Everyday we hear sounds from various

sources like humans, birds, bells, machines,

vehicles, televisions, radios etc. Sound is a

form of energy which produces a sensation

of hearing in our ears. There are also other

forms of energy like mechanical energy, light

energy, etc. We have talked about mechanical

energy in the previous chapters. You have

been taught about conservation of energy,

which states that we can neither create nor

destroy energy. We can just change it from

one form to another. When you clap, a sound

is produced. Can you produce sound without

utilising your energy? Which form of energy

did you use to produce sound? In this

chapter we are going to learn how sound is

produced and how it is transmitted through

a medium and received by our ears.

11.1 Production of Sound

Activity _____________ 11.1

· Take a tuning fork and set it vibrating

by striking its prong on a rubber pad.

Bring it near your ear.

· Do you hear any sound?

· Touch one of the prongs of the vibrating

tuning fork with your finger and share

your experience with your friends.

· Now, suspend a table tennis ball or a

small plastic ball by a thread from a

support [Take a big needle and a

thread, put a knot at one end of the

thread, and then with the help of the

needle pass the thread through the

ball]. Touch the ball gently with the

prong of a vibrating tuning

fork (Fig. 11.1).

· Observe what happens and discuss

with your friends.

Activity _____________11.2

· Fill water in a beaker or a glass up to

the brim. Gently touch the water surface

with one of the prongs of the vibrating

tuning fork, as shown in Fig. 11.2.

· Next dip the prongs of the vibrating

tuning fork in water, as shown in Fig.

11.3.

· Observe what happens in both the

cases.

· Discuss with your friends why this

happens.

Fig. 11.1: Vibrating tuning fork just touching the

suspended table tennis ball.

Fig. 11.2: One of the prongs of the vibrating tuning

fork touching the water surface.

OUNDOUND

OUND

hapter

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SCIENCE128

the vibrating object to the ear. A particle of

the medium in contact with the vibrating

object is first displaced from its equilibrium

position. It then exerts a force on the adjacent

particle. As a result of which the adjacent

particle gets displaced from its position of

rest. After displacing the adjacent particle the

first particle comes back to its original

position. This process continues in the

medium till the sound reaches your ear. The

disturbance created by a source of sound in

the medium travels through the medium and

not the particles of the medium.

A wave is a disturbance that moves

through a medium when the particles of the

medium set neighbouring particles into

motion. They in turn produce similar motion

in others. The particles of the medium do not

move forward themselves, but the

disturbance is carried forward. This is what

happens during propagation of sound in a

medium, hence sound can be visualised as a

wave. Sound waves are characterised by the

motion of particles in the medium and are

called mechanical waves.

Air is the most common medium through

which sound travels. When a vibrating object

moves forward, it pushes and compresses the

air in front of it creating a region of high

pressure. This region is called a compression

(C), as shown in Fig. 11.4. This compression

starts to move away from the vibrating object.

When the vibrating object moves backwards,

it creates a region of low pressure called

rarefaction (R), as shown in Fig. 11.4. As the

object moves back and forth rapidly, a series

of compressions and rarefactions is created in

the air. These make the sound wave that

Fig. 11.3: Both the prongs of the vibrating tuning

fork dipped in water

From the above activities what do you

conclude? Can you produce sound without

a vibrating object?

In the above activities we have produced

sound by striking the tuning fork. We can

also produce sound by plucking, scratching,

rubbing, blowing or shaking different objects.

As per the above activities what do we do to

the objects? We set the objects vibrating and

produce sound. Vibration means a kind of

rapid to and fro motion of an object. The

sound of the human voice is produced due

to vibrations in the vocal cords. When a bird

flaps its wings, do you hear any sound? Think

how the buzzing sound accompanying a bee

is produced. A stretched rubber band when

plucked vibrates and produces sound. If you

have never done this, then do it and observe

the vibration of the stretched rubber band.

Activity _____________ 11.3

· Make a list of different types of

musical instruments and discuss

with your friends which part of the

instrument vibrates to produce

sound.

11.2 Propagation of Sound

Sound is produced by vibrating objects. The

matter or substance through which sound

is transmitted is called a medium. It can be

solid, liquid or gas. Sound moves through a

medium from the point of generation to the

listener. When an object vibrates, it sets the

particles of the medium around it vibrating.

The particles do not travel all the way from

Fig. 11.4: A vibrating object creating a series of

compressions (C) and rarefactions (R) in

the medium.

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SOUND 129

propagates through the medium.

Compression is the region of high pressure

and rarefaction is the region of low pressure.

Pressure is related to the number of particles

of a medium in a given volume. More density

of the particles in the medium gives more

pressure and vice versa. Thus, propagation

of sound can be visualised as propagation of

density variations or pressure variations in

the medium.

uestion

1. How does the sound produced by

a vibrating object in a medium

reach your ear?

2. Explain how sound is produced

by your school bell.

3. Why are sound waves called

mechanical waves?

4. Suppose you and your friend are

on the moon. Will you be able to

hear any sound produced by

your friend?

11.2.1 SOUND WAVES ARE LONGITUDINAL

WAVES

Activity _____________ 11.4

· Take a slinky. Ask your friend to hold

one end. You hold the other end.

Now stretch the slinky as shown in

Fig. 11.5(a). Then give it a sharp push

towards your friend.

· What do you notice? If you move your

hand pushing and pulling the slinky

alternatively, what will you observe?

· If you mark a dot on the slinky, you

will observe that the dot on the slinky

will move back and forth parallel to

the direction of the propagation of the

disturbance.

The regions where the coils become closer

are called compressions (C) and the regions

where the coils are further apart are called

rarefactions (R). As we already know, sound

propagates in the medium as a series of

compressions and rarefactions. Now, we can

compare the propagation of disturbance in a

slinky with the sound propagation in the

medium. These waves are called longitudinal

waves. In these waves the individual particles

of the medium move in a direction parallel to

the direction of propagation of the disturbance.

The particles do not move from one place to

another but they simply oscillate back and

forth about their position of rest. This is

exactly how a sound wave propagates, hence

sound waves are longitudinal waves.

There is also another type of wave, called

a transverse wave. In a transverse wave

particles do not oscillate along the direction

of wave propagation but oscillate up and down

about their mean position as the wave travels.

Thus, a transverse wave is the one in which

the individual particles of the medium move

about their mean positions in a direction

perpendicular to the direction of wave

propagation. When we drop a pebble in a

pond, the waves you see on the water surface

is an example of transverse wave. Light is a

transverse wave but for light, the oscillations

are not of the medium particles or their

pressure or density— it is not a mechanical

wave. You will come to know more about

transverse waves in higher classes.

11.2.2 CHARACTERISTICS OF A SOUND

WAVE

We can describe a sound wave by its

· frequency

· amplitude and

· speed.

A sound wave in graphic form is shown in

Fig. 11.6(c), which represents how density and

pressure change when the sound wave moves

in the medium. The density as well as the

pressure of the medium at a given time varies

with distance, above and below the average

value of density and pressure. Fig. 11.6(a) and

(b)

(a)

Fig. 11.5: Longitudinal wave in a slinky.

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SCIENCE130

Heinrich Rudolph Hertz

was born on 22 February

1857 in Hamburg,

Germany and educated at

the University of Berlin. He

confirmed J.C. Maxwell’s

electromagnetic theory by

his experiments. He laid the

foundation for future

development of radio, telephone, telegraph

and even television. He also discovered the

photoelectric effect which was later

explained by Albert Einstein. The SI unit of

frequency was named as hertz in his honour.

Fig. 11.6(b) represent the density and

pressure variations, respectively, as a sound

wave propagates in the medium.

Compressions are the regions where

particles are crowded together and

represented by the upper portion of the curve

in Fig. 11.6(c). The peak represents the region

of maximum compression. Thus,

compressions are regions where density as

well as pressure is high. Rarefactions are the

regions of low pressure where particles are

spread apart and are represented by the

valley, that is, the lower portion of the curve

in Fig. 11.6(c). A peak is called the crest and a

valley is called the trough of a wave.

The distance between two consecutive

compressions (C) or two consecutive

rarefactions (R) is called the wavelength, as

shown in Fig. 11.6(c), The wavelength is

usually represented by l (Greek letter

lambda). Its SI unit is metre (m).

H. R. Hertz

Fig. 11.6: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents

graphically the density and pressure variations.

Frequency tells us how frequently an

event occurs. Suppose you are beating a

drum. How many times you are beating the

drum in unit time is called the frequency of

your beating the drum. We know that when

sound is propagated through a medium, the

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SOUND 131

How the brain interprets the frequency

of an emitted sound is called its pitch. The

faster the vibration of the source, the

higher is the frequency and the higher is

the pitch, as shown in Fig. 11.7. Thus, a

high pitch sound corresponds to more

number of compressions and rarefactions

passing a fixed point per unit time.

Objects of different sizes and conditions

vibrate at different frequencies to produce

sounds of different pitch.

The magnitude of the maximum

disturbance in the medium on either side of

the mean value is called the amplitude of the

wave. It is usually represented by the letter A,

as shown in Fig. 11.6(c). For sound its unit

will be that of density or pressure.

The loudness or softness of a sound is

determined basically by its amplitude. The

amplitude of the sound wave depends upon

the force with which an object is made to

vibrate. If we strike a table lightly, we hear a

soft sound because we produce a sound wave

density of the medium oscillates between a

maximum value and a minimum value. The

change in density from the maximum value

to the minimum value, then again to the

maximum value, makes one complete

oscillation. The number of such oscillations

per unit time is the frequency of the sound

wave. If we can count the number of the

compressions or rarefactions that cross us

per unit time, we will get the frequency of

the sound wave. It is usually represented by

n (Greek letter, nu). Its SI unit is hertz

(symbol, Hz).

The time taken by two consecutive

compressions or rarefactions to cross a fixed

point is called the time period of the wave. In

other words, we can say that the time taken

for one complete oscillation is called the time

period of the sound wave. It is represented by

the symbol T. Its SI unit is second (s).

Frequency and time period are related as

follows:

A violin and a flute may both be played at

the same time in an orchestra. Both sounds

travel through the same medium, that is, air

and arrive at our ear at the same time. Both

sounds travel at the same speed irrespective

of the source. But the sounds we receive are

different. This is due to the different

characteristics associated with the sound.

Pitch is one of the characteristics.

Fig. 11.7: Low pitch sound has low frequency and

high pitch of sound has high frequency.

Fig. 11.8: Soft sound has small amplitude and

louder sound has large amplitude.

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SCIENCE132

uestions

1. Which wave property determines

(a) loudness, (b) pitch?

2. Guess which sound has a higher

pitch: guitar or car horn?

The speed of sound is defined as the

distance which a point on a wave, such as a

compression or a rarefaction, travels per unit

time.

We know,

speed, v = distance / time

Example 11.1 A sound wave has a

frequency of 2 kHz and wave length

35 cm. How long will it take to travel

1.5 km?

Solution:

Given,

Frequency,

= 2 kHz = 2000 Hz

Wavelength, l = 35 cm = 0.35 m

We know that speed, v of the wave

= wavelength



frequency

v = l n

= 0.35 m 2000 Hz = 700 m/s

The time taken by the wave to travel a

distance, d of 1.5 km is

Thus sound will take 2.1 s to travel a

distance of 1.5 km.

uestions

1. What are wavelength, frequency,

time period and amplitude of a

sound wave?

2. How are the wavelength and

frequency of a sound wave

related to its speed?

3. Calculate the wavelength of a

sound wave whose frequency is

220 Hz and speed is 440 m/s in

a given medium.

4. A person is listening to a tone of

500 Hz sitting at a distance of

450 m from the source of the

sound. What is the time interval

between successive compressions

from the source?

The amount of sound energy passing each

second through unit area is called the intensity

of sound. We sometimes use the terms

“loudness” and “intensity” interchangeably,

but they are not the same. Loudness is a

measure of the response of the ear to the sound.

Even when two sounds are of equal intensity,

we may hear one as louder than the other

simply because our ear detects it better.

of less energy (amplitude). If we hit the table

hard we hear a louder sound. Can you tell

why? A sound wave spreads out from its

source. As it moves away from the source its

amplitude as well as its loudness decreases.

Louder sound can travel a larger distance as

it is associated with higher energy. Fig. 11.8

shows the wave shapes of a loud and a soft

sound of the same frequency.

The quality or timber of sound is that

characteristic which enables us to distinguish

one sound from another having the same pitch

and loudness. The sound which is more

pleasant is said to be of a rich quality. A sound

of single frequency is called a tone. The sound

which is produced due to a mixture of several

frequencies is called a note and is pleasant to

listen to. Noise is unpleasant to the ear! Music

is pleasant to hear and is of rich quality.

Here l is the wavelength of the sound wave. It

is the distance travelled by the sound wave in

one time period (T) of the wave. Thus,

v = λ ν

or v = λ ν

That is, speed = wavelength



frequency.

The speed of sound remains almost the

same for all frequencies in a given medium

under the same physical conditions.

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SOUND 133

uestion

1. Distinguish between loudness

and intensity of sound.

uestion

1. In which of the three media, air,

water or iron, does sound travel

the fastest at a particular

temperature?

11.3 Reflection of Sound

Sound bounces off a solid or a liquid like a

rubber ball bounces off a wall. Like light, sound

gets reflected at the surface of a solid or liquid

and follows the same laws of reflection as you

have studied in earlier classes. The directions

in which the sound is incident and is reflected

make equal angles with the normal to the

reflecting surface at the point of incidence, and

the three are in the same plane. An obstacle of

large size which may be polished or rough is

needed for the reflection of sound waves.

Activity _____________11.5

· Take two identical pipes, as shown in

Fig. 11.9. You can make the pipes

using chart paper. The length of the

pipes should be sufficiently long

as shown.

· Arrange them on a table near a wall.

· Keep a clock near the open end of one

of the pipes and try to hear the sound

of the clock through the other pipe.

· Adjust the position of the pipes so

that you can best hear the sound of

the clock.

· Now, measure the angles of incidence

and reflection and see the

relationship between the angles.

· Lift the pipe on the right vertically

to a small height and observe

what happens.

(In place of a clock, a mobile phone

on vibrating mode may also be used.)

11.2.3 SPEED OF SOUND IN DIFFERENT

MEDIA

Sound propagates through a medium at a finite

speed. The sound of a thunder is heard a little

later than the flash of light is seen. So, we can

make out that sound travels with a speed

which is much less than the speed of light. The

speed of sound depends on the properties of

the medium through which it travels. You will

learn about this dependence in higher classes.

The speed of sound in a medium depends on

temperature of the medium. The speed of

sound decreases when we go from solid to

gaseous state. In any medium as we increase

the temperature, the speed of sound increases.

For example, the speed of sound in air is 331

m s

–1

at 0

ºC and 344 m s

–1

at 22 ºC. The speeds

of sound at a particular temperature in various

media are listed in Table 11.1. You need not

memorise the values.

Table 11.1: Speed of sound in

different media at 25 ºC

State Substance Speed in m/s

Solids Aluminium 6420

Nickel 6040

Steel 5960

Iron 5950

Brass 4700

Glass (Flint) 3980

Liquids Water (Sea) 1531

Water (distilled) 1498

Ethanol 1207

Methanol 1103

Gases Hydrogen 1284

Helium 965

Air 346

Oxygen 316

Sulphur dioxide 213

Fig. 11.9: Reflection of sound

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SCIENCE134

11.3.1 ECHO

If we shout or clap near a suitable reflecting

object such as a tall building or a

mountain,we will hear the same sound

again a little later. This sound which we

hear is called an echo. The sensation of

sound persists in our brain for about 0.1

s. To hear a distinct echo the time interval

between the original sound and the

reflected one must be at least 0.1s. If we

take the speed of sound to be 344 m/s at a

given temperature, say at 22 ºC in air, the

sound must go to the obstacle and reach

back the ear of the listener on reflection after

0.1s. Hence, the total distance covered by

the sound from the point of generation to

the reflecting surface and back should be

at least (344 m/s) ´ 0.1 s = 34.4 m. Thus,

for hearing distinct echoes, the minimum

distance of the obstacle from the source of

sound must be half of this distance, that

is, 17.2 m. This distance will change with

the temperature of air. Echoes may be heard

more than once due to successive or

multiple reflections. The rolling of thunder

is due to the successive reflections of the

sound from a number of reflecting surfaces,

such as the clouds and the land.

11.3.2 REVERBERATION

A sound created in a big hall will persist

by repeated reflection from the walls until

it is reduced to a value where it is no longer

audible. The repeated reflection that

results in this persistence of sound is

called reverberation. In an auditorium or

big hall excessive reverberation is highly

undesirable. To reduce reverberation, the

roof and walls of the auditorium are

generally covered with sound-absorbent

materials like compressed fibreboard,

rough plaster or draperies. The seat

materials are also selected on the basis of

their sound absorbing properties.

Example 11.2 A person clapped his hands

near a cliff and heard the echo after 2 s.

What is the distance of the cliff from the

person if the speed of the sound, v is

taken as 346 m s

–1

Solution:

Given,

Speed of sound, v = 346 m s

–1

Time taken for hearing the echo,

t = 2 s

Distance travelled by the sound

= v ´ t = 346 m s

–1

´ 2 s = 692 m

In 2 s sound has to travel twice the

distance between the cliff and the

person. Hence, the distance between the

cliff and the person

= 692 m/2 = 346 m.

Horn

Megaphone

Fig 11.10: A megaphone and a horn.

uestion

1. An echo is heard in 3 s. What is

the distance of the reflecting

surface from the source, given that

the speed of sound is 342 m s

–1

11.3.3 USES OF MULTIPLE REFLECTION

OF SOUND

1. Megaphones or loudhailers, horns,

musical instruments such as trumpets

and shehanais, are all designed to send

sound in a particular direction without

spreading it in all directions, as shown

in Fig 11.10.

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SOUND 135

2. Stethoscope is a medical instrument

used for listening to sounds produced

within the body, mainly in the heart or

lungs. In stethoscopes the sound of the

patient’s heartbeat reaches the doctor’s

ears by multiple reflection of sound, as

shown in Fig.11.11.

Fig. 11.12: Curved ceiling of a conference hall.

Fig. 11.13: Sound board used in a big hall.

uestion

1. Why are the ceilings of concert

halls curved?

11.4 Range of Hearing

The audible range of sound for human beings

extends from about 20 Hz to 20000 Hz (one

Hz = one cycle/s). Children under the age of

five and some animals, such as dogs can hear

up to 25 kHz (1 kHz = 1000 Hz). As people

grow older their ears become less sensitive to

higher frequencies. Sounds of frequencies

below 20 Hz are called infrasonic sound or

infrasound. If we could hear infrasound we

would hear the vibrations of a pendulum just

as we hear the vibrations of the wings of a bee.

Rhinoceroses communicate using infrasound

of frequency as low as 5 Hz. Whales and

elephants produce sound in the infrasound

range. It is observed that some animals get

disturbed before earthquakes. Earthquakes

produce low-frequency infrasound before the

main shock waves begin which possibly alert

the animals. Frequencies higher than 20 kHz

are called ultrasonic sound or ultrasound.

Ultrasound is produced by animals such as

dolphins, bats and porpoises. Moths of certain

families have very sensitive hearing equipment.

These moths can hear the high frequency

Fig.11.11: Stethoscope

3. Generally the ceilings of concert halls,

conference halls and cinema halls are

curved so that sound after reflection

reaches all corners of the hall, as

shown in Fig 11.12. Sometimes a

curved soundboard may be placed

behind the stage so that the sound,

after reflecting from the sound board,

spreads evenly across the width of the

hall (Fig 11.13).

In these instruments, a tube followed

by a conical opening reflects sound

successively to guide most of the

sound waves from the source in the

forward direction towards the

audience.

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SCIENCE136

squeaks of the bat and know when a bat

is flying nearby, and are able to escape

capture. Rats also play games by

producing ultrasound.

Hearing Aid: People with hearing loss may

need a hearing aid. A hearing aid is an

electronic, battery operated device. The

hearing aid receives sound through a

microphone. The microphone converts the

sound waves to electrical signals. These

electrical signals are amplified by an

amplifier. The amplified electrical signals

are given to a speaker of the hearing aid.

The speaker converts the amplified

electrical signal to sound and sends to the

ear for clear hearing.

in construction of big structures like

buildings, bridges, machines and also

scientific equipment. The cracks or

holes inside the metal blocks, which

are invisible from outside reduces the

strength of the structure. Ultrasonic

waves are allowed to pass through the

metal block and detectors are used to

detect the transmitted waves. If there

is even a small defect, the ultrasound

gets reflected back indicating the

presence of the flaw or defect, as shown

in Fig. 11.14.

Metallic components are generally used

uestions

1. What is the audible range of the

average human ear?

2. What is the range of frequencies

associated with

(a) Infrasound?

(b) Ultrasound?

11.5 Applications of Ultrasound

Ultrasounds are high frequency waves.

Ultrasounds are able to travel along well-

defined paths even in the presence of

obstacles. Ultrasounds are used extensively

in industries and for medical purposes.

• Ultrasound is generally used to clean

parts located in hard-to-reach places,

for example, spiral tube, odd shaped

parts, electronic components, etc.

Objects to be cleaned are placed in a

cleaning solution and ultrasonic waves

are sent into the solution. Due to

the high frequency, the particles of

dust, grease and dirt get detached and

drop out. The objects thus get

thoroughly cleaned.

• Ultrasounds can be used to detect

cracks and flaws in metal blocks.

Fig 11.14: Ultrasound is reflected back from the

defective locations inside a metal block.

Ordinary sound of longer wavelengths

cannot be used for such purpose as it will

bend around the corners of the defective

location and enter the detector.

• Ultrasonic waves are made to reflect

from various parts of the heart and

form the image of the heart. This tech-

nique is called ‘echocardiography’.

• Ultrasound scanner is an instrument

which uses ultrasonic waves for

getting images of internal organs of the

human body. A doctor may image the

patient’s organs, such as the liver, gall

bladder, uterus, kidney, etc. It helps

the doctor to detect abnormalities,

such as stones in the gall bladder and

kidney or tumours in different organs.

In this technique the ultrasonic waves

travel through the tissues of the body

and get reflected from a region where

there is a change of tissue density.

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SCIENCE136

squeaks of the bat and know when a bat

is flying nearby, and are able to escape

capture. Rats also play games by

producing ultrasound.

Hearing Aid: People with hearing loss may

need a hearing aid. A hearing aid is an

electronic, battery operated device. The

hearing aid receives sound through a

microphone. The microphone converts the

sound waves to electrical signals. These

electrical signals are amplified by an

amplifier. The amplified electrical signals

are given to a speaker of the hearing aid.

The speaker converts the amplified

electrical signal to sound and sends to the

ear for clear hearing.

in construction of big structures like

buildings, bridges, machines and also

scientific equipment. The cracks or

holes inside the metal blocks, which

are invisible from outside reduces the

strength of the structure. Ultrasonic

waves are allowed to pass through the

metal block and detectors are used to

detect the transmitted waves. If there

is even a small defect, the ultrasound

gets reflected back indicating the

presence of the flaw or defect, as shown

in Fig. 11.14.

Metallic components are generally used

uestions

1. What is the audible range of the

average human ear?

2. What is the range of frequencies

associated with

(a) Infrasound?

(b) Ultrasound?

11.5 Applications of Ultrasound

Ultrasounds are high frequency waves.

Ultrasounds are able to travel along well-

defined paths even in the presence of

obstacles. Ultrasounds are used extensively

in industries and for medical purposes.

• Ultrasound is generally used to clean

parts located in hard-to-reach places,

for example, spiral tube, odd shaped

parts, electronic components, etc.

Objects to be cleaned are placed in a

cleaning solution and ultrasonic waves

are sent into the solution. Due to

the high frequency, the particles of

dust, grease and dirt get detached and

drop out. The objects thus get

thoroughly cleaned.

• Ultrasounds can be used to detect

cracks and flaws in metal blocks.

Fig 11.14: Ultrasound is reflected back from the

defective locations inside a metal block.

Ordinary sound of longer wavelengths

cannot be used for such purpose as it will

bend around the corners of the defective

location and enter the detector.

• Ultrasonic waves are made to reflect

from various parts of the heart and

form the image of the heart. This tech-

nique is called ‘echocardiography’.

• Ultrasound scanner is an instrument

which uses ultrasonic waves for

getting images of internal organs of the

human body. A doctor may image the

patient’s organs, such as the liver, gall

bladder, uterus, kidney, etc. It helps

the doctor to detect abnormalities,

such as stones in the gall bladder and

kidney or tumours in different organs.

In this technique the ultrasonic waves

travel through the tissues of the body

and get reflected from a region where

there is a change of tissue density.

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SOUND 137

What

you have

learnt

• Sound is produced due to vibration of different objects.

• Sound travels as a longitudinal wave through a material

medium.

• Sound travels as successive compressions and rarefactions

in the medium.

• In sound propagation, it is the energy of the sound that

travels and not the particles of the medium.

• The change in density from one maximum value to the

minimum value and again to the maximum value makes

one complete oscillation.

• The distance between two consecutive compressions or two

consecutive rarefactions is called the wavelength, λ.

• The time taken by the wave for one complete oscillation of

the density or pressure of the medium is called the time

period, T.

• The number of complete oscillations per unit time is called

the frequency (ν),

• The speed v, frequency ν, and wavelength λ, of sound are

related by the equation, v = λν.

• The speed of sound depends primarily on the nature and

the temperature of the transmitting medium.

• The law of reflection of sound states that the directions in

which the sound is incident and reflected make equal angles

with the normal to the reflecting surface at the point of

incidence and the three lie in the same plane.

• For hearing a distinct sound, the time interval between the

original sound and the reflected one must be at least 0.1 s.

• The persistence of sound in an auditorium is the result of

repeated reflections of sound and is called reverberation.

These waves are then converted into

electrical signals that are used to

generate images of the organ. These

images are then displayed on a monitor

or printed on a film. This technique

is called ‘ultrasonography’.

Ultrasonography is also used for

examination of the foetus during

pregnancy to detect congenial defects

and growth abnormalities.

• Ultrasound may be employed to break

small ‘stones’ formed in the kidneys

into fine grains. These grains later get

flushed out with urine.

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SCIENCE138

· Loudness is a physiological response of the ear to the intensity

of sound.

· The amount of sound energy passing each second through

unit area is called the intensity of sound.

· The audible range of hearing for average human beings is in

the frequency range of 20 Hz – 20 kHz.

· Sound waves with frequencies below the audible range are

termed “infrasonic” and those above the audible range are

termed “ultrasonic”.

· Ultrasound has many medical and industrial applications.

Exercises

1. What is sound and how is it produced?

2. Describe with the help of a diagram, how compressions and

rarefactions are produced in air near a source of sound.

3. Why is sound wave called a longitudinal wave?

4. Which characteristic of the sound helps you to identify your

friend by his voice while sitting with others in a dark room?

5. Flash and thunder are produced simultaneously. But

thunder is heard a few seconds after the flash is seen, why?

6. A person has a hearing range from 20 Hz to 20 kHz. What

are the typical wavelengths of sound waves in air

corresponding to these two frequencies? Take the speed of

sound in air as 344 m s

–1

7. Two children are at opposite ends of an aluminium rod. One

strikes the end of the rod with a stone. Find the ratio of

times taken by the sound wave in air and in aluminium to

reach the second child.

8. The frequency of a source of sound is 100 Hz. How many

times does it vibrate in a minute?

9. Does sound follow the same laws of reflection as light does?

Explain.

10. When a sound is reflected from a distant object, an echo is

produced. Let the distance between the reflecting surface

and the source of sound production remains the same. Do

you hear echo sound on a hotter day?

11. Give two practical applications of reflection of sound waves.

12. A stone is dropped from the top of a tower 500 m high into a

pond of water at the base of the tower. When is the splash

heard at the top? Given, g = 10 m s

–2

and speed of sound =

340 m s

–1

· Sound properties such as pitch, loudness and quality are

determined by the corresponding wave properties.

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SOUND 139

13. A sound wave travels at a speed of 339 m s

–1

. If its wavelength

is 1.5 cm, what is the frequency of the wave? Will it be

audible?

14. What is reverberation? How can it be reduced?

15. What is loudness of sound? What factors does it depend on?

16. How is ultrasound used for cleaning?

17. Explain how defects in a metal block can be detected using

ultrasound.

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