NCERT Textbook Chapter 12 - Sound , Science, Class 9 Class 9 Notes | EduRev

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Class 9 : NCERT Textbook Chapter 12 - Sound , Science, Class 9 Class 9 Notes | EduRev

 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, heat
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 ear.
12.1 Production of Sound
Activity _____________12.1
• 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. 12.1).
• Observe what happens and discuss
with your friends.
Activity _____________12.2
• 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. 12.2.
• Next dip the prongs of the vibrating
tuning fork in water, as shown in Fig.
12.3.
• Observe what happens in both the
cases.
• Discuss with your friends why this
happens.
Fig. 12.1: Vibrating tuning fork just touching the
suspended table tennis ball.
Fig. 12.2:  One of the prongs of the vibrating tuning
fork touching the water surface.
12 12
12 12 12
S S S S SOUND OUND OUND OUND OUND
Chapter
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, heat
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 ear.
12.1 Production of Sound
Activity _____________12.1
• 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. 12.1).
• Observe what happens and discuss
with your friends.
Activity _____________12.2
• 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. 12.2.
• Next dip the prongs of the vibrating
tuning fork in water, as shown in Fig.
12.3.
• Observe what happens in both the
cases.
• Discuss with your friends why this
happens.
Fig. 12.1: Vibrating tuning fork just touching the
suspended table tennis ball.
Fig. 12.2:  One of the prongs of the vibrating tuning
fork touching the water surface.
12 12
12 12 12
S S S S SOUND OUND OUND OUND OUND
Chapter
plucked vibrates and produces sound. If you
have never done this, then do it and observe
the vibration of the stretched rubber band.
Activity _____________12.3
• Make a list of different types of musical
instruments and discuss with your
friends which part of the instrument
vibrates to produce sound.
12.2 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
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
Fig. 12.3: 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
Fig. 12.4: A beam of light from a light source is made  to fall on a mirror . The reflected light is falling on the wall.
Can sound make a light spot dance?
Take a tin can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch
it over the can, then wrap a rubber band around the balloon. Take a small piece of mirror.
Use a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit
to fall on the mirror. After reflection the light spot is seen on the wall, as shown in Fig.
12.4. Talk or shout directly into the open end of the can and observe the dancing light spot
on the wall. Discuss with your friends what makes the light spot dance.
SOUND 161
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, heat
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 ear.
12.1 Production of Sound
Activity _____________12.1
• 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. 12.1).
• Observe what happens and discuss
with your friends.
Activity _____________12.2
• 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. 12.2.
• Next dip the prongs of the vibrating
tuning fork in water, as shown in Fig.
12.3.
• Observe what happens in both the
cases.
• Discuss with your friends why this
happens.
Fig. 12.1: Vibrating tuning fork just touching the
suspended table tennis ball.
Fig. 12.2:  One of the prongs of the vibrating tuning
fork touching the water surface.
12 12
12 12 12
S S S S SOUND OUND OUND OUND OUND
Chapter
plucked vibrates and produces sound. If you
have never done this, then do it and observe
the vibration of the stretched rubber band.
Activity _____________12.3
• Make a list of different types of musical
instruments and discuss with your
friends which part of the instrument
vibrates to produce sound.
12.2 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
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
Fig. 12.3: 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
Fig. 12.4: A beam of light from a light source is made  to fall on a mirror . The reflected light is falling on the wall.
Can sound make a light spot dance?
Take a tin can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch
it over the can, then wrap a rubber band around the balloon. Take a small piece of mirror.
Use a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit
to fall on the mirror. After reflection the light spot is seen on the wall, as shown in Fig.
12.4. Talk or shout directly into the open end of the can and observe the dancing light spot
on the wall. Discuss with your friends what makes the light spot dance.
SOUND 161 SCIENCE 162
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. 12.5. 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. 12.5. 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
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?
12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL
Sound is a mechanical wave and needs a
material medium like air, water, steel etc. for
its propagation. It cannot travel through
vacuum, which can be demonstrated by the
following experiment.
Take an electric bell and an airtight glass
bell jar. The electric bell is suspended inside
the airtight bell jar. The bell jar is connected
to a vacuum pump, as shown in Fig. 12.6. If
you press the switch you will be able to hear
the bell. Now start the vacuum pump. When
the air in the jar is pumped out gradually,
the sound becomes fainter, although the
same current is passing through the bell.
After some time when less air is left inside
the bell jar you will hear a very feeble sound.
What will happen if the air is removed
completely? Will you still be able to hear the
sound of the bell?
Fig. 12.5: A vibrating object creating a series of
compressions (C) and rarefactions (R) in
the medium.
Q
Fig. 12.6: Bell jar experiment showing sound cannot
travel in vacuum.
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, heat
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 ear.
12.1 Production of Sound
Activity _____________12.1
• 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. 12.1).
• Observe what happens and discuss
with your friends.
Activity _____________12.2
• 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. 12.2.
• Next dip the prongs of the vibrating
tuning fork in water, as shown in Fig.
12.3.
• Observe what happens in both the
cases.
• Discuss with your friends why this
happens.
Fig. 12.1: Vibrating tuning fork just touching the
suspended table tennis ball.
Fig. 12.2:  One of the prongs of the vibrating tuning
fork touching the water surface.
12 12
12 12 12
S S S S SOUND OUND OUND OUND OUND
Chapter
plucked vibrates and produces sound. If you
have never done this, then do it and observe
the vibration of the stretched rubber band.
Activity _____________12.3
• Make a list of different types of musical
instruments and discuss with your
friends which part of the instrument
vibrates to produce sound.
12.2 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
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
Fig. 12.3: 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
Fig. 12.4: A beam of light from a light source is made  to fall on a mirror . The reflected light is falling on the wall.
Can sound make a light spot dance?
Take a tin can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch
it over the can, then wrap a rubber band around the balloon. Take a small piece of mirror.
Use a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit
to fall on the mirror. After reflection the light spot is seen on the wall, as shown in Fig.
12.4. Talk or shout directly into the open end of the can and observe the dancing light spot
on the wall. Discuss with your friends what makes the light spot dance.
SOUND 161 SCIENCE 162
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. 12.5. 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. 12.5. 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
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?
12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL
Sound is a mechanical wave and needs a
material medium like air, water, steel etc. for
its propagation. It cannot travel through
vacuum, which can be demonstrated by the
following experiment.
Take an electric bell and an airtight glass
bell jar. The electric bell is suspended inside
the airtight bell jar. The bell jar is connected
to a vacuum pump, as shown in Fig. 12.6. If
you press the switch you will be able to hear
the bell. Now start the vacuum pump. When
the air in the jar is pumped out gradually,
the sound becomes fainter, although the
same current is passing through the bell.
After some time when less air is left inside
the bell jar you will hear a very feeble sound.
What will happen if the air is removed
completely? Will you still be able to hear the
sound of the bell?
Fig. 12.5: A vibrating object creating a series of
compressions (C) and rarefactions (R) in
the medium.
Q
Fig. 12.6: Bell jar experiment showing sound cannot
travel in vacuum.
SOUND 163
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 line 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. 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.
12.2.3 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. 12.8(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. 12.8(a) and Fig. 12.8(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. 12.8(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
uestions
1. Explain how sound is produced
by your school bell.
2. Why are sound waves called
mechanical waves?
3. Suppose you and your friend are
on the moon. Will you be able to
hear any sound produced by
your friend?
12.2.2 SOUND WAVES ARE LONGITUDINAL
WAVES
Activity _____________12.4
• Take a slinky.  Ask your friend to hold
one end. You hold the other end.
Now stretch the slinky as shown in
Fig. 12.7 (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.
Fig. 12.7: Longitudinal wave in a slinky.
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
Q
(a)
(b)
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, heat
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 ear.
12.1 Production of Sound
Activity _____________12.1
• 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. 12.1).
• Observe what happens and discuss
with your friends.
Activity _____________12.2
• 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. 12.2.
• Next dip the prongs of the vibrating
tuning fork in water, as shown in Fig.
12.3.
• Observe what happens in both the
cases.
• Discuss with your friends why this
happens.
Fig. 12.1: Vibrating tuning fork just touching the
suspended table tennis ball.
Fig. 12.2:  One of the prongs of the vibrating tuning
fork touching the water surface.
12 12
12 12 12
S S S S SOUND OUND OUND OUND OUND
Chapter
plucked vibrates and produces sound. If you
have never done this, then do it and observe
the vibration of the stretched rubber band.
Activity _____________12.3
• Make a list of different types of musical
instruments and discuss with your
friends which part of the instrument
vibrates to produce sound.
12.2 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
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
Fig. 12.3: 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
Fig. 12.4: A beam of light from a light source is made  to fall on a mirror . The reflected light is falling on the wall.
Can sound make a light spot dance?
Take a tin can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch
it over the can, then wrap a rubber band around the balloon. Take a small piece of mirror.
Use a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit
to fall on the mirror. After reflection the light spot is seen on the wall, as shown in Fig.
12.4. Talk or shout directly into the open end of the can and observe the dancing light spot
on the wall. Discuss with your friends what makes the light spot dance.
SOUND 161 SCIENCE 162
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. 12.5. 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. 12.5. 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
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?
12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL
Sound is a mechanical wave and needs a
material medium like air, water, steel etc. for
its propagation. It cannot travel through
vacuum, which can be demonstrated by the
following experiment.
Take an electric bell and an airtight glass
bell jar. The electric bell is suspended inside
the airtight bell jar. The bell jar is connected
to a vacuum pump, as shown in Fig. 12.6. If
you press the switch you will be able to hear
the bell. Now start the vacuum pump. When
the air in the jar is pumped out gradually,
the sound becomes fainter, although the
same current is passing through the bell.
After some time when less air is left inside
the bell jar you will hear a very feeble sound.
What will happen if the air is removed
completely? Will you still be able to hear the
sound of the bell?
Fig. 12.5: A vibrating object creating a series of
compressions (C) and rarefactions (R) in
the medium.
Q
Fig. 12.6: Bell jar experiment showing sound cannot
travel in vacuum.
SOUND 163
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 line 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. 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.
12.2.3 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. 12.8(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. 12.8(a) and Fig. 12.8(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. 12.8(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
uestions
1. Explain how sound is produced
by your school bell.
2. Why are sound waves called
mechanical waves?
3. Suppose you and your friend are
on the moon. Will you be able to
hear any sound produced by
your friend?
12.2.2 SOUND WAVES ARE LONGITUDINAL
WAVES
Activity _____________12.4
• Take a slinky.  Ask your friend to hold
one end. You hold the other end.
Now stretch the slinky as shown in
Fig. 12.7 (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.
Fig. 12.7: Longitudinal wave in a slinky.
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
Q
(a)
(b)
SCIENCE 164
Frequency tells us how frequently an
event occurs. Suppose you are beating a
drum. How many times you are beating the
drum per unit time is called the frequency of
your beating the drum. We know that when
sound is propagated through a medium, the
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, 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 in the density of
the medium is called the time period of the
valley, that is, the lower portion of the curve
in Fig. 12.8(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. 12.8(c), The wavelength is
usually represented by ? (Greek letter
lambda). Its SI unit is metre (m).
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. 12.8: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents
graphically the density and pressure variations.
H. R. Hertz
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