line in the habitat was fed
from a cylinder and reducing valve situated below the house.
A gauge was used to set and measure the flow into the atmosphere.
The level of carbon dioxide was checked with a "Itingrose" meter
and oxygen with a paramagnetic meter, every four hours. Once
146
a day these were in turn checked against chalical indicator
tubes. This type of tube was also used for adaily check
of carbon monoNide, hydrogen, stibine and arsnine. Although
the former was present in a concentration of five parts per
million, no trace of the latter impurities was detected.
The louel Cell
_It a late stage in the development of this habitat, a
submersible fuel battering manufactured by the -2:Aectric Power
Storage Co., Ltd., became available. This was self contained
unit using compressed hydrogen and or.sygen. This 16 cell
battery was housed in a light metal cylinder and was automatically
pressurised to ambient sea—bed pressure with nitrogen. The
battery, which could be handled by two divers, was placed on
the sea—bed below the habitat and was capable of supplying
electrical power up to 100 watts. The gas cylinders were
replaced weekly. Because of the ease of using this battery,
the L2: was normally operated from the fuelcell in preference
to its in.ternal lead battery.
Operation
The site chosen was lAarfa Point in the north of
The shore facility was eivided into the main camp situated
about 100 yards from the sea and the lan:ing place where the
divers entered the water. The accompanying table shows the
main items of specialist ecluipment that were eAployed in this
operation. The first habitat was moored at 30 feet in
70 feet of rater on August 5th. However, when preparations
were in hand for the first diver to be nut under saturation
conditions a sudden storm destroyed the habitat complete
with fittings and life support module and damaged the shore
camp.
It was not until August 31st that the first pair of
divers were ready to take un occupation in a second habitat.
._'ter the first storm it was realised that 30 feet
was an awkward depth to face unsettled weather; it was not
deep enouh to ride out a storm, yet the length of decompression
required made a ranici escane impossible. The choice now
open was to operate much deeper at 00 feet (the was
desined for a manimum of 70 feet with nitrogen) where the
effect of wave motion woul be much less or to reduce the
operating denth to 20 feet where a rapid a;ort would be possible
leaving the support party time to deflrte the haAitat and
stow it safely on the sea—bed. The lack of information on
decompression after saturation with nitrogen mintures detennined
that the latter course be adopted.
;Jomewhat to the surprise of the occupants, and the
author who spent four days living in the habitat, it was
comparitively easy to work and sleep underwater. The imowled2,:e
147
148
that the breathing atmosphere and safety was dependant on
oneself and not on a third party whom may not have been seen
since leaving the surface several days previous, appeared to
instil a sense of security.
The inside of the fabric was generally wet with
condensation but the scientific apparatus and life support
equipment did not collect moisture. After a freshwater shower
the human body could be dried with a towel and would remain
dry. No trouble vas experienced when using writing paper or
books. However, as has been reported on previous experiments
of this nature, cold was the main problem. Although the
water temperature was 25° C and air temperature 23°C, the
divers found that it could take several hours to ware up
after a relatively short dive. This was somewhat puzzling
as on shore, where the air temperature was the same on cloudy
days, there was no similar problem.
Throughout the experiment the partial pressure of carbon
dioxide was around and that of oxygen 20 of one atmosphere.
:11 divers spent one hour free orinning at 10 feet for
decompression after one or more days in the habitat.
Acknowledcralent
This appendix refers to the work of the joint
Imperial/Enfield College expedition of 1969. The Imperial
College team was led by lir. P. Newman who was also the
expedition diving officer. Mr. D. Baume led the Enfield
College team. Financial backing came from many bodies and
included the Royal Geogra9hicalsociety- and Imperial College
Exploration I3oard. Consi:lerable assistance to this project
was also given by many organisations in the u.r., 1:alta
and the continent of Europe.
149
150
REFERENCES
1. The British sub—aqua Club diving manual, Eaton publications 1966.
2. Bennett,P.B. ix Elliott,D.N. "The physiology and medicine of
diving and compressed air work" Bailliere Tindall a Cassell 1969
3. Miles,S "Underwater medicine" Staples Press 196G.
4. Williams,S "Underwater breathing apparatus" The physiology and
medicine of diving and compressed air work, Ed. Dennett,P.B.
Elliott,D.H. Bailliere Tindall & Cassell 1969.
5. Baddeley,A.j. "Diver performance and the interaction of stresses"
Underwater Assoc. Report 1966-67, Iliffe.
6. Dugan,J eNplores the sea" 'Tarnish Hamilton 1956.
7. Sims, Rear Admiral W. "Victory at sea" J. Murray 1920.
S. Derktay,H.O. Gazey,B.K. « Teer,C.A. "Underwater communication
past present and future" J. of sound allibr. 7,1. G4 (1968)
9. Mason,D. "U—boat the secret menace" Macdonald u Co 1965.
10. Tucker,D.G. & Gazey,D.L. "Applied underwater acoustics"
PerTamon 1066.
11. Derktay,11.0. a Gazey,D.K. "Communication aspects of underwater
telemetry" iladio a: Electronic Eng. 33,295 (1967)
12. Gazey,B.E. & Morris,J.C. "An underwater acoustic telephone for
free swimming divers" Electronic Eng. June 1964.
13. This is the U.S. Navy AN/UPC-1 system and is used by the NATO
Navies.
14. This equipment is produced by the Aquasonics Co in the U.S.A.
15. Hamilton,P.M. "Underwater hearing thresholds" J.A.S.A.
29,792 (1957)
151
16. Wainwright,W.N. "On comparison of hearing thresholds in air
and in water" J.A.S.A. 30,1025 (1958)
17. Montague,W.E. & Strickland,J.F. "Sensitivity of the water
immersed ear to high and low level tones" J.A.S.A. 33,1376 (1961)
18. Brandt,J.F. & Nollien,11 "Underwater hearing thresholds in man
as a function of depth" J.A.S.A. 46,803 (1969)
19. Bauer,B.B. & Torick,E.L. "Experimental studies in underwater
directional communication" J.A.S.A. 40,25 (1966)
20. Webb,H.J. Webb,J.R. "An underwater audio communicator"
I.E.E.E. Trans. AU-14, 127 (1966)
21. Sims,C.C. "Development of the USRL type J.9. transducer" USTI!,
research report 49, March 20 1959. (Also J.A.S.A. 32,1305 (1960))
22. Liartelli,L. Reinberg,C. "Electronic self contained apparatus
for sound or voice communication" Italian Patent No 617081 (1961)
23. I, orse,P.M. "Vibration and sound" McGraw Hill 1948 sections
27.15 & 27.4.
24. Ldater,J.C. Yrank,N.H. "Electromagnetism" :.1cGraw :ill 1947,
nap. 10.
25. Dunn,H.L. "The calculation of vowel resonances and an electrical
vocal tract" J.A.S.A. 22,740 (1950)
26. Stover,.. "Technique for correcting helium speech distortion"
J.A.S.A. 41,70 (1967)
27. Golden,11.11. "Improving naturalness and intelligibility of
helium-oxygen speech using vocoder techniques" J.A.S.A.
40,621 (1966)
28. Annon "Underwater radio communication" wireless World Feb. 1966
152
29. Needy,K.K. "Divers communication improved" science 158,321 (1066)
30. Nyquist,J "Thermal agitation of electric charge in conductors"
Phys. fev. 32,110 (1928)
31. A throat microphone is used with the 'Aquaphone' made by
Aquaphone Ltd of Poole Dorset.
32. Toby,P. & Dinsdale,J. "Transistor audio power amplifier"
:'tireless World November 1961.
33. Report on enquiry- into Sealab 3 accident. Ocean Industry May 1969
34. Zwislocki,J. "Ear protectors" Handboidi of noise control Ed.
C.M. Harris, McGraw Hill
35. Batteau,D.`11. "The role of the pinny in human localization" Proc.
Roy. _;oc. 168,158 (1967)
36. Nordlund,B Fritzell,B "The influence of azimuth on speech
signals" Acta Oto—laryngologica 56,632 (1963)
37. Hirsh, I.J. "Relation between localization and intelligibility"
J.A.S.A. 22,196 (1950)
08. Cherry, C. "On human communication" Science Editions 1961
Chap. 7 4.3.
39. Hollien,H Doherty,2—T. "Speech intelligibility of diving masks
and mouthcups" 47,127 (1970)
40. Nordlund,B. "studies of steriophonic hearing" Univ. of
Goteborg 1963.
41. LAgnalling and homing by underwater sound; for small
craft and commando swimmers" Classified paper, data taken from
':Iainwrifr,ht (16)
42. Feinstein,S.H. "ihunan hearing underwater, are things as bad
as they seem?" J.A.S.A.40,1561 (1966)
43. Reference manual of transistor circuits, Mullard Ltd 1961.
44. Sivian,L.J. "On hearing in water versas hearing in air"
J.A.S.A. 19,461 (1947)
45. Smith,P.F. conduction, bone conduction and underwater
hearing thresholds in man" J.A.S.A. 44,389 (1068)
46. Stephens,R.W.B. & Dates,A.E. "Acoustics and Vibrational
Physics" Arnold 1966.
47. "The sealab 2 human behaviour program" Project Sealab report
Office of Naval Research report ACR-124.
4S. Hawkins,J.E. Stevens,S.S. "J,Lasking of pure tones and speech
by white noise" J.A.S.A. 22,6 (1950)
49. Thurlow,W.R. & Runge,P.S. "Effect of induced head movements on
localization of direction of sound" J.A.S.A. 42,480 (1967)
50. Topping,J. "Errors of observation and their treatment"
Institute of Physics 1950.
51. Lllump,1!..G. :i;ady,71.R. "Some measurements of interaural time
difference thresholds" J.A.S.A. 28,860 (1956)
59. :;ureter, 7. "The influence of gas bubbles on the generation
of underwater sound" Phi) Thesis, University of London 1007.
53. Eauer,D.B. a Di:Javtia,11..L. "Transmission of directional
perception" I.E.E.E. Trans. AU-13 5. (1965)
54. Pollack, I. fz. Picket,J.M. "Sterionhonic listening and speech
intelligibility against voice babble" J.A.S.A. 30,131 (1950
153
55. MacInnis,J.D. "Living under the sea" Scientific American
214 (3)1 24 (March 1966)
56. Barton,it. "International Oceanics" Hydrospace 2 (4),10
(December 1969)
57. Dwight,;1.3. "Tables of Integrals and other Mathematical Data"
:Macmillan 1961
154
LIST OF SYMBOLS.
A a Area
C Capacitance
c Velocity of sound
d Interaural distance, Skin depth for E.M. radiation
db decibel
✓ (10 Farad 0Jicrofarad)
f frequency
H (mH) Henry (Millihenry)
I Acoustic intensity
k Boltzmanns constant (1.38 x 10-23
Joule/degree)
1 length
L Inductance
O Volume flow of fluid
R Resistance
✓ ILadius vector
t Time
LT Particle velocity of fluid
✓ Voltage
1:ensity
CP :tesistivity standard deviation
Ls-) Angular frequency = 21-rf
(Ks.) Ohm (hillohm)
Permeability
155
156
GLOSSARY 0? DPTING TEIES
Equipment
Aqualung Apparatus for providing the diver with
compressed air for breathing .
Closed-circuit A breathing system where all the exhaled gases
are purified and breathed again
Counterlung A gas-tight bag into which the exhaled gases
pass in a closed or semi-closed circuit breathing
set.
Demand valve The component of an aqualung which regulates the
supply of air to the diver's breathing
requirements. (See fig 1.1)
Demand valve, A breathing system where the demand valve is
single-hose positioned close to the diver's mouth. It is
coupled to the air cylinder by a single high-
pressure flexible hose.
T:emand valve, A breathing system where the demand valve is
twin-hose positioned behind the diver's head and coupled
to a rubber mouthpiece by way two large diameter
low pressure rubber hoses. (une for inhalation,
the other for exhalation)
Foam rubber Cellular neoprene sheet, generally about gun
thick, used for making protective clothing
Full-face-mask A single diving mask covering the whole face.
Helmet Either a rigid gas-tight dome covering the
157
Mouth—mask,
or mouth cup
Mouth—bit
entire head, or a close fitting neoprene hood
covering the head but not the face.
Rubber cup covering the mouth, designed to allow
the diver to articulate underwater. Used in
conjunction with a mask covering the eyes and
nose.
Rubber 'bit' held between the diver's teeth and
coupled to his breathing apparatus. Used in
conjunction with a mask covering the eyes and
nose.
Non—return Device for allowing the passage of gas in one
valve. direction only
Scuba Self Contained Underwater Breathing apparatus.
Normally used to refer to aqualung equipment.
Semi--closed Breathing system where part of the exhaled gases
circuit. are purified and breathed again.
Physiology
Clear the ears The conscious act of equalising the pressure
in the middle ear. (Normally by swallowing or
blowing into the nose)
Decompression Reducing the absolute pressure on the human body
slowly. Failure to observe a safe "decompression
schedule" may result in decompression sickness
often called 'the diver's bends'
Eustachion tubes The connection between the miaile car and back
of mouth. Although normally closed, the
eustachion tubes open during the act of 'clear—
ing the earst
Hypothermia Loss of body heat.
Inert, or The gas that is used to dilute oxygen in a
dilutant gas. breathing mixture. Nitrogen and helium are the
most common.
Narcosis Many inert gases and in particular, nitrogen,
(inert gas) have a narcotic effect when breathed under
pressure. (Helium is often said to be free from
this trouble)
158
ADDITIONAL MA= ;11:1, P71-23ENTED WITH
M,TTQ 11.„
"Voice communication between divers" Underwater Association
report 1966-67.
"Communication between divers" Oceanology International 69
(Proceedings of the S.U.T. conference Brighton 1069)
"Audio communication between free divers" 'Underwater Acoustics'
(Ed. Stephens 11.W.B.) Wiley (in press)
Noise Message
Underwater Association Report 1966-67 47
Voice communication between divers
B. RAY Physics Department, Imperial College, London
SUM MARY The report describes work on various problems
connected with communication between divers. The recordings and tests were made at Marfa Quay, Malta. The distortions introduced by speaking into a face mask are considered and a model proposed to explain them. Apparatus that was used to enable divers to communicate over ranges in excess of 100 yards is described. The relative performance of systems using only the unaided ear for reception are compared with carrier wave methods. Finally, the paper considers the problem of directional bearing underwater.
INTRODUCTION There are two possible means of "wire-less" speech
communication through water. The simplest is to amplify the human voice and send it through the water in such a way that the unaided human ear can be used for reception. This is, of course, just the way a public address system works in air. Alternatively, the speech signal can be used to modulate some carrier. Possible carriers include ultrasonics, laser
beams, and electric and magnetic fields. The carrier system requires some form of receiver, and one loses the advantage of interruption or two people talking at once. A send-receive switch is normally required.
Except where otherwise stated this paper refers to the former "direct speech" method of communication. A block diagram of the system used is shown in Fig. 1. Fig. 2 shows a "direct speech" communi-cator developed at Imperial College.
FORMATION OF WORDS A problem common to all kinds of communication
systems, with or without wires, is the physics of speech inside a small closed volume. Test recordings were made with various kinds of masks and mouth-pieces, as it has been reported (Webb, 1966) that some kinds have a distinct advantage in communication work. In our tests the microphone inside the mask was in each case directiy connected to a surface tape recorder. The diver read a test paragraph as distinctly as possible.
Table I overleaf provides a subjective impression of the quality of the recordings.
DIRECT
VOICE
1 Perception
i 1
Voice Microphone Prop ag ation in Amplifier through Ear
Mask Transducer Water
Fig. I Block diagram of the direct speech communication system.
sec 2n f11 gm cm
48 B. RAY
s •
• • ----- • •••
r
ti
Fig. 2 A "direct speech• - communicator developed at Imperial College.
A suggested explanation of these observations involves the acoustic properties of the mask and the concept of acoustic inductive (mass-controlled) and capacitative (stiffness-controlled) impedance. In air
Table I.
Conditions Comments
All masks in air Complt!tely intelligible
Full face mask underwater
Occasional words missed, meaning clear
Mouth mask underwater
Many mtences had to be repeated to under-stand them
Speaking directly into water by removing a conventional mouth "bit"
Some divers could con-vey simple words but most could achieve no communication
J
the walls of the mask will vibrate as a membrane and transmit some sound whereas, in water, the air-rubber-water impedance mismatch will prevent appreciable transmission and the mask will act as a small closed cavity. To a first approximation the acoustic impedance of such a cavity will be capaci-tative and of the form:—
PCI —4 -- Xe
Where for body temperature and atmospheric pressure :—
Density p =1.14 x /0-' 3 gm/cc Velocity c = 3.53 x 104 cm/.sec Volume of mask = V cc
Hence X, is proportional to 1/V (I) Dunn (1950) has shown how the formation of
vowels by the throat, mouth and lips can be repre-sented by a transmission line (Fig. 3).
Dunn reduced this to a simple electrical lumped
l
i
t
(a.)
Voice communication between divers 49
Throat
Mouth Lips
Fig. 3 Mechanised model of the vocal tract (Dunn, 1950).
Air
(115) Fig. 4 (a) Electrical model of the vocal tract (Dunn, 1950). (b) Modified model of the vocal tract including the effect of the mask. circuit (Fig. 4a). However, this model only refers to speaking into free air. It is proposed to modify the model by replacing the mainly inductive loading of the air by the mainly capacitative loading provided by the mask (Fig. 4b).
It can be shown that the number of resonances in the system does not change from the original four. The capacitative load of the mask does not add an extra resonance; it merely modifies the ones already there. This will be true whatever the values of the components.
With this modification to the model it is possible to account for the better intelligibility achieved with the full-face mask when computed with the smaller ones. In order to comnare the free air and under-water face mask electrical models we can, to a first approximation. add the capacity of the mask to that of the mouth to form a new "ficticious" mouth capacity. Hence from equation (1) the new mouth cavity thus generated will be given by:-
1 = 1 4. 1 V
fic. V V
mask mouth For the English language typical mouth volumes
for the vowels in the words "eat". "lost", and "boots" are 7,90, and 35 cc respectively (From X-ray analysis,
Dunn, 1950). If these volumes are compared with 1200 cc for the full-face mask, 200 cc for the mouth mask, and only a few cc when speaking directly into water, we have a possible explanation for the differing effects of these masks on speech.
A more detailed analysis shows that the low frequency components of the voice will be affected most. Supporting evidence was obtained when underwater speech was filtered. In normal speech there is a frequency such that if one electrically removes all lower or all higher frequencies, then the intelligibility remains the same. This frequency is usually found between 1500 and 1800 c/s. When recordings from the mouth mask were filtered this frequency was estimated to be between 2000 and 2500 c/s. This suggests that the mask has caused more distortion to the lower frequencies.
At the time of writing this is being further tested with recordings made under laboratory conditions.
MICROPHONE The choice of microphone was relatively simple.
In a voice frequency system both throat and bone conduction microphones, being themselves in contact with the water, are prone to acoustic feedback through the water from the transmitter. Lip microphones, isolated by an air cavity from the amplified signal, do not suffer this defect and hence were used throughout the experiments.
AMPLIFIER A transistorised amplifier of conventional class B
design was employed. It was housed in a case 12 in. long made from in. wall "Perspex" tube. "0" ring seals were used throughout, and the design proved entirely satisfactory. The mean electrical output v'hilct the diver was speaking was about two watts.
TRANSDUCERS Several types of transducer were tried; the most
successful was a moving coil design. Pressure com-pensation was achieved by an internal bladder open to the water.
Both fel- diver to surface communication and for subsequent analysis of acoustic signals in water. a smali hydrophone was employed. This gave a very low output, and it was found convenient to anchor the hydrophone to the bottom with a preamplifier. The preamplifier was encapsulated in epoxy resin. and power was fed to it down the cable used to convey the amplified signals to the surfac... re:order.
NOISE Under very quiet cl;nditions the range of a com-
municator, relying only on the unaided ear for reception, may only be limited by the signal Calling below the threshold cf hearing. However, in general,
mask
Surface
Bottom
Fig. 5 Propogation of sound between one transmitter and two receivers at (A) and (B)
50
B. kart
Table II
Origin of noise Type of noise Masking effect Effect on
MHZ carrier system at 25 yds.
Biological "Snapping shrimps"
Impulsive noise
Very little Very considerable
Sea state Low frequency continuous
Very little None
Thermal High frequency None (below threshold)
_Theoretical only
Receiving diver's demand valve
Wide band Complete masking
Very considerable
Receiving diver's exhalation
Low frequency intermittent
Masks weak signals
Small
Other human noises and movements
Bone conduction continuous
Masks weak signals
Small
As one moved to the limit of the range of the transmitter it was found necessary to remain motion-less to hear the signals. The auditory canals of the diver are closed by the presence of water, and this condition gives rise to an im-provement in hearing by bone conduction (between 15 and 25 db.. Zwislocki, 1957). Hence body and equipment movement noises are carried through the body and be-come important underwater. These sounds, along with exhalation bub-ble noises, were found to be the main limitation to the ranee of direct voice communication with the working diver. Although a commercial system was found to be limited to about 10 ft., the amplifier and transducer described already, allowed reliable communi-cation over ranees in r.xcess of 100 ft. Used between surface and diver clarity of signals enabled them to be heard beyond 300 ft.
the steady deterioration of the signal to noise ratio as one moves away from the transmitter will set a maximum range. The ability of different noises to mask speech differs. A continuous noise is worse than an impulsive one. Column three of Table II shows the effect of various noises on a direct audio system. The effect on an ultrasonic carrier communicator can be seen from the last column. This system con-tained both transmitter and receiver and had to be operated rather like an ordinary walkie-talkie.
As can be seen, the three serious noise sources are the demand valve hiss, low and medium frequency noises gen-erated by movement in the water and exhalation noise. It would seem unlikely that there is any way of silencing the conventional cit-mand valve sufficiently to avoid masking incoming signals It follows that for reliable com-munication both parties must synchronize their breathing rate. Ironically this is aided by the valve noise received from the other party. It was for this reason that no electronic means were used to prevent breathing noises being trans-mitted.
RECEPTION Fig. 5 illustrates one transmitter and two possible
receiving positions. Two main propagation paths are drawn between the transmitter and the diver in position (A). One of these is a surface reflection. Surface reflections suffer very little attenuation but are reversed in phase. It follows from this that there will be a zone close to the surface where there is almost complete cancellation of reception or transmission. In practice it was found useless for a
Voice communication between divers 51
diver to attempt communication within two feet of the surface. Another result of this was that music heard within a few feet of the surface lacked bass frequencies compared with the same test piece heard at a deeper depth. The paths from the transmitter to position (B) differ considerably in length. Signals arriving at (B) will arrive at different times and cause "multiple path distortion". This gave rise to con-siderable distortion of signals recorded close inshore over a rocky bottom, whereas in 70 ft. of water over a sandy bottom, signals were far less distorted.
It is generally impractical to design an electronic circuit to remove multiple path distortion. However, it should be clear from the figure that if the diver (B) possessed some form of directional receiver he would be able to differentiate to some extent the direct path from the others. On land man is able to tell the direction of a sound and ignore echoes. The extent to which man can tell the direction of a sound underwater is important in a communication link.
To test directional hearing a wide-band sound source—music was used as nodal patterns would have precluded the use of a narrow-band source such as a tone—was placed 15 ft. below the surface. The subject divers were placed about 30 ft. away at the same depth. With their eyes closed the subjects were first spun around and then asked to point to the direction of the source. No protective clothing was worn on the head. The divers were requested to hold their heads still while performing this test. The results of this were not conclusive; the seven subjects pointed to the correct hemisphere on 32 out of the 50 tests. Better tests are at present being planned for the future.
CONCLUSIONS The difficulties in evaluating communication systems
in the underwater environment are considerable. Tests involving the human element will be affected by a whole range of unusual problems.
There was a tendency for divers to face each ()thee
at close range and discuss simple topics such as depth, weather, etc. This gave an over-favourable first impression that did not stand up to more critical analysis. Likewise, tests performed in swimming baths or with surface operators hanging equipment from boats should not be directly compared with tests using divers in the open sea.
REFERENCES DUNN, H. K. (1950) The calculation of vowel
resonances in an electrical vocal tract. J. Acous. Soc. Am. 22. 740.
ZWISLOCKI, J. (1957) Ear protectors, Handbook of Noise Control (McGraw Hill), 8-12, 8-13. (Ed. C. M. Harris).
WEBB, H. J., and WEBB, J. R. (1966) An under-water audio communicator. LE.R.E. Transac. on Audio and Electroacoustics, AU-14, 127.
ACKNOWLEDGEMENTS We would like to thank the Imperial College
Exploration Board, The Royal Geographical Society and the many other organizations who gave us financial and material support. We would particularly like to thank Dr. R. W. B. Stephens for his valuable help both in London and in Malta.
MEMBERS OF 1966 IMPERIAL COLLEGE MALTA EXPEDITION
A. D. Baume P. Jenkins. B.Sc.. A.R.C.S. A. P. Kingwell, B.Sc., A.C.G.I. R. Leonard J. Love, B.Sc., A.C.G.I. B. Ray, B.Sc., A.R.C.S. R. Wharton, B.Sc., A.C.G.I.
Reprinted from, "Oceanology International CO" (Proceedings of
the Society for Underwater Technology ..Collference, Brighton 1969.)
C C12. TUN I CAT ICN B ETWEEN D rirms B. Ray (Imperial College)
INTRODUCTION.
In normal circumstances the task of a communication engineer commences at the microphone and finishes at the human ear. Underwater the environment is so hostile that the act of speaking or for that matter hearing cannct be assumed. Communication travels frcm thought to mind and any system requires judging on this basis. There are many occasions underwater when the best solution today to a co=munieation problem lies with visual signals or tugs on a lifeline. However it is proposed to limit this discussion to voice communication. In general this can be represented by Fig.l.
BEFORE EVEN SPEECH.
The diver is subject to many physiological conditions such as cold, ahx-iety, narcosis, carbon dioxide build-up, and anoxia, that have the effect, in the first place, of slowing down and distorting mental processes. The communication equipment must not aggravate these troubles by encumbering the diver, nor must a significant mental effort be required to operate it. No record appears to exist of an equipment actually leading to the drown-ing of a diver, but there are examples of systems that went some way in this direction (1).
THE FORMATION OF WORDS.
The main differences between speaking into a face-mask or helmet under-water and speaking in free air, are tabulated in Fig.2. Of these,nuinbers one and six are the most serious. Many people have heard demonstrations of 'helium speech', which will be dealt with in a later paragraph, out the problems associated with the volume of.the mask are less well known. It is best demonstrated by listening to the signal received from a 'fully equipped diver who is standing with his breathing set submerged but with his head above the surface of the water.. As he submerges completely there is a noticeable drop in intelligibility. In air there is appreciable transmission through the relatively thin walls of the face-mask, which reduces the acoustic impedance of the mask cavity, as seen from the lips, below that of an infinitely rigid mask. When,the mask is completely sub-merged, traasmission through the walls is negligible: and the mask can be regarded as a closed cavity, pro--tucing a high acoustic impedance at the lower speech frequencies.
It has been sunested that this high impedance will produce speech distort-ions, the magnitude of which vary inversely with the volume of the mask (2). Two possible solutions to the problem are proposed. The obvious one is to increase the size of the mask. A lightweight inner-mash can be inserted covering the nose and mouth to prevent a large 'dead-space' as the latter leads to possible carbon dioxide build-up (3). The second is to add an acoustic feedback path by placing an amplifier and loudspeaker inside the mask. This, if designed properly, should reduce the acoustic impedance
Fig 1. The communj.cation chain.
PsyFholog!Fal + Thc4nt .1 a noise i
+ Articulation g 1 Ecrs
t /
4
Speech in mask Acoustic noise, 4. I demand voive,etc. 4,- ._,
4,
Microphone
Trons/ miss:cn
tlese in sea
Propagation ; -
Dover L
Rece; ver
Ecrphone Acoustic noise, bubbles, breathing, etc.
Eor
4- Percepticn ro se
If necessary
'Diver IX
Fig. 3. View seen by cameraman in free choice experiments.
The sound source is beyond the top left hand corner of
the photograph.
1.7rWr.' 7, • 7prreqt,":rr.<9',64.:,
Fig. 21 Comparison between spea underwater and speaking in free air
Difference I Effect Contrary factors Possible solutions
Mask has a finite volume coust;c impedance differs from air
III
Larger volume can cause problems :with CO! build up
(a) Inner mask (b) Electronic feedback
The nose is generally blocked, or in a separate cavity
Uvula may be closed, giving rise to a chaitge in vowel sounds
Requirement to 'clear' the ears Mask that allows nose to be pinched for car clearing, but with nose normally free in main cavity
The air path between mouth and cars does not exist (except with a helmet)
The feedback between voice and hearing is radically altered
1
(a) Helmet covering whole head (b) 'Side-tone' fed to cars
Water (or rubber) surrounding the chest and throat
from throat or chest
Change in damping of the throat cavity and lack of radiation • - .
Mechanical restriction of mask on face muscles
Difficulty in producing certain speech sounds 1
Better masks
Increased velocity of sound in breathing gas when using helium—oxygen mixtures nitrogen
'Donald Duck.' effect on voice
•
At deeper depths air cannot be used (inert gas narcosis)
(a) Within limits it may be pos-sible to add small amounts of
(b) Unscrambler Increase in ambient pressure Believed to cause a lack of
fricatives and inability to whistle
Small pressure fluctuations caused by breathingequipment (Generally about I to 2 in. water gauge)
'Difficulty' in articulation Reliability of breathing equipment sets a limit on the sensitivity of the non-return valves used in the construction
Better design of breathing apparatus
Fig. 4.
Masking effect Masking effect Origin of Noise Description on Direct Audio on Carrier System
r
Sea state,and waves on shore
Continuous Low frequency
Very little None
4 - Receiving Divert Diverb Wide band iComplete Complete Demand Valve i iMasking iMasking
4- -4- ..t. "Snapping iImpulsive Small Considerable Shrimps" Biological + I-- L. Human movement & Continuous Masks weak Small bubble noise Bone Conduct—
ion Signals
Reverberation iRelated to Not obvious * Very * Signal Noticable
4.
I-
L
* As reverberation is related to the signal it is difficult to assess the masking effect.
seen from the lips. Although this technique has been used for sound absorb-tlon in air (4), there are practical difficulties in applying it underwater.
The problem of 'helium speech' has become a familiar one in recent years. The velocity of sound in helium is nearly three times greater than in air and in mixtures of helium and oxygen (heliox) it lies between the two. The -effect--of the changed velocity of sound in the breathing gas is that the frequencies of the speech foments are higher in heliox. However, this increase in frequency is in itself proportional to the original frequency, and here occurs the first difficulty in unscrambling helium speech. It is of no help to lower the absolute frequencies by a fixed number of Hertz (a relatively straight forward operation). One popular method (5) at present is to break the speech signal up into a number of frequency bands and to reduce the frequency of each by a different, fixed amount. It has been stat-ed that in saturation diving there is some change in the characteristics of a diver's speech after two or three days, leading to some improvement in intelligibility.
One practical problem in diver communication is that the effect of adding two distorting factors (fig 2), which taken individually may be acceptable, can be disastrous. One example of this is a simple wire comunication sys- • tem between divers and the Sealab 11 habitat based at 205 feet. The report (6) stated, "The transmission was completely garbled and unintelligible. The same type of intercom had been tested at 10 ft at the U.S. Navy Mine Defense Laboratory, with very good results."
This particular system used a bone conduction microphone that relied on sound transmitted through the bone structure of the head. Although this is .the simplest type to operate underwater (7), a more conventional air micro-phone inside the mask will give better reproduction, especially of the high-er voice frequencies. The only other alternative, a throat microphone, normally provides far too low a standard of reproduction.
TRANSMISSION & RECEPTION.
The simplest, and often most efficient, method of transmission is to use a pair of wires between the diver and receiver. If it is convenient to combine the telephone line with an air- or life-line, then this method is the obvious choice (8). However, when the diver is not attached to any form of line, a 'wire-less' system is required. The simplest method of wire-less communic-ation is to amplify the diver's voice and transmit it through ehe water in such a way that the unaided human ear can be used for reception. This will be described by the term 'direct audio', and is, of course, the equivalent to an ordinary public address system.
Alternatively the speech signal can be used to modulate some carrier. Possible carriers include ultrasonic radiation, optical beams, electric and magnetic fields, and very low frequency radio waves. Although this latter has been used for some time for communication with submarines, aerial requ-irements render this quite impractical for divers. A system using the conduction field generated by two electrodes on the divers back is available (9). Optical sytems will, in areas where there is diving interest, be
limited in distance by the amount of suspended matter in the sea water. In many cases this will be a matter of a few feet. The majority of diver comm-unication equipment available uses an ultrasonic carrier at a frequency of between 8 and 150 KHz. Most forms of modulation familiar to the radio engineer are to be found. The range over which communication is possible varies from a few hundred metres to several kilometres. It is not proposed to enlarge on ultrasonic communication here as this subject has been well covered elsewhere. (10),(11).
NOISE.
The ability of different noises to mask speech differs. Fig.4. shows the effect of various noises that were experienced by the author while using compressed air diving equipment in the Mediterranean. Their effect on a 'direct audio' communicator is compared with that on a carrier system, (3 KHz, SSB modulation). The equipment was tested over a range of 25 metres on a rocky shore in sea state 2 to 3. As can be seen, the three serious noise sources were the breathing apparatus, the diver's equipment and cloth-ing and reverberation. It would seem unlikely that there is any way of silencing the conventional demand valve sufficiently to avoid masking in-coming signals. It follows that for reliable communication both parties must synchronize their breathing. Ironically this is aided by the valve noise received from the other party. It was often found necessary to remain motionless to hear the signals. The auditory canals of a diver are closed by the presence of water, and this condition gives rise to an im-provement in hearing by bone conduction (between 15 & 20 db (12)). Hence, in addition to external noises, body and equipment movement sounds are trans-mitted through the body and become important underwater. It was concluded that whereas noise generated by the receiving diver, set the limit to the range of a 'direct audio' system, externalnoise and reverberation were the limiting factors with the carrier equipment. The fact that the 'direct audio' system was superior over short ranges in noisy reverberant conditions leads one to the possibility that its advantages lie with the use of the unaided human ears underwater.
DIVER HEARING.
The free diver usually has his ears open to the water and they normally fill with water, leaving only a small bubble of air on the drum. Pressure on the inside of the ear must be equalised to the external pressure by the diver opening his eustachian tubes periodically (swallowing or blowing into the nose are the usual methods). Failure to equalise properly can cause loss in hearing sensitivity and complete neglect can lead to a rupture of the drum (13).
It is observed that in everyday life there are many cases where.the human brain and ears are required to understand a speech sizmal in the presence of considerable noise. The brain appears to be able to perform this function partly from the nature of language and partly through the use of two ears. This ability is well demonstrated by the 'cocktail party' effect (14), A person 'listening' to several conversations simultaneously has little diff-iculty in isolating them .and understanding which ever one he pleases:
However, this ability is lost when, for example, listening to a recording of the same sound. If this same ability exists underwater then there is good reason for using a binaural receiver or a method utilising both of the unaided ears.
EXPERIMENTS IN BINAURAL PERCEPTION UNDERWATER.
A series of experiments on binaural hearing were carried out by a team from Imperial College in 1968. Although two main experiments were planned, it was found necessary to design a third.while in the field.
The accuracy with which a diver can perceive the direction of a sound source (in the horizontal plane) was measured. The subject diver was suspended in free water by adjusting his weight so that he was bouyant and tying him, by means of a rope around his ankle, to the sea-bed. He was blindfolded and asked to point to the direct- ion of a wide-band source. A plan-view photograph was used to record the indication. A preliminary survey of the results suggests that the diver was correct on most occasions. (Photograph - Fig.3.)
b/ A seated diver was asked to judge from which of two positions a sound originated. The seat and the two possible positions formed a triangle on the sea-bed. The diver had merely to raise his right or left hand to indicate his choice. When the angle between the source positions was greater than 200 few divers had difficulty in indicating correctly. While performing these tests several subjects reported there being a marked difference in the level of sound between facing towards and away from the source. To test this impression a simple audiometer was constructed. It consisted.of a square wave source of 400 Hz, the amplitude of which could be varied in 3.5 db.steps. Five.subjects were tested with this instrument and all showed a difference of between 3.5 db and 7 db between front and back, the forward direction being the more sensitive. The result was not affected by removing the air cylinder and demand valve from the diver's back.
Quantative aspects of these experiments will be available at the time of the conference.
CONCLUSIONS.
At the present timethe most serious problem in voice communication between divers is the act of speaking underwater. Far more research is needed on an interdisciplinary front.
Binaural hearing underwater has been largely overlooked. This is puzzling when one considers that often the diver's sense of touch and vision can be reduced to a very low level of efficiency. The use of the unaided ears,,or possibly some form of binaural receiver (15), appears to offer advantages particularly in the reverberant conditions that are likely to be met in- shore or in the vicinity of wrecks, construction work, etc. Binaural hearing offers a simple way of navigating underwater. It was found that after some practice divers were able to find a hidden acoustic marker. It remains to be seen whether this can be put to use under cowercial conditions.
ACKNOWLEDOENTS.
The author wishes to acknowledge the help and guidance of Dr R.W.B. Stephens. Financial support from the Minnesota Mining & Manufacturing Co.Ltd. enabled the author to pursue this study. The trials that were carried out in the Mediterranean were supported by the IMperial College Exploration'Board.
REFERENCES.
1. O.N.R. Report A.C.R.-124, "Project Sealab Report" p143 'Swimmer intercom' Office cf Naval Research (1967)
2. Ray B. "Voice Communication Between Divers" Underwater Assoc. Report 1966-7, p47, Pub. Iliffe Science &*Technology Publications Ltd.
3. A mask of this type is made by, Draeger Normalair Ltd. 4. Stephens R.W.B. & Bate A.E. "Acoustics & Vibrational Physics" 15.18
Arnold 1966. 5. Golden R.M. "Improving Naturalness & Intelligibility of Helium-
Oxygen Speech Using Vocoder Techniques" J. Acoust..Soc. Amer.40,621. (1966)
6. O.N.R. Report A.C.R.-124, "Project Sealab Report" p144 'Swimmer intercom' Office of Naval Research (1967)
7. Needy K.K. "Divers Communication Improved" Science 153,321 (1966). 8. An example of a combined life-line/telephone is, Siebe Gorman/McMurdo
Duck-Set. 9. An electric conduction communicator was marketed by Andrew & Dalton
Ltd. 10. Tucker D.G. & Gazey B.K. "Applied Underwater Acoustics" Pergamon 1966. 11. Berktay H.O. & Gazey B.K. "Communication Aspects of Underwater
Telemetry" Proc. I.E.R.E. Conference on Electronic Engineering in Oceanography 1966.
12. Zwislocki J. "Ear protectors" Handbook of Noise Control 8-12 (Ed C.M.Harris) McGraw Hill 1957.
13. Miles S."Underwater Medicine" p85 Staples Press 1966. Cherry C. "On Human Communication" Chap.7,4.3. Science Editions 1961.
15. Bauer B.B.,DiMattia A.L.,& Resenheck A.J. "Transmission of Directional Perception" I.E.E.E. Trans. on Audio AU-13, 5, (1965).
'Underwater Acoustics' (Ed Stephens R.W.B.) Wiley 1970.
8
Audio Communication between Free Divers
B. Ray Physics Department, Imperial College of Science and Technology, London
.•
8.1 Introduction .
8.2 The problem . . . .
241
243 8.2.1 Before words arc farmed 243 8.2.2 !Vold formation tender water • . . • 244 8.2.3 Problems associated with breathing mixture . 244 8.2.4 Acoustic impedance of the facemask . . 247
8.3 Microphones . . . . • 247 8.3./ Transmission through water • 248
8.4 Reception and noise 250 8.4.1 Hearing 251
8.5 Conclusions 252
References 253
8.1 Introduction Before looking at the communication problems encountered by men
working under the sea, it is necessary to examine the equipment and tech-niques that are used. This introductory section will be devoted to a brief examination of diving technology.
Until the present decade nearly all serious underwater work was per-formed using a system in which the diver was connected to the surface at all times. The diver wears a 'Hard-Hat' diving dress, which comprises a heavy copper helmet attached to a rubber suit of generous proportions. together with the necessary lead weights and heavy lead boots to keep the diver stable on the sea floor. In Hard-Hat. as with other diving systems, the human body is exposed to the ambient water pressure: however it all the cavities within the human body are 'equalized' to this pressure then the diver may be quite unaware of the absolute pressure. Since the body will not tolerate a pressure differential and if the diver has failed to equalize the pressure in his lungs, ears or sinuses by more than a few feet water gauge,
241
242 Underwater Acoustics
the results can be very serious. It is imperative, therefore, in diving equip-ment to provide breathing gas at the ambient water pressure; in the Hard-Hat system this requirement is met by an air line to a surface compressor which continually supplies the diver with excess air. The correct air pressure is then maintained in the diving dress by expelling this air into the water through a valve on the helmet. Communication with the surface is maintained by a telephone cable alongside the air line. There is ample space inside the copper helmet to fit an intercom, using a conventional moving-coil loudspeaker and microphone.
Since World War 11 compressed air breathing apparatus has become available. The heart of a compressed-air diving set is the 'demand-valve'. This is usually a one or two stage pressure reducer attached to a high-pressure air cylinder. It is designed to provide air at a pressure that will remain within about 2 in water gauge of the pressure at its sensing dia-phragm. Because of the sensitivity of the demand valve, it is necessary to return the exhaled air to a point close to the sensing diaphragm before releasing it through a non-return valve. The self-contained compressed-air diver may wear a mask covering his eyes and nose and hold his breathing hoses between his teeth with a mouth bit; he may wear a mask covering his eyes and nose and a second cup-shaped mask over his mouth; or he may use a 'full-face' mask covering the whole of the face. For protection against the cold a close-fitting rubber suit is normally worn and a pair of fins give the free diver far more mobility than his predecessors. The acoustic problems brought about by the use of a mask will be dealt with later. However, at this stage it should be noted that broad-band noise will be produced by the pressure reduction and this will completely mask most hearing. In the exhalation process there will be the operation of several non-return valves and the production and release of bubbles.
The equipment described so far is not suitable for work below about 250 ft. For deep diving it is usual to use some form of closed- or semi-closed-circuit breathing equipment. In this apparatus the breathing gas, which is now probably a mixture of oxygen and helium, is recirculated and used again. The exhaled gases pass over a chemical to remove carbon dioxide. A small bleed of high-pressure oxygen makes up for the amount of oxygen used by the human body, and the gases finally pass into a flexible bag. This bag is known as the counter-lung, from which inhalation takes place. Although the ancillary equipment tends to be similar to that of the com-pressed-air diver, the level of inhalation and exhalation noise is very much less.
Tiiis is often called SCUBA in the U.S. and AQU‘`.1 UNG in the U.K. These terms :will not be used here as they are trade marks in certain countries.
Audio Communication between Free Divers- 243
8.2 The problem The use of free divers and modern diving techniques however, has
brought about some new problems with communication. The most impor-tant are listed below
(1) The free diver cannot use a cable telephone. (2) The free diver's 'helmet' fits his head far closer than does the 'Hard-
Hat', so creating problems in forming words into a confined, or non-existent cavity, and hearing a signal under similar conditions.
(3) The modern diver may well be using a breathing gas that differs markedly from air. The main reason for this is that air, or to be precise, the nitrogen in the air, has a narcotic effect under pressure. It is generally not. practical to use oxygen/nitrogen breathing mixtures at depths below 250;f and either helium or hydrogen are normally used as the inert gas for deeper diving; both helium and hydrogen have a velocity of sound that is greater than that in air.
In the context of these problems it is important not to take too much for granted. The communication engineer must consider all communication methods and must not reject such possibilities as hand signals. However it is proposed to limit this discussion to voice communication. In general this can be represented as in Fig. 8.1.
8.2.1 Before words are formed
The diver has many physiological enemies such as cold, anxiety, the build-up of exhaled carbon dioxide, anoxia due to inefficient breathing equipment, and narcosis; these may all have the effect, in the first place. of slowing down die mental processes of the diver. It is important that the communication equipment does not aggravate these troubles, nor must operating the equipment require considerable mental effort.
One interesting example of this last part is the use of 'mouth-masks', that, is rubber cup-shaped mouldings designed to strap over the mouth and lips. Their object is to remove the requirement for the diver to hold a conven-tional 'bit' between his teeth. Although these devices do, in many cases, offer improved comfort and clearer communication. they can also cause prob-lems in sealing the mask to the face. When a mouth-mask was used in a U.S. Navy saturation diving project' this problem was magnified to a daniag ous degree by the back pressure exerted by the breathing apparatus. Need-less to say, no intelligible communication was received from these divers!
The author has himself experienced conditions under which he was un-able to remember which end of a vertical rope led to the surface; the reader is left to imagine how reliably one could operate a transmitter under these circumstances.
Psychological noise
Acoustic noise, demand valve, etc.
► Thought
Articulation
f Speechiin mask
Microphone•
Transmission
4-
1
Ears 4
-J
Receiver
If necessary ;
Earp.hone
Ea▪ r
Psychological ▪ Perception noise"
Acoustic noise, bubbles, breathing, etc.
244 Underwater Acoustics
Propagation
Noise in sea
Diver I
'Diver It
Fig. 8.1
8.2.2 Word formation under water The main differences between speaking underwater and speaking in air are
shown in Table 8.1 which should be largely self-explanatory. The 'Contrary factors' column gives the reasons why the obvious solution may not be a suitable one. For example, one may try to overcome problems caused by the limited volume of the mask by increasing the volume, but this un-fortunately can often give rise to pockets of expired air becoming trapped in the additional volume.
It is proposed to limit the discussion to iv:0 of the more important problems.
8.2.3 Problems associated with breathing inixture Before discussing the effects of using breathing gases other than air, it may
be useful to examine the reasons that drive physiologists to look for exotic mixtures. In general it is the partial pressure of the constituents of the
Table 8.1 Comparison between speaking underwater and speaking in free air
Difference
Effect
Contrary factors
Possible solutions
Mask has a finite volume
The nose is generally blocked, or in a separate cavity
The air path between mouth and ears does not exist (except with a helmet)
Water (or rubber) surrounding the chest and throat
Acoustic impedance differs from air
Uvula may be closed, giving rise to a change in vowel sounds
The feedback between voice and hearing is radically altered
Change in damping of the throat cavity and lack of radiation from throat or chest
Larger volume can cause (a) Inner mask problems with CO2 build up (b) Electronic feedback
Requirement to 'clear' the cars Mask that allows nose to be pinched for ear clearing, but with nose normally free-in main cavity
.(a) Helmet covering whole head (b) 'Side-tone' fed to cars
Mechanical restriction of mask oa lace 111LISCICS
Increased velocity of :-,odnd in breathing gas when using helium-oxygen mixtures
Difficulty in producing certain speech sounds
'Donald Duck' effect on voice
Better masks
At deeper depths air cannot be (a) Within limits it may be pos• used (inert gas narcosis) sible to add small amounts of
nitrogen (b) Unscrambler
Increase in ambient pressure Believed to cause a lack of fricatives and inability to whistle
Small pressure Iluctuations 'Difficulty' in articulation Reliability of breathing Bet.:r design of breathing caused by breathing equipment. equipment sets a limit on the apparatus (Generally about I to 2 in sensitivity of the non-return water gauge) valves used in the construction
ul
246 Underwater Acoustics
inhaled gas that define its suitability for breathing. Oxygen is poisonous above 2 atmospheres partial pressure and nitrogen has a narcotic effect above about 7 atmospheres. Hence it should be clear that below about 200 ft it is necessary to reduce the percentage of oxygen and to use a substitute for nitrogen. The most common substitutes are hydrogen and helium. In mixtures containing either of these gases the sound velocity is greater than in air.f
The classical model for the human voice considers the larynx as a wide-band noise generator with the mouth, throat and•nose cavities acting as resonators. The end result is a frequency spectrum whose envelope is defined by the vocal tract and whose fine structure is a function of the larynx waveform. The effect of the increased velocity of sound in helium/ hydrogen mixtures is to shift the envelope shape up the frequency spectrum. The larynx pitch is not affected to a significant degree. The maxima of the frequency spectrum are termed the `formants' of the voice and the most obvious effect of breathing a helium or hydrogen mixture is an increase in the frequency of these formants, the increase being in proportion to the change in the velocity of sound in the exhaled breath.
From the foregoing discussion it should be clear that in order to render helium speech intelligible it is pointless to reduce the component frequencies of speech by a fixed number of cycles per second (a relatively straight-forward operation). What is required is a proportional frequency reduction, which is just what happens when a tape recorder is played back at slow speed. Although under some circumstances it may be useful to use a tape recorder for this purpose, the distortion of the time scale normally negates any advantage gained. One practical method does, in fact, use a time-stretching technique to correct the frequency spectra. However a part of each separate sound is discarded so that the overall time scale remains true.2 At the time of writing the most popular method of unscrambling the speech of divers is to break the speech signal into a number of frequency bands and • to reduce the frequency of each by a different, fixed, amount. The larger the number of frequency bands the closer will be the approximation to the ideal.
Unfortunately, at present the performance of even the best instruments leaves a considerable amount to be desired. It appears that apart from shift- ing the formants. the gas mixture distorts speech in other ways. The role played by the absolute pressure is still in doubt; a mixture of 2! per cent oxygen, 79 per cent helium has the predicted effect at atmospheric pressure and can be unscrambled remarkably well. At 20 atmospheres pressure, a suitable breathing gas would contain less than 5 per cent oxygen and the
t The reduction of the oxygen percentage makes it possible to design a diving system that avoids explosive combinations of oxy-hydrogzn.
Audio Conimunication between Free Divers 247
velocity of sound in this would be higher than in the 21/79 mixture that was used at the surface. ro the untrained observer the surface 21/79 speech would appear strange but understandable; however, the speech from 20 atmospheres would be completely unintelligible, and might not be recog-nized as the human voice.
The problems of making measurements under high pressures are many, for not only does there exist some doubt as to the nature of speech recorded under these conditions, but there are limitations on the gas mixtures that can be used in the case of human subjects. Furthermore, experiments at 20 atmospheres are very expensive to perform because it will take several days to decompress a subject from even a short exposure to this pressure.
8.2.4 Acoustic impedance of the facemask If one listens to the signals received from a fully equipped diver who is
standing with his breathing set submerged but with his head above the surface of the water, as he submerges his body completely, there will be a noticeable drop in intelligibility. In air there is appreciable transmission through the relatively thin walls of the facemask, which reduces the acoustic impedance of the mask cavity, as seen from the lips, below that or an infinitely rigid mask. When the mask is completely submerged, trans-mission through the walls is negligible and the mask can be regarded as a closed cavity having a high acoustic impedance. Although this argument strictly only holds for frequencies below the cavity resonance of the mask, the conclusion, namely that the mask will produce an effect which is. inversely proportional to its volume (i.e. proportional to its impedance), appears to provide a useful guide to the choice of masks for practical communication purposes3.
Two possible solutions to this problem will be considered. The obNious. one is to increase the volume of the mask. If this is done, then some light-weight inner mask can be inserted to prevent a large dead-space as the latter leads to the possibility of carbon dioxide build-up. The second is to add an acoustic feedback path by placing a microphone, amplifier and loudspeaker inside the mask. It should be possible to design such a system to reduce the acoustic impedance seen from tile lips. Although this technique has been used for sound absorption in air there are practical difficulties in applying it underwater.
8.3 Microphones Three types of microphone are in use by divers; they are air, bone-con-
duction and throat. All of them have particular problems. The air micro-phone, positioned in the mask, stands the best chance of receiving
Moulded rubber Peizo- electric
ceramic Reducible air volume, Fixed air
V volume, v
Seal
—Resin
--Electrodynamic microphone insert
Cable
Flexible membrane
248 Underwater Acoustics
intelligible signals. However, it must be designed to withstand a large p: e;-sure change and frequent splashing with water. A design for a pressure-compensated air microphone is given in Fig. 8.2. By contrast the bone-conduction microphone is a vibration pick ups and can more readily be made as an encapsulated pressure-resistant device. Throat microphones are not normally used underwater as the quality of reproduction is seldom sufficient to allow intelligible communication.
The choice between air and bone-conduction may be influenced by their different performance with locally generated noise. The main interference
Examples of underwater microphones
Bone-conduction Air microphone designea for use inside a face mask microphone
Showing the effect of pressure on this microphone : the membrane con move to equalize pressure.
Max;mum depth capability =, v — metres
Lead out wires
Fig. 8.2
experienced when using an air microphone is the sound generated by the breathing set, particularly the inhalation noise. On the other hand it is the noise caused by the exhalation bubbles that is the main limitation of the bone-conduction microphone.
8.3.1 Transmission through water Referring again to Fig. 8. I it can be seen that the stage has now been
reached where an electrical representation exists of the words spoken by the transmitting diver. Some methods that can be used to transmit a speech signal through water are listed in Table 8.2. The simplest, and often most ef9cient of these is to use a pair of wires between the transmitter and the receiver. If it is convenient to combine the telephone line with an air- or life-
Table 8.2 'Telephony transmission systems
Method
Propagation
Radiator
Receiver Main limitation
Typical range
Two wires Electric current
Telephone
Telephone
The inconvenience of No limitation the wires
Direct audio Audia frequency Piston sound
The car Noise generated by 0-30 m sound
(loudspeaker)
the receiving diver and his breathing set
Ultrasonic Modulated high frequency sound 8 -200 kl lz
Resonant piezo-electric transducer
Resonant piezo- Reverberation and electric transducer multi-path
distortion
0.1-5 km
Two spaced Interference due to electrodes some- any electrical what aligned to the installation with transmitting eat th currents electrodes
Electric field
Optical
Magnetic field
(a) Audio frequency Two spaced
electric current electrodes field
(b) Modulated high frequency field
Visible light (green Laser or semi- window) conducting diode
Magnetic induction Large coil
Photodiode or photocell
Coiil somewhat aligned to trans-miting coil
Scattering and absorption of transmitted beam. Transmitters and recekers are generally highly directional
A few metres with early models. Information rather obscure on later ones
0-100 m (clear water) 0-5 m (coastal water)
Very low frequency EM waves Aerial (impractically Aerial radio
large for diver)
250 Underwater Acoustics
line, then this method is the obvious choice. However when the diver is not attached to any form of line a wire-less system is required.
The simplest method of wire-less communication is to amplify the voice of the diver and to transmit it through the water in such a way that the unaided human ear can be used for reception. This has been described by the term 'Direct Audio', and can be considered as the equivalent of an ordinary public-address system. The advantages of a direct audio trans-mission are that it is simple, and that a two-way conversation is possible without the need for a send/receive switch. On the debit side, the range is limited and the performance is dependent on the protective clothing that the diver is wearing around the ears.
The most popular method of transmission is to modulate an ultrasonic carrier. Ordinary amplitude modulation (AM), frequency modulation (FM)7, and single sideband AM systems are all in use, and there have been experiments with pulse modulation. The choice of modulation is governed by its ability to resist multiple-path distortion as this will be the predomi-nant interference in water depths of interest to divers. Frequency modula-tion has a natural resistance to the arrival of spurious multipath signals that are of small amplitude compared with the carrier, high-amplitude signals will cause serious distortion. In contrast an AM system will demodulate all the incoming signals and present the listener with the reverberation. The ear is not unaccustomed to dealing with a reverberant signal and there will be further discussion on this later.
Although some of the first communication sets to be marketed used the electric conduction field set-up by two spaced electrodes on the back of the diver, and electric fields are generated by certain fish, this method is less popular today. The advantage of being free from acoustic limit•itions was offset by limited range and 'dead spots' when the transmitting and receiving `aerials' were at right angles. The introduction of high-frequency carrier conduction field sets with automatic gain control at the receiver may well revive this technique.
8.4 Reception and noise Diver-to-diver communication is inherently a short-range technique.
Range limitations that arc due to poor signal to ambient noise (sea-state. thermal, biological, etc.) raiio are seldom of interest: it is, in general, the acoustic noise locally aenerated at the receiving end that limits the per-formance of these diver commurication systems. The major part of this often stems from the breathing equipment; however, this may not present as serious a problem as might be .-:xpected from acoustic measurements. Because the act of breathing may often completely mask incoming signals,
Audio Conummication between Free Divers 25T
it may be necessary to synchronize the breathing of the two divers; this is often helped by the valve noise received from the other party. With most forms of face mask the auditory canals are closed by the presence of water. This condition gives rise to an improvement in hearing by bone conduction of between 15 to 20 dB8. The effect of this is that body movement sounds and noise generated by movement of clothing and equipment on the body become significant underwater. A simple example of this is the effect of scratching the back of the head; in air this is unlikely to interfere with normal hearing, whereas underwater, this may well mask voice com-munication.
Table 8.3 Threshold of hearing underwater Comparison of published figures at 2 kHz. The divers are not wearing a
protective rubber helmet.
Reference Value of threshold Authors no. ref. 0.0002 dyne
Ide, J. M. (1944) (obtained from Ref. 15) 73 dB Hamilton, P. M. (1957) 14 53 dB Wainwright, W. N. (1958) 15 82 dB Montague, W. E., and Strickland, J. F. (1961) 10 70-80 dB Brandt, J. F., and Hollien, H. (1967) 16 60-70 dB Zwislocki, J. (1957) 17 Threshold of bone
conduction hearing in air 46 dB
There is a further source of noise that is significant in air-filled structures such as sea-bed shelters and underwater laboratories. By nature of their design these structures are often highly reverberant: this is particularly so in the case of some of the smaller flexible ones. Here the 'Q' of the gas-filled shelter may approach that of a bell on land. Excess gas is often continuously vented from such structures and the release of bubbles appears to be an excellent way of exciting their resonances. The noise level inside can be sufficient to mask telephone conversation and to cause considerable annoyance to the occupants.
8.4.1 Hearing The diver must equalize the pressure across the ear 'drum. This is nor-
mally accomplished by allowing the outer ear to be open to the water or helmet cavity, and equalizing the pressure in the middle ear by opening the eustachian tubes periodically (swallowing or blowing into the nose are the usual methods). Failure to equalize properly can cause loss in hearing sensitivity, and complete neglect can lead to rupture of the drums.
252 Underwater Acoustics
Normally when man swims under water there is a bubble of air trapped in the auditory canal; the part played by this is uncertain, as is the mechanism by which sound reaches the inner ear. It is possible that almost all hearing underwater is by bone conduction, and the similarity between the under-water and bone-conduction hearing thresholds have been used as evidence to support this". Alternatively the sound may be transmitted along the auditory canal in much the same way as in air; the ability to hear direc-tional information is the evidence in this case. Table 8.3 compares some of the published values for underwater thresholds at 2 kHz. The author is of the opinion that it is not meaningful to consider underwater hearing as either tympanic or bone conduction. These terms apply to hearing in air where the difference in acoustic properties between human flesh-and-bone, and gas, is such that one can consider a sound wave as travelling in one or the other. Under water it may well be more reasonable to consider the two hearing organs as existing in an infinite fluid. This ignores the water—skin boundary completely. The middle ear, sinuses and other air cavities are impedance discontinuities in the neighbourhood of the receptors and must b3 taken into account. Directional hearing and changes in angular sensi-tivity can be explained while retaining some similarity with bone conduction hearing.
It is possible for an underwater swimmer to tell the direction from which a sound originates11. This suggests that there may be considerable advantages to be gained in underwater communication by using a binaural receiver, as it is well known that binaural hearing in air helps in the under-standing of a speech signal in the presence of noise or reverberation. This ability is well demonstrated by the 'cocktail-party effect'12 ; a person `listening' to several conversations simultaneously has little difficulty in isolating them and understanding whichever one he pleases. This ability, however, is lost when listening to a monophonic recording of the same sound.
Although the use of binaural receivers has been proposed before13, the only available equipment that allows binaural reception are the 'direct audio' communicators. These, of course, use (both) the unaided ear(s) for reception. It has been reported3 that under some conditions communica-ion is possible with 'direct audio' equipment where carrier systems failed
due to reverberation.
8.5 Conclusions In the experience of the author, most equipment designed for diver com-
munication fails through mechanical design, either leakage (poor seal design), corrosion (wrong combinations of metals), or it encumbers the
Audio Con►nunicution between Free .111ver:s 253
diver (bouyancy index, physical size or method of operation). Having eliminated equipment that cannot be reliably tested outside the laboratory, the second most common failing is in the acoustic design at the transmitting end: facemask, microphone and breathing set.
It is only after the mechanical and acoustic design has been considered need one look to the ability of the equipment to transmit the voice signal through the water; yet it is in this area where almost all the evaluation of devices is done. It is of little wonder that a communicator that performs well in the test tank all too often fails to be of practical worth.
Although it has been emphasized that the whole of the diver's equipment must be designed with communication in mind, it is, of course, well known that the human brain is very tolerant to a considerable amount of speech distortion. It is possible to make up for a weak link in the communication chain providing all the other links are capable of passing on the distortion produced in order to give the person at the receiving end the best chance of. 'reading' the signal. For this reason, helium speech plus a pair of wires, a poor facemask plus a pair of wires, or a person speaking in air with a poor submarine telephone, may all be intelligible. Whereas combinations that use two weak links are seldom useful; their encumberance generally out-weighs any communication advantages they may offer the diver.
References I O.N.R. Report A.C.R.-124, Project Sealab Report, p. 143, 'Swimmer
Inrercom' Office of Naval Research, Washington, D.C. (1967). 2 Stover, W. R., 'Technique for Correcting Helium Speech Distortion'.
J. Acoust. Soc. Am. 41, 70 (1967). 3 Ray, B., 'Voice Communication Between Divers', Underwater Association
Report 1966-67, p. 47, Iliffe, London. 4 Stephens, R. W. B., and A. E. Bates, Acoustics and Vibrational Physics 15, 18,
Arnold, London, 1966. 5 Needy, K. K., 'Divers Communication Improved', Science 153, 321 (1966). 6 Webb, H. J., and J. R. Webb, 'An Underwater Audio Communicator',
Inst. Elec. Electron. Engrs Trans. AU-14, 127 (1966). 7 Gazey, B. K., and J. C. Morris, 'An Underwater Acoustic Telephone for
Free-swimming Divers'. Electron. Eng., June, 1964. 8 Zwislocki, J., 'Ear Protectors'. Handbook of Noise Control (C. M. Harris,
ed.), pp. 8-12, McGraw-Hill, New York, 1957. 9 Miles, S., Underwater Medicine, p. 85. Staples, London. 1966.
10 Montague, W. E., and J. F. Strickland, 'Sensitivity of the Water Immersed Ear to High- and Low-level Tones'. J. Acoust. Soc. Am. 33. 1376 (1961).
11 Ray, B., 'Communication Between Divers', Oceanology International 69 (Proceedings of the Society for Underwater Technology Conference) (1969).
12 Cherry, C., On Human Comnwnication, Ch. 7, 4.3, Science Editions, New York, 1961.
254 Underwater Acoustics
13 Bauer, B. B., A. L. DiMattia and A. J. Rosenheck, 'Transmission of Directional Perception', but. Elec. Electron. Engrs Trans. AU-13, 5 (1965).
14
Hamilton, P. M., 'Underwater Hearing Thresholds', J. Acoust. Soc. Am. 29, 792 (1957).
15 Wainwright, W. N., 'On Comparison or Hearing Thresholds in Air and in Water', J. Acoust. Soc. Am. 30, 1025 (1958).
16 Brandt, J. F., and H. Hollien, 'Underwater Hearing Thresholds in Man', J. Acoust. Soc. Am. 42, 966 (1967).
17
Zwislocki, J., 'In Search of the Bone-conduction Threshold in a Free Sound Field', J. Acoust. Soc, Am. 29, 795 (1957).
Recommended