Speech communication under conditions of deafness or loud noise

SPEECH COMMUNICATION UNDER CONDITIONS OF DEAFNESS ORLOUD NOISE

By W. G. RADLEY, C.B.E., Ph.D., Member.(The paper was first received 6th June, and in revised form 25th August, 1947. // was read before a joint meeting of THE INSTITUTION andTHE PHYSICAL SOCIETY 4th December, 1947, the SCOTTISH CENTRE 21th January, the EAST MIDLAND CENTRE 10th February, the NORTH-EASTERN

RADIO AND MEASUREMENTS GROUP 16th February, the SOUTH MIDLAND CENTRE 15/ March, and the SOUTHERN CENTRE \Oth March, 1948.)

SUMMARYDeafness and loud noise have the same effect on the reception of

speech through telephone receivers: they increase the minimumdistinguishable speech signal. When either condition becomes severe,satisfactory intelligibility is attained only if the speech is reproducedvery loudly, and at times with intensities approaching those which giverise to a sensation of pain. This common feature is a reason forassociating in the paper two otherwise distinct experimental researches.In the first part the range of hearing loss for which hearing aids maybe useful is indicated, and their design then considered in a generalway. A theoretical study indicated that, when speech signals have tobe very loud in order to be distinguished, the greatest intelligibility canusually be expected if the amplification varies with frequency in a givenway. Experiments which confirmed this conclusion by tests with alarge number of deaf people are described,and the most useful amplifica-tion/frequency characteristic for a hearing aid is given. In the secondpart data are given concerning the noise existing in some armouredfighting vehicles, and the extent to which it may be excluded by ear-pads. The overall amplification/frequency characteristic of an inter-communication, or radiotelephone, system is not considered in suchdetail as is that for a hearing aid, but subjective tests are describedwhich showed that, for the best results, the response should have nosudden changes with frequency.

(1) INTRODUCTIONDuring the past twenty years much attention has been given to

subjective methods of assessing the value of speech transmissionsystems,1"4 it being argued that the transmission of intelligenceby speech involves mental reactions which, with our presentstate of knowledge, cannot be determined by any objectivemeasurement. The techniques of syllable, word and sentencearticulation tests are fairly well known to all telephone engineers,and the nearness of the results they yield to those obtained in theordinary everyday use of a telephone system has often beendiscussed. By means of subjective tests the degradation ofintelligibility resulting from such impairments as restriction of theband of frequencies transmitted, room noise or inadequatevolume of the received speech, are fairly well appreciated forcircuits of the quality encountered in the normal telephonenetwork.

When, however, the listener's hearing is impaired, or he is ina very noisy location, the intelligibility of speech transmitted overthe system is often low; satisfactory communication is attainedonly by making the speech power available to the listenersufficient to overcome his deafness or the masking noise. Thisnecessary use of high-power outputs from the telephone receiversis one reason for associating these two cases in the present paper.Another is that both have been studied during the past few yearsby committees which have had a number of common memberswhose work has elucidated many points concerned with thesuccessful reception of speech in difficult conditions. In parti-cular, experimental work carried out under their guidance hasserved to indicate the optimum transmission (amplification/fre-quency) characteristics for the two systems. It is the object of

Dr. Radley is at the Post Office Research Station.

this paper to put this work on record, for, although such charac-teristics may have been used before, there has been no publishedexperimental evidence of the reasons for their success.

(2) HEARING AIDS(2.1) The Extent and Magnitude of Deafness

According to a recent American statement one person in everyten suffers from some temporary or permanent loss of hearingsense. The proportion for whom the loss of hearing is suffi-ciently severe to make the use of electrical amplification necessaryis, however, only of the order of one in a hundred of the totalpopulation.

In the course of clinical investigations carried out on behalf ofthe Electro-Acoustics Committee of the Medical ResearchCouncil, three methods of assessing the degree of deafness weresystematically employed:—

(i) as the loss in sensitivity to pure tones, determined by a pure-toneaudiometer test and expressed as the average loss in decibelsfor tones over the frequency range 1 000-2 000 c/s,

(ii) as the amplification in decibels of speech sounds above the normallevel required to give 40 % word articulation, determined by aspeech audiometer test (this value was described as the speechdeafness),

(iii) as the social disability owing to the deafness. The latter is usuallyexpressed in terms of a grading due to BeasleyS and is deter-mined by interrogation:—Grade I—difficulty in hearing in theatre or church,Grade II—difficulty in hearing "ordinary" conversation,Grade III—difficulty in hearing loud conversation, or in

hearing telephone conversation with a good connection.

A conventional type of pure-tone audiometer was used forthe first method of test. For the second the subject was seatedwith his ear resting against a padded ring at a fixed distance froma loudspeaker. The word lists used for this test were reproducedfrom disc-recordings, through a good-quality system with a rangeof amplification sufficient to give reproduction at a level rangingfrom just below normal threshold to 110 db above it. It wasfound experimentally that 40% success for word articulationcorresponded to a sentence intelligibility of 90%; with wordarticulation scores below 40 % sentence intelligibility fell rapidly.The 40% level for word articulation was therefore adopted ascritical. Speech audiograms (A, B and C) for three subjectswhose deafness corresponded, respectively, to each of the threeBeasley gradings are shown in Fig. 1, and in every case the lossfor speech is taken as being represented by the displacement tothe left of the normal word-articulation curve measured at the40% level.

It will be noted that in the range of intensity levels covered bythis Figure, three are marked as corresponding to "loud con-versation," "ordinary conversation" and "church level." Thefirst two of these were determined partly by means of sound-level measurements. In particular, the level for ordinaryconversation was determined experimentally by substituting atest crew of speakers for reproductions from recordings througha loudspeaker. The speech audiometer used in this way in

201 ]

202 RADLEY: S P E E C H C O M M U N I C A T I O N U N D E R CONDITIONS O F D E A F N E S S OR L O U D N O I S E

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Fig. 1.—Speech audiograms for representative subjects.

conjunction with the speech-deafness chart makes possible anobjective evaluation of the Beasley grading of deaf subjects.

Some 200 subjects attending a clinic were tested in each of theways mentioned above. Certain relationships between their"pure tone deafness," "speech deafness" and Beasley grading,assessed objectively in the manner described, are given inTable 1.

In a few cases the subject's disability takes the form of a dis-tortion when pure tones are heard as noises or as complex tones;in other cases hearing losses are due to abnormalities of thecentral nervous system which prevent understanding of certainwords, although there is little loss of hearing acuity for pure tones.In all such cases little or no help can be obtained from the use ofa hearing aid.

(2.2) General Considerations relative to the Design of aHearing Aid

From the preceding section it will be apparent that hearing isnot ordinarily considered to be sufficiently impaired to requireassistance until the increase in the threshold of audibility for puretones is such that the loss of sensitivity for speech is of the orderof 30 db. For a person with normal hearing, the threshold ofaudibility approximates to the lower curve of Fig. 2. Unfor-tunately, there is seldom, if ever, any corresponding increase inthe level at which speech sounds produce a sensation of pain ordiscomfort; this level is indicated for pure tones by the uppercurve of Fig. 2. There is thus a physiological limit to themaximum acoustic power which a hearing aid should deliver.In practice this is seldom exceeded, as the design is madecheaper by decreasing the limit to the output of undistorted

Table 1

Beasley grading (objective assessment)

Grade I

Grade II

Grade III and higher grades

Speechdeafness

18-33 db

34-47 db

> 4 8 d b

No. ofsubjects

26

50

124

Average pure tone loss (1 000-2 000 c/s)

Number of subjects

<20db

4

1

—

20-40 db

18

25

21

40-60 db

4

24

71

60-80 db

—

—

29

> 80 db

—

—

3

It will be noted from this Table that 18 of the 26 subjects inGrade I (70%) come within the 20-40 db category of pure-toneaudiometric loss. The proportion of subjects in Grade i nfalling within this category of pure tone loss, although lower, isstill fairly considerable (17%). The fact that the individualscomposing the fraction, though much deafer in terms of socialdisability than those of Grade I, may be indistinguishable fromthe latter in terms of pure-tone loss, indicates that pure-toneaudiometer tests do not provide an entirely reliable index ofinability to hear speech.

The Beasley grading of the 200 subjects was also effected sub-jectively by interrogation. If the objective method of gradingby means of the speech audiometer be accepted as valid, itwould appear from Table 1 that difficulty in hearing ordinaryconversation is predictable in 87% of the subjects whereas only68 % admitted such difficulty. This difference is almost certainlya reflection of the tendency of deaf subjects to minimize theirdisability.

Loss of sensitivity is not usually uniform for sounds at allfrequencies. The majority of subjects, particularly elderlypersons, become very deaf to sounds at high frequencies whilecontinuing to hear those at lower frequencies relatively well. Amuch smaller proportion suffer from relative deafness to soundsin the middle of, or at the low-frequency end of the range ofauditory frequencies. Provided that they have sufficient residualhearing, all these subjects can be given assistance by a hearing aidconsisting of a microphone, amplifier with frequency-selectivegain, and telephone receiver.

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Fig. 2.—Equivalent-loudness contours.

speech power. It must be remembered, however, that a hearingaid which provides adequate protection to the user by limitationof the maximum output will, when it is also giving considerableamplification, frequently sound noisy. This is due to the factthat, although both speech and background room-noise areamplified, the speech components alone will usually be limited tothe general deterioration of the signal/noise ratio.

In practice, leakage of sound from the telephone receiver backto the microphone may give rise to howling. This sets a furtherlimit to the acoustic gain which can be provided. Owing to themuch better .seal which the insert-type receiver makes with theear, hearing aids using them can be made to give more amplifica-tion than those using flat, external receivers.

RADLEY: SPEECH COMMUNICATION UNDER CONDITIONS OF DEAFNESS OR LOUD NOISE 203

The amplification provided by all British and American hearing-aids examined has been found to vary considerably with thefrequency of the input sound, and with many it has varied widelyfor comparatively small changes in this frequency. The bandof frequencies effectively amplified has been usually restricted to500-3 000 c/s, and distortion, due to limited power-handlingcapacity of the output stage of the amplifier, has been evidentin all cases when outputs were below the pain level. All themore powerful hearing aids have been liable to howl when thetelephone receiver has not been firmly held to, or in, the ear.

(2.3) Theoretical Consideration of the Optimum PerformanceCharacteristics for a Hearing Aid

In 1944 the Medical Research Council appointed a committeeto advise upon the design, performance and application of electro-acoustic equipment used in the investigation and alleviation ofdeafness, and to institute such fundamental investigations aswere considered necessary in this connection.8 One of the firsttasks undertaken by this committee was the development andspecification of a standard hearing aid which could be readilymanufactured. A necessary preliminary to this was the deter-mination of the performance characteristics which should makethe hearing aid of the greatest service to the majority of deafpeople.

It will have been gathered from what has preceded that com-plete restoration of any considerable hearing loss, to the extentthat full sensitivity is obtained for the total range of intensityand frequency discernible by a person with normal hearing, istheoretically impossible. The greatest inconvenience and losssuffered by deaf people, however, undoubtedly arises from theirinability to hear and understand speech. Their inability to hearproperly and appreciate other sounds, such as music, is ofrelatively less importance to their normal working and social life.Determination of the optimum performance characteristic for ahearing aid is therefore narrowed, in the first place, to that whichwill result in its giving the greatest intelligibility for speech.

It has been proposed that the amplification provided by ahearing aid should vary with frequency in such a way that theuser's loss of hearing is cancelled at all frequencies. "Fitting"of the hearing aid to the audiogram showing his threshold ofhearing for sounds at different frequencies is usually suggested.Recent experiments have, however, indicated that such a simpleconception is not generally applicable.6-7 An American report,seen only after the British work had been completed, summarizesone objection to it by pointing out that listening is usually atlevels well above the threshold and where the subject's responseis less abnormal. Another suggestion has been to the effect thatit is an equal-loudness contour (Fig. 2) at the level of comfort-able listening, and not at the threshold of hearing, that shouldbe fitted."*

The foregoing type of argument is based on the belief that thetonal quality of the reproduced speech should resemble thatheard by a person of normal hearing; it will carry weight only ifthe deafness is so slight that good articulation is obtained withreproduced speech volumes that are not uncomfortably loud.It will not be found to apply when the hearing loss is serious.

In the Appendix there is given in outline a theoretical studywhich was made for the committee of the Medical ResearchCouncil. In this study it is assumed that a mixture of sounds,such as speech, will cause pain or discomfort when the averagepower exceeds a definite amount. The problem is to find thefrequency-distribution of the permissible power to give thehighest articulation-efficiency, and to find the overall amplifica-tion/frequency characteristic of the system to attain this end. Inthe solution of this problem, use was made of the availableknowledge concerning the contribution to the total articulation-

efficiency made by a frequency band of given width, and con-cerning the variation of this contribution with the place of theband within the audio-frequency spectrum. The results of thestudy are given by Figs. 12, 13 and 15. It will be seen that formost cases of fairly severe deafness it is advantageous for theamplification to decrease progressively with decrease of frequencybelow 1 000 c/s.

The general conclusion reached, that comparatively littleenergy should be expended at those frequencies which are notcapable of contributing greatly to the intelligibility of speech, isof considerable importance to all conditions where communica-tion is possible only with very loud signals.

(2.4) Experimental Confirmation of the Optimum PerformanceCharacteristics for a Hearing Aid

The committee decided that their final recommendation as tothe most useful amplification/frequency characteristic for ahearing aid should, however, result from experiment. For thispurpose the examination of patients at two clinics* was supple-mented by experiments with high-quality speech-reproducingsystems which were built at the Post Office Research Station.The latter were designed to give the most perfect reproductionpossible from high-quality telephone receivers worn by thesubjects under test. Facilities were incorporated for introducingselective amplification of a kind likely to benefit a variety of deafsubjects. The experiments were supplemented as far as possibleby tests on commercially available hearing-aids of British andAmerican manufacture.

The many technical and psychological difficulties presented bysubjective tests of this kind are well known to all telephoneengineers, and these difficulties were not lessened in the presentcase when, instead of the tests being made by trained crews, theyhad to be made by deaf persons who had no previous experienceof this kind. Therefore changes due to the deaf persons becom-ing more practised as the test proceeded had to be carefullywatched, and the tests so arranged as to eliminate these and otherunwanted effects. It therefore became desirable that the testsshould be made with a large number of persons, in addition totheir being comprehensive in the inclusion of persons with varyingdegrees of deafness.

In order to control the spread of results as far as possible, itwas decided to substitute recorded speech for the directly spokenword throughout the tests. Records were therefore made by twomale and two female speakers, chosen after several auditions soas to cover a fairly wide range of voice pitch. Each record com-prised 50 familiar English single-syllable words, 25 spoken by amale and 25 by a female, recorded at intervals of four seconds.Before the records were used they were carefully tested forequality of difficulty by a trained crew over a range of low speechlevels, so that the percentage of words heard correctly was ofthe same order as those heard correctly by a deaf person.

Since the quantity to be measured was the ability of the subjectto understand spoken words as received through the speech-reproducing system, and since such ability is a function ofapparent loudness as well as the effective amplification/frequencycharacteristic of the transmission system, a decision had to bemade regarding the loudness level at which the amplified soundsshould be presented to the deaf persons. After consideration ofother methods, it was decided that each subject should choosefor himself, and for each condition of test, what he consideredto be the best loudness. This he did while a special record, con-taining words of the kind used in the subsequent test, but spokenat much shorter intervals, was reproduced to him. In additionto the tests carried out at the chosen level, other tests were usually

* Department of Education of the Deaf, Manchester University; and OtologJcalResearch Unit, National Hospital, Queen Square, London.

204 RADLEY: SPEECH COMMUNICATION UNDER CONDITIONS OF DEAFNESS OR LOUD NOISE

made with the speech level above and below the chosen level.This was done to provide for a mistaken choice by the deafperson. It was considered that in adopting the loudness levelselected by the subject himself, conditions were brought as nearas possible to those of a hearing aid in use; the results could alsobe more readily compared and used in studying the furtherproblem of relating the individual optimum to the individualdeafness characteristics.

The first series of tests was designed to determine the best shapefor the low-frequency end of the amplification/frequency charac-teristic. For this purpose tests were made with the response ofthe system substantially uniform throughout and also with it cutat the low-frequency end in various ways. Subsequently, furthertests were carried out to determine the extent to which theresponse of the system could be restricted at the high-frequencyend without serious loss of intelligibility. At a still later stage ofthe work, observations were made of the effect of powerlimitation.

The tests were carried out upon 63 subjects at one clinic and165 at the other. Among the 228 subjects examined, the deafnessvaried from a slight impairment of hearing to one showing anaudiometric loss of some 90 db as averaged over the frequencyrange 500-4 000 c/s. The subjects tested could be arbitrarilyclassified in a variety of ways of which the simplest were:—

(a) those whose hearing loss did not average more than 45 db overthe frequency range 500-4 000 c/s, and

(b) those whose hearing loss was greater than this value.Generally speaking, the subjects in category (b) were also thosewith a rapidly increasing loss at the higher frequencies.

The experimental work just described showed that a small,but quite definite, improvement in intelligibility was obtainedfor most subjects in category (b) when the amplification wasdecreased for frequencies below 750 c/s at a rate of approximately12 db per octave. This confirmed the conclusions reached as aresult of the earlier theoretical study and given in the Appendix.For the less-deaf subjects no significant change in intelligibilitywas apparent between a condition when the amplification wasrestricted in this way and one in which it was uniform down toquite low frequencies. The tests also showed that the amplifica-tion provided for frequencies above 4 000 c/s could be reduced ata rate of at least 18 db per octave. In order to obtain the bestresults, the amplification for frequencies 750-4 000 c/s shouldeither be uniform, or increase smoothly with frequency to a peakvalue at 4 000 c/s, which is 12 db greater than the amplificationat 750 c/s. It did not seem that any practical advantage wouldarise from the provision of an intermediate setting in hearing-aidequipment. An analysis of the results also failed to indicate thatany advantage would be gained by departing from the amplifica-tion/frequency characteristics given above for subjects having ahearing loss varying widely with frequency. As a further resultof the experimental work it appeared that a hearing aid should becapable of providing an overall acoustic amplification of at least40 db, and that the input and output sound pressures shouldremain proportional to one another up to an output of 200dynes/cm2. These measurements should be made at a frequencyof 750 c/s.

(2.5) Realization of the Optimum Performance Characteristic ina Practical Hearing Aid

It has been demonstrated that the performance characteristicsgiven in the preceding two Sections can be achieved in a practicalhearing aid using, for example, a crystal microphone and eitheran insert crystal receiver or a flat external receiver of an electro-magnetic type.

Fig. 3(o), taken from the report of the committee, showsthe objectively measured characteristics of a hearing aid con-

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good results. \ (b) American commercial aid (insert receiver).Clinical tests gave / (c) British commercial aid (external receiver).

fair results. \ (d) British commercial aid (external receiver).Clinical tests gave / (<?) British commercial aid (external receiver).

poor results. \ (/) British commercial aid (external receiver).

structed experimentally to have this performance.8 The dottedlines enclosing the curve represent suggested specificationtolerances necessary to make provision for the variations incomponents (particularly microphones and receivers) used inconstructing commercial hearing aids. With the microphonesand receivers that are available at present, small resonances maybe unavoidable, but any large and sudden change in amplificationcannot occur without seriously affecting the usefulness of thehearing aid. Fig. 3, (b) to (/), shows the characteristics, ob-jectively measured in the same way, of a number of representativeand commercially available British and American hearing aids.The clinical results obtained with these aids have been indicated,as they are instructive.

With few exceptions, the British aids available to the endof 1945 employed telephone receivers of the flat external typewith a resonant diaphragm, though limited use has also beenmade of external crystal receivers; the few insert receivers havebeen of the moving-iron type. Modern American hearing aids,on the other hand, make wide use of insert receivers of boththe crystal and moving-iron types. Bone-conduction receiversare used to a certain extent. However, even when the ear isseverely diseased or damaged, they rarely give better results thanthe more usual types which are intended to transmit sound byair vibration rather than directly by mechanical vibration.Crystal microphones are employed in practically all modernhearing aids having one or more stages of valve amplification.

(3) RECEPTION OF SPEECH IN VERY NOISY LOCATIONS(3.1) Nature and Magnitude of the Problem

The second problem arises in locations where airborne noiseis so loud that speech sounds of normal intensity become indis-


tinguishable even to a listener with good hearing. This problembecame of urgent practical importance a few years ago, modernforms of warfare having resulted in the development of armouredfighting vehicles and aircraft in which both telephonic inter-communication and radio communication are made difficult byvery loud noise picked up by the speaker's microphone orpenetrating to the listener's ears despite closely fitting earphones.

In order to obtain representative data on the magnitude of thenoise, measurements were made in a number of British armouredfighting vehicles11 in use in 1941 when the vehicles were travellingover a variety of road surfaces, at various speeds, and utilizingdifferent gears, as most likely to occur in practice. As some ofthe conditions could be maintained only for comparatively shortperiods and, in any case, made subjective comparisons impossible,objective measurements were made. The sound-level meter usedconformed closely to the specification for such instrumentswhich was proposed by the American Standards Association^ in1936 and recommended for adoption by the European AdvisoryCommittee on International Telephony (C.C.I.F.) in 1938; itsresponse varied with frequency in accordance with the 70-dbequal-loudness contour of Fig. 2. Measurements were madein the turret at the level of the commander's head, and closeto the driver's head. Armoured fighting vehicles normallytravel with covers open and fight with them closed; measurementswere therefore made for both conditions. In all cases the fullcomplement of personnel was present in the vehicle; this isimportant since the noise level is dependent on the amount ofacoustic absorption provided by members of the crew. Theresults are given in Table 2 and are expressed as decibels above

(3.2) Means of Improving the Signal/Noise RatioA committee was set up in 1941 to advise the Ministry of

Supply on research and development of microphones and tele-phone receivers for military purposes. This committee paidconsiderable attention to instruments to be used in noisylocations.

(3.2.1) Means of excluding Airborne Noise from the Listener's Ear.Different types of closely fitting pad surrounding the telephone

receiver and closing the gap between it and the head have beendeveloped. Some of the better ones shown in Fig. 4 were madethe subject of test.u The extent to which external, airbornesound was excluded by these ear-pads was measured in twoways.

(a) The observer wore the padded telephone receivers, in whicha series of pure tones was generated, and determined the minimumaudible level for each tone. This was done (i) when he was in asilent room and (ii) in ambient noise at a level of 125 db andsimilar in composition to that existing in an armoured fightingvehicle. The average shift of threshold between the two deter-minations for a selection of the ear-pads illustrated in Fig. 4 isshown in Fig. 5. It will be observed that tones at frequenciesaround 500 c/s had to be increased in level by 80 or 90 db inorder to make them audible above the noise penetrating theear-pads, i.e. at these frequencies the attenuation to external noisewas small.

(6) The observer sat in a non-reflecting room at some distancefrom a loudspeaker generating test tones. The minimum levelof each tone which was just audible was determined (i) when

Table 2

Type of vehicle

Tnfantry Tank Mk. II (Matilda) ..Infantry Tank Mk. HI (Valentine)Infantry Tank Mk. IV (Churchill)Cruiser Tank Mk. V (Covenanter)Cruiser Tank Mk. VI (Crusader)..Light Tank Mk. VII (Tetrach) ..

measurementswere made,

101010221520

Noise measured, db

' In turret

Open

113111114114.112100

Closed

112105119113120t102

In driver's compartment

Open

117106116110110104

Closed

119*112120113117114

Chief source of noise

None can be singled outExhaust and gearTrackGear and air compressorExhaust and gearGear and exhaust

• At 2 m.p.h. in bottom gear,t At 17 m.p.h. in top gear.

the generally agreed reference sound-intensity level for noisemeasurements, which is 0 • 0002 dyne/cm2. They have not beenexpressed as phons as this would have implied a subjective methodof test.

Similar measurements made in typical aircraft would have givenreadings of the order of 125 db in single-engine fighters and of theorder of 115 db in multi-engine bombers. In order that.the magni-tude of the noise may be appreciated, Table 3 gives noise levelsthat are commonly encountered in some familiar surroundings.

Table 3

Locution or source of noise Average level, db

Aero engine at 20 ftPneumatic drill at 10 ftTrain in tunnelMotor bus, inside noiseCity office with windows closedQuiet suburban gardenRustle of leaves in very gentle breeze .

1201009570603010

not wearing the padded telephone receivers and (ii) when wearingthem. The sound exclusion afforded by the ear-pad was thenexpressed as the mean shift of threshold when measurementswere made in four frequency bands, each approximately anoctave wide, between 200 and 4 000 c/s. Measured in this waythe rubber ear-pad [Fig. 4(b)] gave 14 db attenuation to externalnoise; approximately the same attenuation could be obtainedwith the velvet-faced pad [Fig. 4(«)], but only with much morepressure and after careful adjustment. Values of 10 and 11 dbhave been quoted for the American pad shown in Fig. 4(J), butup to 21 db has been obtained with that shown in Fig. 4(r). Thelast consists of a "dough-nut" of chamois leather filled withkapok. This ear-pad was used by the R.A.F. and assembled aspart of a closed helmet.

Even if perfection in the fitting of an ear-pad with very greatattenuation were attained, there would be considerable penetra-tion of noise to the ear by bone conduction. This cannot beeliminated. In practice, however, it is extremely difficult toapproach this limit; in particular, attenuation at low frequenciesis very small.


Fig. 4.—Typical telephone receiver ear-pads.(«) Sorbo-rubber covered with velvet.(b) Rubber.

(e) Rubber with hollow rubber rim.

(c) Chamois, filled with kapok.(d) Rubber with Sorbo-rubber rim.

db50

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70

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X XK x *

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200 5000500 1000 2000Frequency,c/s

lrig. 5.—External noise penetrating ear-pads.Ear-pads (a) and (fo) [(</) with increased pressure].

— • Ear-pad («•).•• :•; x Ear-pads (d) and (<>).

(3.2.2) Means of reducing the Noise entering the Circuit through theMicrophone.

If the speaker, as well as the listener, is in a noisy location,unwanted sound entering the mouthpiece of the microphone maymask the speech signal and thus degrade the intelligibility of thetelephone circuit still further. Satisfactory use then depends onthe degree to which the noise can be excluded from the micro-phone by its mouthpiece, and to which the former can berendered insensitive to components of frequency dominant inthe noise. A microphone fitted to a face-piece and worn overthe lip has been adopted by the American Army. This micro-phone was designed as a pressure-gradient type, and therefore todiscriminate in favour of speech from the closely adjacentmouth against noise from a more distant source. This purposeappears to have been achieved to a considerable extent, althoughmeasurements of the amount of the discrimination have notbeen made in this country except subjectively as regards theeffect on speech transmission in loud noise.

With carbon microphones, non-linearity enters to spoil furtherthe performance in loud noise. As a result of this non-linearity,carbon microphones not only reproduce the signal and the noise

which are present in the mouthpiece, but also generate additionaltones (intermodulation products) which fall within the samefrequency band as the useful ones; in particular, tones aregenerated which correspond to modulation of the speech by thenoise, and these greatly interfere with the recognition of the signal.In addition, carbon microphones saturate when the total sound-input exceeds a definite value, so that an increase of noise levelmay cause an actual drop in the level of the reproduced signal.As a general rule, the normal inset-type of carbon microphoneshould not be used in locations where the noise level exceeds90 db.

Throat microphones (sometimes called laryngophones) whichoperate by contact with the soft part of the throat on either sideof the larynx have often been used in noisy locations. Theyexclude most of the airborne sound (and leave the hands free), buteven the best types do not give such good quality as many goodmouth microphones. This is due to the fact that sounds presentin the mouth and throat are attenuated in passing through theflesh of the throat much more at the higher speech frequenciesthan at the lower. Other speech sounds formed by the lips andteeth, and those emanating from the nasal cavities, have to followa still longer path, with high attenuation before they can operatea throat microphone which, being in direct contact with thelarynx, is mostly subjected to large-amplitude low-frequencyvibrations.

(3.3) Experimental Determination of the Optimum PerformanceCharacteristics of Instruments and Circuits used in VeryLoud Noise

During the course of its work, the committee gave considerableattention to the problem of assessing the speech transmissionefficiency of a telephone circuit to be used for military purposes.It came to the conclusion that a method which expresses theresults in terms of the percentage of simple sentences of whichthe sense is correctly received, is the most meaningful measureof this property both to the specialist and to the non-specialistmind. A similar conclusion was reached in America, a NationalResearch Council report3® on articulation methods stating that"the results of tests made with the lists of sentences lead directlyto a prediction of the practical efficiency of a communicationsystem."


A form of sentence test known as "immediate appreciation" hasrecently been described.14* This is claimed to require com-paratively little time or training of the testing crew. It has beenused, therefore, for assessing military circuits in the field underworking conditions, but it would have been difficult to hold theconditions in an armoured fighting vehicle constant for sufficientlylong for certainty to be attached to the results of any field test.

In order to make subjective tests of the intercommunicationand radiotelephone systems under practical conditions possible,the noise in a type of armoured fighting vehicle of considerableimportance was measured, recorded and analysed when thevehicle was travelling at high speed with the turret and driver'scompartment closed. The record was then continuously re-produced in a laboratory but, owing to the considerable powerassociated with the low-frequency components, it was necessaryto supplement the loudspeaker and amplifier output by amechanical source of sound. By analysis over the whole soundspectrum, comparison with the measurements made in the vehicleand adjustment of the electrical and mechanical sources of sound,it was possible to reproduce in this laboratory a very close copyof the noise conditions in the particular vehicle.

A practised crew being available, the more usual form ofsubjective test employed by the Post Office and other telephoneadministrations was used. This consists of the determinationof the percentage of meaningless syllables, usually inserted in acarrier sentence, which are correctly received. The form of testis not inappropriate to the type of information usually passedover intercommunication and radio circuits when armouredfighting vehicles are in action.

Speakers and listeners were connected either by means of astandard intercommunication system or by means of the corre-sponding radio transmitter and receiver. In the latter case, thetransmitter and receiver were linked by a coaxial line andelectrical noise was added artificially, corresponding both asregards magnitude and the frequency of its components to theradio interference normally experienced on such circuits, so asto make the laboratory conditions completely realistic.

Speakers and listeners used the various microphones andtelephone receivers for which data were required; these wereinterchanged in such a way as to make their comparison possible.The noise conditions set up in the laboratory were so bad that,with transmission over the radio link, syllable articulation wasonly of the order of 10-15 % (corresponding to 38-55 % sentenceintelligibility). With similar instruments a corresponding figureover the intercommunication circuit was 28 %, the better resultbeing due to the absence of interference and distortion introducedin the radio link. Statistical analysis of the results as the testsproceeded gave confidence that they yielded reliable indicationsof significant differences between the various telephone instru-ments tested.

(34) Conclusions regarding the Optimum Performance Charac-teristics of Instruments and Circuits used in Very Loud NoiseThe experimental work described in the preceding Section led

to the conclusion that the overall response of the transmissionsystem, including the microphone and telephone receiver,should have no sudden change in efficiency over that part of thespeech-frequency range transmitted by the communication chan-nel. The reasons for this have since become more apparent as aresult of the theoretical study made later in connection with hear-ing aids, and given in the Appendix. With conditions such as existin armoured fighting vehicles, the minimum intensity which asignal must have in order that it can be heard is much increased.Fig. 6 illustrates this. The lower curve is that originally givenby Harvey Fletcher1 as representing the average individual's

• This method was first described by Grinsted.28

160

140

120

§100

| 80

.8 60

£ 4-0

20

0

-20

Threshold of hearing when wearing earphones— — in silence ' '—

0 100 200 300 1000 2000 5000 10000Frequency, c/5

Fig. 6.—Effect of noise on opt imum signal level.

threshold of hearing for pure tones. The upper curve wasderived experimentally from tests with a number of observersand shows the threshold, also for pure tones, when these aremasked by noise of the type and magnitude that penetrates tothe ear of a listener wearing padded earphones in an armouredfighting vehicle. The sound-intensity level at which pain, oran unpleasant sensation, is first experienced is not so clearlydefined. For pure tones it is in the region of 120-140 db.When listening to sounds, such as speech, containing componentsat a number of different frequencies, all of them contribute to theoccurrence of pain. An attempt has been made to indicate thisby the shaded area on Fig. 6. This leads to the conclusionthat the overall amplification of the system should not vary withfrequency in such a way as to cause those components whichdo not contribute greatly to the intelligibility of speech to bereproduced with enhanced intensity. If such peaks in theamplification/frequency characteristic tend to cause prematureover-loading of the ear, the signal level may be reduced inconsequence. This will result in deterioration of the signal/noiseratio, possibly to the extent that certain components may becomeinaudible. It follows also that, with very loud noise, adjustmentof the received-signal level is rather critical.

Similar studies were made in the U.S.A., notably in thePsycho-Acoustic Laboratory at Harvard University, and in theBell Telephone Laboratories; they give general support to theconclusions reached in this country.

(3.S) Realization of the Optimum Performance Characteristics inIntercommunication and Radio Systems

In practice the desirable overall response was most readilyreached by the use of moving-coil microphones and telephonereceivers of comparatively robust and cheap design. Thesetypes were adopted as standard for vise in British armouredfighting vehicles. It is possible that alternative types of tele-phone instrument might have given a performance approximatingmore closely to that desired, had the remaining components of theintercommunication and radio systems been designed for theoptimum overall response. In particular, a comparatively simpleand robust type of moving-iron unit has been used in Britishaircraft under somewhat comparable conditions of noise.

(4) CONCLUSIONSMuch still remains to be done in connection with the two major

problems discussed in the paper. More work is required onthe problem of enabling the partially deaf to hear speech.Research of a fundamental nature is necessary in order toestablish a correlation between certain objective measurements


and subjective impressions, especially when insert and bone-conduction receivers are used. The applications of the latterrequire further investigation. The work concerned with speechcommunication in very noisy locations was carried out hurriedlywith the immediate object of making a selection between thetelephone instruments then available. It revealed certain prin-ciples which must be observed in the design of a communicationsystem which is to be useful under the conditions of the problem.It is possible, however, that further study, along the lines of thetheoretical study made in the Appendix, of the optimum charac-teristics for a hearing aid would indicate means of obtainingsome further increase in the articulation efficiency of the systembefore the speech power becomes such as to cause pain.

What has been done already, however, has made clear certainprinciples which must be observed in the design of the two typesof speech-transmission system with which the paper is concerned,if they are to give the best results.

(5) ACKNOWLEDGMENTSThe author wishes to thank the Engineer-in-Chief of the

Post Office for permission to publish the paper; also the MedicalResearch Council and the Chief Scientist, Ministry of Supply.He also gratefully acknowledges the help he has received fromhis colleagues at the Post Office Research Station who havecarried out much of the experimental work, and from those otherengineers and physicists who have served with him on the twocommittees. He would especially mention his indebtednessto Mr. J. O. Ackroyd for the Appendix and to Dr. C. S. Hall-pike for the results of much of the clinical work using speechaudiometers.

(6) BIBLIOGRAPHY(1) FLETCHER, H.: "Speech and Hearing" (Van Nostrand, 1929).(2) FLETCHER, H., and STEINBERG, J. C : "Articulation Testing

Methods," Bell System Technical Journal, 1929, 8, p. 806.(3) Reports on Articulation Testing Methods: I. National

Research Council (U.S.A.), Committee on Sound Control(1942), II. Office of Scientific Research and Develop-ment, Psycho-Acoustic Laboratory, Harvard (1944).*

(4) Report on Intelligibility Measurement: Office of Scientific—Research and Development, National Defence Research

Committee, Psychological Corporation, New York(1944).*

(5) BEASLEY, W. C.: "The General Problem of Deafness in thePopulation," Laryngoscope, 1940, 50, p. 856.

(6) EWING, A. W. G., EWING, J. R., and LITTLER, T. S.:Medical Research Council, Special Report Series No. 219(1937).

(7) FOWLER, E. P.: Transactions of the American OtologicalSociety, 1936, 26, p. 275.

(8) "Hearing Aids and Audiometers," Medical ResearchCouncil, Special Report Series No. 263 (1947).

(9) FRY, D. B., and KERRIDGE, P. M. T.: "Tests for the Hearingof Speech by Deaf People," Lancet, 1939, 1, p. 106.

(10) KNUDSEN, V. O., and WATSON, N. A.: "Selective Amplifica-tion in Hearing Aids," Journal of the Acoustical Society ofAmerica, 1940, 11, p. 406.

(11) Research Report No. 11681 of the British Post Office:"Noise in Armoured Fighting Vehicles," January, 1942.*

(12) "American Tentative Standards for Noise Measurements andAmerican Tentative Standards for Sound Level Meters,"Journal of the Acoustical Society of America, 1936, 8,pp. 143 and 147.

(13) Research Report No. 11889 of the British Post Office:"Telephone Headgear Assembly for Armoured FightingVehicles," February, 1943.*

* These are restricted reports and not generally available.

(14) WIGAN, E. R., and RULE, F. T. (TO be published inPart III of the Journal)

(15) Research Report No. 11728 of the British Post Office:"Microphones and Receivers for Use with No. 19 WirelessSet," February, 1943.*

(16) FRENCH, N. R., and STEINBERG, J. C : "Factors Governingthe Intelligibility of Speech Sounds," Journal of theAcoustical Society of America, 1947, 19, p. 90.

(17) COLLARD, J.: "Calculation of the Articulation of a Tele-phone Circuit," Electrical Communication, 1930, 8, p. 141.

(18) DUNN, H. K., and WHITE, S. D.: "Statistical Measurementson Conversational Speech," Journal of the AcousticalSociety of America, 1940, 11, p. 278.

(19) FLETCHER, H.: "Auditory Patterns," Reviews of ModernPhysics, 1940,12, p. 47.

(20) HOTH, D. F.: "Room Noise Spectra at Subscriber's Tele-phone Locations," Journal of the Acoustical Society ofAmerica, 1941, 12, p. 499.

(21) LOYE, D. P., and MORGAN, K. F.: "Sound Picture Recordingand Reproducing Characteristics," Journal of the Societyof Motion Picture Engineers, 1939, 32, p. 631.

(22) POCOCK, L. C : 1938, unpublished.(23) POCOCK, L. C.: "Calculation of Articulation for Effective

Rating of Telephone Circuits," Electrical Communication,1939,18, p. 120.

(24) POCOCK, L. C : "Microphones and Receivers," Journal OJthe British Institute of Radio Engineers, 1943, Nos. 3-5,p. 197.

(25) Research Report No. 7605 of the British Post Office: "TheCorrelation of Articulation Results Obtained by DifferentTesting Crews and the Calculation of Articulation,"March, 1936.*

(26) SrviAN, L. J., and WHITE, S. D.: "Minimum Audible SoundFields," Journal of the Acoustical Society of America,1935, 4, p. 288.

(27) STEINBERG, J. C , MONTGOMERY, H. C , and GARDNER,M. B.: "Results of the World's Fair Hearing Tests,"ibid., 1940,12, p. 291.

(28) GRINSTED, W. H.: "The Statistical Assessment of Standardsof Telephone Transmission," Engineering Supplement tothe Siemens Magazine, 1937, No. 140, p. 1.

(7) APPENDIXTheoretical Consideration of the Optimum Frequency Response

Characteristic of a Hearing AidContributed by J. O. ACKROYD, M.A., B.Sc.(Eng.)

(Crown copyright reserved; reproduced with the permission of theController of H.M. Stationery Office.)

A subject having a severe hearing loss cannot hear speechwith perfect intelligibility even when assisted by a hearing aid,since the amplification used must be limited to avoid pain fromthe loudest sounds. As a result the faintest speech soundscannot be heard. The purpose of this Appendix is to determinethe best amplification/frequency characteristic for a hearing aidused by such a subject.

It is a convenient approximation to assume that the ability ofreproduced speech to cause pain or discomfort is determinedsolely by its average power, and that if the latter is fixed, theformer is unaffected by changes of the frequency characteristic.The pain threshold for pure tones is known to be substantiallyindependent of frequency over the audible range and even atsub-audible frequencies as low as 1 c/s. But the ability ofspeech to cause pain is probably determined by the highest valuereached by its power, averaged over a short period of time ofthe order of £ sec. The ratio of this quantity to the average

• These are restricted reports and not generally available.


power is indeed somewhat dependent on the frequency charac-teristic, but not to such an extent as. will seriously affect theconclusions found in this Appendix.*

(7.1) The Calculation of ArticulationIt has been shown by Collard17 that the "articulation

efficiency" of a system can be calculated from a knowledge ofits overall frequency/response characteristic, of the energyspectrum of the voice, and of the characteristics of the listeners'hearing. The result of the calculation cannot, of course, beexpected to agree with the result of a measurement using asingle testing crew, but it is usually found to fall well withinthe range of measurements obtained by different crews. It isfelt, therefore, to be justifiable to use the method to supply theanswer to a problem which would require a prohibitively greatnumber of measurements to solve experimentally, but which oncefound will be capable of experimental verification.

The spectrum can be divided into narrow frequency bands,each of which contributes its quota to the total "band articula-tion," B. (The "band articulation" defines the goodness of thesystem, and is related in a known way to the "word articulation,""syllable articulation" and "idea intelligibility.") The maximumpossible contribution of a given band is proportional to itswidth (in cycles per second), and depends on its location in thefrequency scale; for a narrow band of width 8/the maximum

contribution is —? 8/ = (say) B'hf. The variation of B' withdf

frequency is shown in Fig. 7 for English speech ;f it differs some-0

200 300400500 1000 2000 5000 10000Frequency, c/;s

Fig. 7.—Variation with frequency of the maximum contribution to theband articulation of a given band.

what for the two sexes. It can be seen that a band of givenwidth is more useful in the middle frequency range than at eitherhigh or low frequencies.

The contribution by a given band may be less than its maxi-mum possible value B'8f by a factor P. This is less than unitywhen some of the sounds in the band have inadequate energy tooverride the threshold of hearing (or the unwanted noise).Fig. 8 shows the dependence of P on the ratio of the intensity ofthe loudest sounds in a band to the intensity the band wouldpossess to be just audible. The spectrum level of noises whichare just audible is shown by the threshold curve of Fig. 9. (Thisshould not be confused with the hearing threshold to individualpure tones normally shown in textbooks.)

(7.2) The Derivation of Optimum Speech Spectrum Curves

Taking into account the factor P, the contribution to the total"band-articulation" by a frequency band of width 8 / is thus

• D u n n and White's give the value o f the power averaged over 1/8-sec intervalswhich is exceeded in 1 % o f such intervals; they also give the power averaged over anextended period. Both quantities were measured with and without a variety of filtersto divide the frequency range. From their data it can be shown that the ratio of thesetwo powers is changed to the extent o f only a few decibels, even when narrow band-passfilters are used. We may infer that n o appreciable change will occur when the fre-quency characteristics advocated here are used.

t The derivation o f this and other curves given here is indicated briefly at the end ofthe Appendix.

RpllLaboratorie(1942)

/

"A

//1/

vfost(

VKfioela xiralor cs

—

'Collard (1934

— —

—1-2

10

08P

Ob

04

02

00 10 20 3 O 4 O 5 0 G O 7 0 8 O 9 OLewi of loudest pealv.dbdbove threshold, ie IOIogB W/Wo

Fig. 8.—Actual contribution of a band as a fraction (P) of its maximumpossible contribution to the band articulation.

60

1 5 0440

\ kNIsr •— m

y1 1 1 1

20

NO 200 300400500 1000 2000Frequency, <#

5000 10000

Fig. 9.—Noise threshold of hearing: maximum spectrum power (percycle) of any noise of considerable bandwidth which is justinaudible.

PB'hf. Let WsSf be the average sound power in the band .Then for a fixed total power in all bands , it can be shown thatthe maximum total "band-ar t icula t ion" will be at tained when inevery band the power is such that the ra te of change of itscontr ibut ion with respect to its power is the same as for allthe o ther bands . I n other words, B'dP/dfVs should have thesame value at all frequencies. If in any band this value ofB'dP\dWs canno t be reached, n o power should be devoted toit. This will be seen to be t rue if we consider two individualbands, designated by subscripts 1 and 2. Suppose (B'dPldWs)x

< (B'dP/dlVs)2. Then the total "band-ar t icula t ion" can beincreased without alteration of the total power by increasing Wsl

with a corresponding decrease of WsX. I t is seen below thatdP/dWs falls as Ws is increased, so for the two bands we areconsidering it will be advantageous t o increase Wsl a t theexpense of WsX until (B'dP/dW^ = (B'dP/dWs)2. This givesthe op t imum condit ion for the two bands . (If equality is no tat tained however far Ws\ is reduced, it can with advantage be re-duced to zero.) The same argument can be applied to anynumber of bands . F o r all bands then, the maximum total"band-ar t icula t ion" is obtained if

„, dPB —— — constant

dW (1)

provided that this can be made so by adjustment of the bandpower. Let WQhf be the average power in a band of width 8/when it is only just audible. Then equation (1) can be written as

1 B>

W d(JVJWQ)= constant .

or 10 log10dP

d(WslWQ)

= 10 log10 Wo - 10 Iog 1 0 5 ' + constant

(2)

(3)


Fig. 8 gives the relation between P and WJWO; from this thecurve of Fig. 10 has been calculated, showing the relation

-10

I 5 0I26O

-80

-90

-1000 10 20 30 40 50 60

db above threshold, 10 lofo (W/Wo)Fig. 10.—Rate of change of the band articulation with respect to the

band power.

< +

Po

\

BeimLabora(1942)

rtQffice

/

fephonetones

itordtt

0f(t

\ X <

\

>ries

%Wl

R)cock/(1939)

©

<

\

\

between dPand (Ws/W0) plotted to logarithmic scales.

With the aid of this curve, equation (3) can be used to find theoptimum shape of the spectrum of the sounds heard for anyarbitrary value of B'dPjdWs.

The relation between . and WJWQ can be expressed

in the formdP fW\~a

= (—?) x constant . . . (4)

Substituting for - dPd(Ws/W0)

optimum condition is given by

from equation (4) in equation (3), the

10 log10 WQ + constantWs = - 10 Iog10£'

- . • . " . (5)

Since the curve of Fig. 10 is not a straight line, the quantitya is not strictly constant and the optimum spectrum-shape is tosome extent dependent on the arbitrary value of B'dP/dWschosen, i.e. on the sensation level of the received speech. Theproblem is simplified if attention is confined to levels not farabove the subject's threshold such as must perforce be used in

< cases of serious deafness. We are then concerned only with theupper part of the curve of Fig. 10 which can be seen to be nearlya straight line for sensation levels of 35 db or less. Equation (4)then holds approximately if the index a is made constant andequal to f .* Making this substitution, equation (5) becomes10 log10 Ws = i x 10 Iog10£' - i x 10 log1G Wo + constant (6)

This equation defines the optimum spectrum shape at thelistener's ear when the total sound power is limited, by the onsetof pain due to the speech, to a value too low for perfect articula-tion to be approached.

(7.3) The Optimum Frequency/Response CharacteristicEquation (6) defined the shape of the optimum spectrum in

terms of the quantities B' and Wo. B' was given in Fig. 7• Fig. 10 shows points calculated from two additional independent sources.^* n

The approximation a = i is in fair agreement with these also.

while the threshold WQ was shown in Fig. 9 for subjects havingnormal hearing. In cases where the hearing loss is uniform,the threshold curve will be parallel to this; for such cases we canimmediately determine the optimum frequency/response charac-teristic if we know the spectrum of the speaker's voice. Fig. 11

GO

.§•6 30

\b0NO 200 300400500 1000 2000 5000 10000

Frequency, c/sFig. 11.—Spectra of normal speech.

shows this energy spectrum for male and female speech and is dueto Morgan and Loye.21

The result is shown in Fig. 12. This is the ideal overall

•

I I I !

Female

V **•-

M M

1-40

Feiale/

1 1 1 1 1 1

\

\

1 1 1 I

KJO 200 500400500 1000 2000Freqjuency,.0

5000 10000

Fig. 12.—Frequency characteristic for maximum intelligibility whenseverely limited by hearing loss (no room noise).

frequency/response characteristic for a hearing aid to be used bya subject having a uniform hearing loss large enough to make theonset of pain prevent a high degree of intelligibility.

It is interesting to observe that considerable differences in theshapes of the male and female spectra have been largely offset bydifferences in the corresponding B' curves, so the final resultsare very similar for either sex or speaker. It is to be expectedthat the precise shape of the curves would be different forabnormally loud or quiet speech where the spectra are known todiffer from the normal curves of Fig. 11. There are no datahowever for the B' curve under these conditions; it is to besupposed that the differences would tend to cancel, as was foundin the comparison between male and female speech.

So far we have confined our attention to the case of a listenerhaving a uniform hearing loss relative to the average of thepopulation. Equation (6) supplies the answer to the question:what modification to the frequency characteristic should be madein the case of a listener whose hearing loss depends on frequency?For example, suppose his hearing loss is independent of frequencybelow 1 000 c/s, but at higher frequencies increases linearly sothat the loss is 30 db greater at 4 000 c/s. By equation (6) theoptimum frequency characteristic will be lower by one-third of30 db at 4 000 c/s as shown by the dotted curves in Fig. 13. Inother words this equation states: most energy should be devotedto the frequencies of greatest importance; only those contributinga considerable amount to the intelligibility justify an increase oftheir power with a corresponding diminishing return, and, if the


-IT10

S-n

1-ffl

.=-30•&>"p-40

Ferns

Uniform \

AMale

i i 1 1

earing l o s s . ; ^ ^ ^ ,

Hearing loss -^'- increasing above—1000c/s(by30dbat 4000 c/s) „

rarwMale

i i

\

Ws dP

Woconstant . (7)

200 300400500 1000 2000Frequency, c/s

5000 10000

Fig. 13.—Frequency characteristic for maximum intelligibility whenseverely limited by hearing loss (no room noise): modificationrequired for a particular case of non-uniform hearing loss.

ear sensitivity is reduced at a given frequency, it is less profitableto expend much power at that frequency.

A subject might have a uniform hearing loss below a certaincritical frequency, while for high frequencies he was totally deaf.In such a case it will be clear that the optimum characteristic willfollow the one already determined up to the critical frequency,while at higher frequencies it should drop sharply so that on thelatter no energy is wasted.

(7.4) Comparison with other Frequency CharacteristicsTo check the validity of the above conclusions, the articulation

efficiency was calculated for three different characteristics,assuming male speech. The first was the calculated optimum,and assuming a particular value of maximum permissible soundpower in relation to the hearing threshold, it gave a value of"band articulation" of 47-5% corresponding to a syllablearticulation of 65%. In the second case a level frequencycharacteristic was assumed; it gave a "band articulation" of26-4% and a syllable articulation of 38%. In the third case afrequency characteristic was assumed such that the spectrumpower (per cycle) was independent of frequency up to 4 800 c/sand above this frequency was zero; this led to band and syllablearticulations of 42 % and 59 % respectively. In each case thesame ratio of the maximum permissible sound power to thehearing threshold was assumed; the value chosen correspondedto a high degree of hearing loss in Beasley's third grade.

(7.5) The Optimum Characteristic in Room NoiseIn the presence of room noise the problem is more complicated.

The hearing threshold to speech may be modified by the noise,and the power (and hence the possibility of pain) may be in-creased. One special case is particularly worthy of considera-tion; we will assume that the room noise is such that whileconversation is being carried on the noise is insufficient tomodify the hearing threshold of the listener, but occasionalperiods of room noise occur of sufficiently high level to causediscomfort. The permissible amplification will thus be limitedby the room noise, and not by the speech.

It was shown in equation (1) that the maximum value of "bandarticulation" B'Pdf v/ill be given for a stipulated total power Wdfwhen at all possible frequencies

B'dP/dW = constant.

As the permissible power is now determined by Wn we have

B'dPldWn = constant

where Wn = spectrum level of the amplified room noise. Theratio Wn\Ws of noise to speech at a given frequency is beyondour control; it will not be affected by changes of amplification.So the condition can be written

But it has already been shown that, approximately,

dP( " y " w * c o n s t a n t -

x constant

So the required condition is

Ws =

101og10 Ws = ±x \0logl0(WJWn) + -5-— i x 10 log10

101og,05'+ constant (8)

Comparison of this equation with equation (6) for the quietroom condition shows that the frequency characteristic thenshown to be the best should be modified by addingf x \0\ogXQWjWn.

The result is therefore dependent on the spectrum of the roomnoise. Fortunately for our purpose, Hoth20 has shown that awide variety of room noises have very similar spectra. Hiscurve for the average room noise shown in Fig. 14 has been used

-40

1 1 1 1 ! 1

. . . .

\

l i l t

100 200 300400500 1000 2000 5000 KXWOFrequency, c/s

Fig. 14.—Spectrum of average room noise (Hoth).

to solve the present problem. (It is perhaps worth commentingthat noises as different in character as those found in the turretof a tank or in the cabin of an aeroplane have spectra quitesimilar to Hoth's curve over the important part of the frequencyrange.)

The resulting optimum frequency characteristics are shown inFig. 15. It will be seen that the curves are generally similar tothose found for the case where no noise was present.

i+IO

-g -10

| - 2 0

•1,-30

Me

/.

'

//

A nvle

I I 1 1 i i i i 1 1

200 300400500 1000 2000Frequency, c/s

5000 10000

Fig. 15.—Frequency characteristic for maximum intelligibility whenseverely limited by hearing loss (occasional room noise). *

(7.6) Conclusions

The op t imum characteristic was shown in Fig. 12 for caseshaving uniform hear ing loss. When the hear ing loss is no tuniform, it has been shown tha t the gain should be reduced by one-third of the increase in hearing loss, at the appropr ia te frequencies.

Startling changes of the overall articulation efficiency will no t


result from minor departures from the optimum response curve.It is one of the advantages of a position on the maximum of anycurve that small departures from the optimum position makeonly minute changes in its height. Furthermore, it emerged inthe calculations (assuming the optimum curve) that both thepower and the "band articulation" are trivial in frequenciesoutside the limits 500-5 000 c/s, so that considerable liberties maybe taken with the curve outside these limits without appreciablychanging the total power or the articulation efficiency.

It will be appreciated that data used in this Appendix wereobtained from tests on subjects having normal hearing. Incases of nerve deafness or other defects of the inner ear there isless justification for using them than in cases of conductiondeafness; the conclusions, however, are in the main confirmedby the articulation tests (described in the body of the paper) whichwere made on subjects having both types of deafness.

(7.7) Derivation of the Constants

The relative importance of different frequencies is given byB'\ the maximum contribution to the "band articulation" of anarrow band of width 8/ is B"6f. The value of B' at eachfrequency is shown in Fig. 7. (For convenience in using thedata, the quantity plotted in this curve is actually 10 logi0i?'.)

It is not proposed to discuss here the method of obtainingthese curves; their derivation has been described in the literature(Collard," 1930, and Pocock,23 1939). The curve for malespeech is the unweighted average of three British determinations(Post Office Research Station^ 1937, and Pocock, 1938 and1939) from measurements made with syllables compounded ofEnglish speech sounds; these three determinations are in reason-able mutual agreement. The curve for female speech is theaverage of two determinations (Collard, 1930, and Pocock, 1938and 1939).

The factor P shown in Fig. 8 gives the reduction of the "bandarticulation" in any band if its energy is inadequate to make allits sounds heard. The solid curve obtained at the Post OfficeResearch Station has been used for the present calculations, for

the reason that it has been found to give the best agreementbetween calculated and measured articulations for a variety ofconditions, many of them using entirely different crews fromthose used to obtain the curve. The broken curves show datafrom two independent sources, namely Collard, 1934 (publishedby Pocock,23 1939), and the Bell Telephone Laboratories, 1942(published by French and Steinberg1* in 1947). Despite thewide apparent differences between them, the curves of the

dPdifferential coefficients , /„ , ,„ , , plotted to logarithmic scales in

d{WIW0)Fig. 10 are quite similar in slope. It is only the slope of thesecurves which concerns us here.

The threshold curve of Fig. 9 has been derived by a somewhatindirect method. It shows the maximum spectrum level of anynoise covering a reasonable bandwidth which just cannot beheard by the average subject. Fletcher*? (1940) has shown thata continuous-spectrum noise will be at the threshold when theenergy measured in a "critical bandwidth" of the noise is equalto that of a pure tone at threshold. He was therefore able toconvert a threshold curve measured in pure tones into oneapplying to continuous-spectrum noises. Steinberg, Mont-gomery and Gardner2' measured a pure-tone threshold whichwas exceeded by 50% of subjects aged 20-29 years. This waspreferred to Fletcher's own threshold curve since a much greaternumber of subjects was used for the tests. Fletcher's data oncritical bandwidths were used to convert the pure-tone thresholdinto one applying to continuous spectrum noises. The data sofar considered have been in terms of free-field sound pressures.Now Sivian and White26 have found a considerable differencebetween the minimum audible sound-field and the minimumaudible sound-pressure when headgear receivers are used; thedifference is considerable at low as well as high frequencies.A correction has accordingly been applied since we are concernedhere with headphone reproduction.

The speech-spectrum curves shown in Fig. 11 are due toMorgan and Loye21 for speech of normal intensity. A Britishdetermination by Pocock2* is in quite close agreement.

DISCUSSION BEFORE A JOINT MEETING OF THE INSTITUTION AND THE PHYSICAL SOCIETY,4TH DECEMBER, 1947

Mr. H. L. Kirke: Deaf persons seem to feel their affliction farmore than blind people, and it is very difficult to persuadethem to wear some form of aid. If something could be done tohelp them overcome that difficulty, they would certainly be ableto benefit a great deal from the results of the work which theauthor has described, and it would be invaluable if this workwould result in the production of an efficient and comparativelycheap hearing-aid set. In addition to efficiency and cheapness,however, there are other qualities which are very important insuch a set; it must be reliable, and a supply of cheap and goodbatteries of long life must be assured.

Although it has hitherto been generally assumed that it wasnecessary to adjust the frequency characteristic of a hearing aidto suit each particular person, that does not in fact appear to benecessary, and it is indeed an advantage to reduce the amplifica-tion at low frequencies, at any rate so far as speech is concerned.Many deaf people, however, take great interest in music, and Iwonder whether any consideration has been given to their listeningto music by means of hearing aids and whether that in any wayaffects the frequency characteristic of the set.

The author points out a fact which is, I think, not commonlyrealized and about which mistaken views are often held, namelythat the curve for the threshold of audibility is not the curve fornormal hearing and that the two curves can be quite different.

Section 3 brings out the fact that noise is on the increase.Everywhere we go, we are experiencing increasingly louder

noises, and that is definitely one of our problems to-day. Thenoises in armoured fighting vehicles and in aircraft are certainlyalmost terrifying. I was interested in the author's remark aboutthe limiting condition of bone conduction, and I wonder whetherit is not possible to reduce the amount of sound which may beconveyed to the bone and thus to the ear.

I have heard it stated that if deaf persons would practise veryfrequently listening to, say, radio programmes or the gramo-phone, their hearing would improve, and I have been showncurves which are said to prove this. Is there any truth in thatassumption?

Dr. C. S. Hallpike: I suppose that it is now generally knownthat the Ministry of Health has not been slow to act upon theElectro-Acoustics Committee's recommendations; in fact, hear-ing aids are now being manufactured according to the Com-mittee's specifications, and it is proposed that these should bedistributed at ear clinics throughout the country as part of thenew Health Service. I believe that these aids will be exceedinglyvaluable for persons suffering from deafness due to disease of themiddle ear, but when the deafness is due to disease of the internalear the position is very much more difficult, both in theory andin practice, and, speaking as an otologist and a physiologist, thisaspect of the problem is very interesting indeed.

Among the fundamental constants of the ear which determineintelligibility are those concerned with frequency and intensitydiscrimination. These constants are governed by the internal

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Speech communication under conditions of deafness or loud noise