NCERT Textbook - Sound | Science Class 9 PDF Download

 Page 1


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.
11
S S S S SOUND OUND OUND OUND OUND
Chapter
2024-25
Page 2


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.
11
S S S S SOUND OUND OUND OUND OUND
Chapter
2024-25
SCIENCE 128
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.
2024-25
Page 3


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.
11
S S S S SOUND OUND OUND OUND OUND
Chapter
2024-25
SCIENCE 128
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.
2024-25
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.
Q
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.
2024-25
Page 4


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.
11
S S S S SOUND OUND OUND OUND OUND
Chapter
2024-25
SCIENCE 128
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.
2024-25
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.
Q
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.
2024-25
SCIENCE 130
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 ? (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
2024-25
Page 5


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.
11
S S S S SOUND OUND OUND OUND OUND
Chapter
2024-25
SCIENCE 128
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.
Q
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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SCIENCE 130
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 ? (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
? (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.
=
1
v
T
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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