Presentation1

If the source vibrate continuously, a continuous sound is produced. In most

cases in ultrasound, the source vibrates briefly, producing a

pulse of sound, which travels through the tissue. After echoes are picked up, another pulse is

sent, and so on. 

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The material in this section gives a brief introduction to some of the terms used in

building acoustics and the test methods used to characterize systems and materials. A very basic understanding of some

fundamentals of acoustics and terms used in building acoustics, is all that is

necessary to understand the material in this and the following

chapters. To emphasize the simplicity of the approach,

equations are kept to a minimum.

Sound is generated by creating a disturbance of the air, which sets up a series of pressure waves fluctuating above and

below the air's normal atmospheric pressure, much as

a stone that falls in water generates expanding ripples on

the surface. Unlike the water waves, however, these pressure

waves propagate in all directions from the source of the

sound. Our ears sense these pressure fluctuations, convert

them to electrical impulses, and send them to our brain, where they are interpreted as sound.

There are many sources of sound in buildings: voices, human activities, external

noises such as traffic, entertainment devices and

machinery. They all generate small rapid variations in pressure about the static

atmospheric pressure; these propagate through the air as

sound waves.

As well as travelling in air, sound can travel as vibrational waves in solids or liquids. The terms airborne and structure-

borne sound are used depending on which medium

the sound is travelling in at the time. For example, the noise

from a radio set may begin as airborne sound, enter the

structure of the building and travel for some distance as

structure-borne sound, and then be radiated again as airborne

sound in another place. (Figure 1) The importance of structure-borne sound will become more apparent when flanking sound transmission is discussed, in

Acoustics in Practice.

Figure 1

Air pressure is usually measured in units of Pascals (Pa). Atmospheric pressure is

about 100 kPa. Sound pressure is a measure of the fluctuation of the air pressure above and

below normal atmospheric pressure as the sound waves

propagate past a listener. Generally, the larger the

fluctuations, the louder the sound.

The pressure variations in an individual sound wave are much less than the static atmospheric

pressure, but the range of sound pressures encountered in

acoustics is very large. The threshold of hearing is assumed

to correspond to pressure fluctuations of 20 microPascals; some individuals will have more acute hearing than this, some less. The threshold of pain in

the ear corresponds to pressure fluctuations of about 200 Pa.

This second value is ten million times the first. These unwieldy numbers are converted to more

convenient ones using a logarithmic scale, the decibel scale. Sound pressure levels are expressed as a number followed by the symbol dB. Sound level metersconvert

electrical signals from a microphone to sound pressure

levels in dB. Table 1 gives some representative sound pressure

levels encountered in a range of situations.

Table 1   Typical sound levels

dB

Jet takeoff, artillery fire, riveting ..................

120 or more

Rock band or very loud orchestra .......................

100120

Unmuffled truck, police whistle ........................

80100

Average radio or TV ....................................

7090

Human voice at 1 m .....................................

5560

Background in private office ...........................

3540

Quiet home .........................................

....

2535

Threshold of hearing ...................................

20

Decibels are more easily related to the response of the human

ear, which also responds logarithmically to sound.The response of our ears, that is, our perception of loudness,

does not increase linearly with a linear increase insound

pressure. For example, a 10 dB increase in sound pressure level

would be perceived as a doubling of the loudness. In

practical situations, level changes of about 3 dB are just

noticeable.

It is very important to remember that decibels and similar

acoustical quantities have properties different from more

conventional units. Sound pressure levels, for example, cannot be added together as

can kilograms. The combination of two noises with average

levels of 60 dB does not give a sound pressure level of 120 dB,

but 63 dB.

The addition of a noise with a level of 70 dB to a room with a level of 80 dB will result in no measureable difference in the

overall level. This does not mean, however, that a large

number of secondary sources can be introduced into an

environment without increasing the overall level. If ten

'negligible' 70 dB noise sources are combined with one 80 dB

noise source, the resulting level will be 83 dB. Fortunately in building acoustics, there is seldom a need to combine

noise levels in this way or to do many complicated calculations

with decibels or other logarithmic units. These

examples merely emphasize the peculiarities of the decibel

scale.

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Sound has a huge impact on our day to day lives. Just think of how much of our

technology involves sending or receiving sounds in

various forms.

Most people don’t fully understand what sound is.

In this section we will be looking at the basic properties of sound.

Sound is a longitudinal wave.

Remember that longitudinal waves are

made up of areas where the wave is compressed

together, and other areas where it is expanded.

This would agree with the way that humans

themselves make sounds. We force air, sometimes

harder, sometimes softer, through our vocal cords.

In the process the air is either squished or allowed

to move freely… making the air into a longitudinal wave!

We will look in detail at three fundamental

characteristics of sound: speed, frequency, and

loudness.

Speed

The speed of sound in air actually depends on the temperature of the air.

As a standard, we will say that the speed of sound is 340m/s at 15°C. If you are not told anything different

in a question, use this value.

If you did need to calculate the speed of sound at a different

temperature, you could use this formula as a rough estimate (you are not

required to memorize this formula)…

v = 331.5m/s + 0.6T

v = velocity of sound (m/s)

T = temperature (°C)

Andy Graves http://www.andrewgraves.biz/

On October 15, 1997 the British built "Thrust SSC" vehicle became the first

land based vehicle to break the sound barrier. To be

official it had to break the sound barrier twice within

one hour. It did this, with an average top speed on the

two runs of Mach 1.020. The runs took place in early in

the day so that the temperature of the air (and the speed of sound) would be lower. As an interesting side note, this record was set one day after the 50th

anniversary of the first supersonic flight made by Chuck Yeager on October 14, 1947 in the "Bell X-1."

Example 1: Determine the speed of sound when it is

–5°C.

v = 331.5m/s + 0.6(-5)v = 331.5m/s + -3m/s

v = 328.5 m/s

You can observe an example of how the speed of sound affects when you

hear it compared to the occurrence of the event that

caused the sound.

During a thunderstorm, watch for a lightning strike. You see it first because light travels at a very high speed

(3.00e8m/s), which is so fast it travels to your eye

from the lightening almost instantly.

Now listen for the thunder. The sound is traveling at a

sluggish (compared to light!) 340m/s behind the

flash of light.

For every 3 seconds that you count between the flash

and the sound there is a distance of about one

kilometre between you and the lightning.

Sound can also travel through solids and liquids,

not just gases.

This is why you can still hear stuff, even if it’s

distorted, when you are under water at a swimming

pool.

The speed of sound in liquids is quicker than in gases, and the speed of sound in solids is even

quicker. • o This is because the atoms are

closer together, so they transfer the sound more efficiently.

You might have even seen people in movies listening for an approaching train by putting an ear on the train tracks and listening for it.

Don’t try this! It’s very dangerous!

• o The sound of the train travels faster and more efficiently through the solid train tracks.

Frequency

If you are doing calculations of the wavelength or

frequency of sound, you still use the standard

formula…

v = f λ

If the sound you are measuring is at a constant

temperature then the velocity will be constant…

about 340 m/s.

What sort of frequencies of sound will you typically

be talking about?

Most often we will be looking at sound waves that humans can actually hear,

which are frequencies from 20 – 20 000 Hz.

Check out the specifications for

headphones printed on the back of the package. They’ll

probably list their range from 20 – 20 000Hz, since that’s what the average

person can hear.

• o 20 Hz would be very deep, low, rumbling sounds.

• o 20 000 Hz would be a very high pitched, squealing sort of noise. (N.B. In music “pitch” means the same as frequency.

NameFrequency Range (Hz) Characteristics

Infrasonic0 - 20 Very low frequencies of sound that

the human ear can’t detect, but you may feel the

rumbling of the waves through your body.

Sonic (AKA Audio) 20 - 20 000 Normal range for

human ears, although not everyone (especially the elderly) will hear to the

extremes of this range.

Ultrasonic 20 000 + Beyond normal hearing for

humans, although some animals (like dogs) hear

part ways into this range. Also used in medicine (e.g. ultrasounds for pregnant

women).

Example 2: My wife and I are listening to my favourite Bugles song, “Video Killed the Radio Star” from the 1980’s. At one point the

singer hits a note that my wife thinks has a

wavelength of 0.014m. I tell her this is impossible…

explain why.

We will assume that the speed of sound is 340 m/s.

That means that we will get…

v = f λf = v / λ

f= (340m/s) / (0.014m)f = 24 286 Hz = 2.4e4 Hz

This frequency is beyond the range of normal human

hearing. We wouldn’t be able to hear it, and it is unlikely that our stereo system could produce a

sound with a frequency that high.

If the speed of sound changed due to a change in the temperature of the air, it would make notes sound

"off key."

This is why an orchestra “warms up” before a

performance.

If a flute was tuned to the right frequency when the

metal is cold, the frequency will change as

the person plays for the first few minutes and the

instrument heats up from their breath.

Every instrument gets played for a few minutes to

make sure that it is at a constant temperature for

the whole performance and is then tuned.

Cover your ears!

Example 3: I am playing the flute (yes, I actually can, I’m just not very good!), and tuned it straight out of the case. The temperature of the flute was 17°C and I

tuned it to 15 000 Hz. I start playing the flute, and by the time I’m a few minutes into the

song I notice that the notes all seem wrong. If the flute has warmed up to my body

temperature (37°C) , determine what my

original tuned note has changed to.

First we need to calculate the speed of sound at

17°C…

v = 331.5m/s + 0.6(17)v = 331.5m/s + 10.2m/s

v = 341.7 m/s

Next we figure out the wavelength of the note I

tuned. This wavelength will remain constant, even if the

flute warms up.

v = f λλ = v/f

λ = (341.7m/s) / (15 000Hz)λ= 0.02278 m

Third, what’s the speed of sound at 37°C (body

temperature)…

v = 331.5m/s + 0.6(37)v = 331.5m/s + 22.2m/s

v = 353.7 m/s

Which means, finally, I can calculate what the

frequency of the note has changed to. This is based on

the constant wavelength and the new speed of

sound.

v = f λf = v / λ

f = = (353.7m/s) / (0.02278m)

f = 15 527Hz

The frequency has jumped up by more than 500Hz!

This will be a very noticeable difference, even for someone that doesn’t

know anything about music… the notes will just

sound wrong.

Loudness

The loudness of a sound depends on the wave’s

amplitude.

This is why a stereo system has an “amplifier”, a

device that increases the amplitude of sound waves.

The louder a sound, the bigger the amplitude.

This is also a way of measuring the amount of

energy the wave has.

The system used to measure the loudness of

sounds is the decibel system, given the unit dB.

The decibel system is based on logarithms, which means for every step up by one, the sound is actually

ten times louder. For example, a 15dB sound is ten times louder than a

14dB sound.

The decibel is actually a fraction of a bel, the original unit for measuring sound (1 db = 0.1 b). The "bel" was

originally named after Alexander Graham Bell, the inventor of the telephone. Because the bel was too

high a value for day to day situations, the decibel became a standard.

Range (dB) DescriptionExamples

0 - 30 Very Quiet This is the threshold of human

hearing, up to the sound of a quiet whisper.

31 - 50 Quiet This is an average quiet house, with

maybe the sound of a fridge running or someone moving

around.

51 - 70 Normal Regular daily sounds like people

talking.

71 - 90 Loud This is the point where a sound becomes annoying or

distracting. Vacuums or a noisy car on a busy street

are at these levels.

91 - 110 Very Loud Most people will try to avoid being in areas this loud. Prolonged exposure can

cause permanent ear damage. Temporary effects,

like "stereo hiss", may happen.

111 + Painful!!! Even limited exposure to levels

this high will cause permanent hearing loss.

You want to know the scary part? Most concerts you go

to will have sound levels between 100 – 130 dB…

easily into the permanent damage range.

Lot’s of old rock stars have permanent hearing loss.

Many modern day musicians wear ear

protection of some sort while in concert.

One of the loudest man-made sounds is created by the space shuttle lifting off. It will generate sounds at an

incredible 215 dB!!! The sound is so loud that it would actually cause damage to the launch

tower, and as a reflected echo, to the shuttle itself.

To absorb the energy, huge amounts of water are

pumped to the base of the launch pad seconds before takeoff. The water absorbs

the sound, as well as a lot of heat. When you see video of

a shuttle launch, most of the white stuff you see

billowing from the launch pad right at takeoff is not

smoke... it's steam! \

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unit03_mechanicalwaves/chp141516_waves/lesson49.htm