Chapter 12
SOUND
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 Fig. 12.1: Vibrating tuning fork just touching the
chapter we are going to learn how sound is suspended table tennis ball.
produced and how it is transmitted through
a medium and received by our ears.
Activity _____________ 12.2
12.1 Production of Sound • 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
Activity _____________ 12.1 tuning fork, as shown in Fig. 12.2.
• Take a tuning fork and set it vibrating • Next dip the prongs of the vibrating
by striking its prong on a rubber pad. tuning fork in water, as shown in Fig.
Bring it near your ear. 12.3.
• Do you hear any sound? • Observe what happens in both the
• Touch one of the prongs of the vibrating cases.
tuning fork with your finger and share • Discuss with your friends why this
your experience with your friends. happens.
• 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
pr ong of a vibrating tuning
fork (Fig. 12.1).
• Observe what happens and discuss
with your friends. Fig. 12.2: One of the prongs of the vibrating tuning
fork touching the water surface.
2020-21
, 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.
Fig. 12.3: Both the prongs of the vibrating tuning fork
12.2 Propagation of Sound
dipped in water. Sound is produced by vibrating objects. The
matter or substance through which sound
From the above activities what do you is transmitted is called a medium. It can be
conclude? Can you produce sound without solid, liquid or gas. Sound moves through a
a vibrating object? medium from the point of generation to the
In the above activities we have produced listener. When an object vibrates, it sets the
sound by striking the tuning fork. We can particles of the medium around it vibrating.
also produce sound by plucking, scratching, The particles do not travel all the way from
rubbing, blowing or shaking different objects. the vibrating object to the ear. A particle of
As per the above activities what do we do to the medium in contact with the vibrating
the objects? We set the objects vibrating and object is first displaced from its equilibrium
produce sound. Vibration means a kind of position. It then exerts a force on the adjacent
rapid to and fro motion of an object. The particle. As a result of which the adjacent
sound of the human voice is produced due particle gets displaced from its position of
to vibrations in the vocal cords. When a bird rest. After displacing the adjacent particle the
flaps its wings, do you hear any sound? Think first particle comes back to its original
position. This process continues in the
how the buzzing sound accompanying a bee
medium till the sound reaches your ear. The
is produced. A stretched rubber band when
disturbance created by a source of sound in
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.
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.
SOUND 161
2020-21
, the medium travels through the medium and
Q
not the particles of the medium.
uestion
A wave is a disturbance that moves 1. How does the sound produced by
through a medium when the particles of the a vibrating object in a medium
medium set neighbouring particles into reach your ear?
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 12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL
medium, hence sound can be visualised as a
wave. Sound waves are characterised by the Sound is a mechanical wave and needs a
motion of particles in the medium and are material medium like air, water, steel etc. for
called mechanical waves. its propagation. It cannot travel through
Air is the most common medium through vacuum, which can be demonstrated by the
which sound travels. When a vibrating object following experiment.
moves forward, it pushes and compresses the Take an electric bell and an airtight glass
air in front of it creating a region of high bell jar. The electric bell is suspended inside
pressure. This region is called a compression
the airtight bell jar. The bell jar is connected
(C), as shown in Fig. 12.5. This compression
to a vacuum pump, as shown in Fig. 12.6. If
starts to move away from the vibrating object.
When the vibrating object moves backwards, you press the switch you will be able to hear
it creates a region of low pressure called the bell. Now start the vacuum pump. When
rarefaction (R), as shown in Fig. 12.5. As the the air in the jar is pumped out gradually,
object moves back and forth rapidly, a series the sound becomes fainter, although the
of compressions and rarefactions is created same current is passing through the bell.
in the air. These make the sound wave that After some time when less air is left inside
propagates through the medium.
the bell jar you will hear a very feeble sound.
Compression is the region of high pressure
What will happen if the air is removed
and rarefaction is the region of low pressure.
Pressure is related to the number of particles completely? Will you still be able to hear the
of a medium in a given volume. More density sound of the bell?
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.
Fig. 12.5: A vibrating object creating a series of
Fig. 12.6: Bell jar experiment showing sound cannot
compressions (C) and rarefactions (R) in
travel in vacuum.
the medium.
162 SCIENCE
2020-21
SOUND
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 Fig. 12.1: Vibrating tuning fork just touching the
chapter we are going to learn how sound is suspended table tennis ball.
produced and how it is transmitted through
a medium and received by our ears.
Activity _____________ 12.2
12.1 Production of Sound • 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
Activity _____________ 12.1 tuning fork, as shown in Fig. 12.2.
• Take a tuning fork and set it vibrating • Next dip the prongs of the vibrating
by striking its prong on a rubber pad. tuning fork in water, as shown in Fig.
Bring it near your ear. 12.3.
• Do you hear any sound? • Observe what happens in both the
• Touch one of the prongs of the vibrating cases.
tuning fork with your finger and share • Discuss with your friends why this
your experience with your friends. happens.
• 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
pr ong of a vibrating tuning
fork (Fig. 12.1).
• Observe what happens and discuss
with your friends. Fig. 12.2: One of the prongs of the vibrating tuning
fork touching the water surface.
2020-21
, 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.
Fig. 12.3: Both the prongs of the vibrating tuning fork
12.2 Propagation of Sound
dipped in water. Sound is produced by vibrating objects. The
matter or substance through which sound
From the above activities what do you is transmitted is called a medium. It can be
conclude? Can you produce sound without solid, liquid or gas. Sound moves through a
a vibrating object? medium from the point of generation to the
In the above activities we have produced listener. When an object vibrates, it sets the
sound by striking the tuning fork. We can particles of the medium around it vibrating.
also produce sound by plucking, scratching, The particles do not travel all the way from
rubbing, blowing or shaking different objects. the vibrating object to the ear. A particle of
As per the above activities what do we do to the medium in contact with the vibrating
the objects? We set the objects vibrating and object is first displaced from its equilibrium
produce sound. Vibration means a kind of position. It then exerts a force on the adjacent
rapid to and fro motion of an object. The particle. As a result of which the adjacent
sound of the human voice is produced due particle gets displaced from its position of
to vibrations in the vocal cords. When a bird rest. After displacing the adjacent particle the
flaps its wings, do you hear any sound? Think first particle comes back to its original
position. This process continues in the
how the buzzing sound accompanying a bee
medium till the sound reaches your ear. The
is produced. A stretched rubber band when
disturbance created by a source of sound in
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.
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.
SOUND 161
2020-21
, the medium travels through the medium and
Q
not the particles of the medium.
uestion
A wave is a disturbance that moves 1. How does the sound produced by
through a medium when the particles of the a vibrating object in a medium
medium set neighbouring particles into reach your ear?
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 12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL
medium, hence sound can be visualised as a
wave. Sound waves are characterised by the Sound is a mechanical wave and needs a
motion of particles in the medium and are material medium like air, water, steel etc. for
called mechanical waves. its propagation. It cannot travel through
Air is the most common medium through vacuum, which can be demonstrated by the
which sound travels. When a vibrating object following experiment.
moves forward, it pushes and compresses the Take an electric bell and an airtight glass
air in front of it creating a region of high bell jar. The electric bell is suspended inside
pressure. This region is called a compression
the airtight bell jar. The bell jar is connected
(C), as shown in Fig. 12.5. This compression
to a vacuum pump, as shown in Fig. 12.6. If
starts to move away from the vibrating object.
When the vibrating object moves backwards, you press the switch you will be able to hear
it creates a region of low pressure called the bell. Now start the vacuum pump. When
rarefaction (R), as shown in Fig. 12.5. As the the air in the jar is pumped out gradually,
object moves back and forth rapidly, a series the sound becomes fainter, although the
of compressions and rarefactions is created same current is passing through the bell.
in the air. These make the sound wave that After some time when less air is left inside
propagates through the medium.
the bell jar you will hear a very feeble sound.
Compression is the region of high pressure
What will happen if the air is removed
and rarefaction is the region of low pressure.
Pressure is related to the number of particles completely? Will you still be able to hear the
of a medium in a given volume. More density sound of the bell?
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.
Fig. 12.5: A vibrating object creating a series of
Fig. 12.6: Bell jar experiment showing sound cannot
compressions (C) and rarefactions (R) in
travel in vacuum.
the medium.
162 SCIENCE
2020-21
