NCERT Textbook: Sound | Science Class 8 PDF Download

 Page 1


SOUND
SOUND
H
ow do you come to know that a
‘period’ is over in your school?
You come to know easily that
someone is at your door when he knocks
or you hear the sound of the doorbell.
Most of the time you can make out that
someone is approaching you by just
hearing the foot steps.
You might have played a game called
hide and seek. In this game a person is
blind-folded and has to catch the
remaining players. How is the blind-
folded person able to guess which player
is closest to her?
Sound plays an important role in our
lives. It helps us to communicate with
one another. We hear a variety of sounds
in our surroundings.
Make a list of sounds you hear in
your surroundings.
In the music room of your school you
hear the sounds produced by musical
instruments like flute, tabla,
harmonium, etc. (Fig. 10.1).
How is sound produced? How does it
travel from one place to another? How
do we hear sound? Why are some sounds
louder than others? We shall discuss
such questions in this chapter.
Fig. 10.1 : Some musical instruments
Tabla
Harmonium
Sitar
Flute
2024-25
Page 2


SOUND
SOUND
H
ow do you come to know that a
‘period’ is over in your school?
You come to know easily that
someone is at your door when he knocks
or you hear the sound of the doorbell.
Most of the time you can make out that
someone is approaching you by just
hearing the foot steps.
You might have played a game called
hide and seek. In this game a person is
blind-folded and has to catch the
remaining players. How is the blind-
folded person able to guess which player
is closest to her?
Sound plays an important role in our
lives. It helps us to communicate with
one another. We hear a variety of sounds
in our surroundings.
Make a list of sounds you hear in
your surroundings.
In the music room of your school you
hear the sounds produced by musical
instruments like flute, tabla,
harmonium, etc. (Fig. 10.1).
How is sound produced? How does it
travel from one place to another? How
do we hear sound? Why are some sounds
louder than others? We shall discuss
such questions in this chapter.
Fig. 10.1 : Some musical instruments
Tabla
Harmonium
Sitar
Flute
2024-25
SCIENCE 124
10.1 Sound is Produced by a
Vibrating Body
Touch the school bell when not in use.
What do you feel? Again touch it when
producing sound. Can you feel it
vibrating?
Activity 10.1
Take a metal plate (or a pan). Hang
it at a convenient place in such a
way that it does not touch any wall.
Now strike it with a stick (Fig.10.2).
Do you hear a sound? Touch the
plate or pan gently with your finger.
Do you feel the vibrations?
Fig. 10.2 : Striking a pan
Again strike the plate with the stick
and hold it tightly with your hands
immediately after striking. Do you
still hear the sound? Touch the
plate after it stops producing sound.
Can you feel the vibrations now?
As you learnt in Class VII the to and
fro or back and forth motion of an
object is called vibration. When a
tightly stretched band is plucked, it
vibrates and produces sound. When it
stops vibrating, it does not produce
any sound.
Activity 10.3
Take a metal dish. Pour water in it.
Strike it at its edge with a spoon
(Fig. 13.4). Do you hear a sound?
Again strike the dish and then
touch it. Can you feel the dish
vibrating? Strike the dish again.
Look at the surface of water. Do you
see any waves there? Now hold the
dish. What change do you observe
on the surface of water? Can you
explain the change? Is there a hint
to connect sound with the
vibrations of a body?
Activity 10.2
Take a rubber band. Put it around
the longer side of a pencil box
(Fig. 10.3). Insert two pencils
between the box and the stretched
rubber. Now, pluck the rubber band
somewhere in the middle. Do you
hear any sound? Does the band
vibrate?
Fig. 10.3 : Plucking the rubber band
2024-25
Page 3


SOUND
SOUND
H
ow do you come to know that a
‘period’ is over in your school?
You come to know easily that
someone is at your door when he knocks
or you hear the sound of the doorbell.
Most of the time you can make out that
someone is approaching you by just
hearing the foot steps.
You might have played a game called
hide and seek. In this game a person is
blind-folded and has to catch the
remaining players. How is the blind-
folded person able to guess which player
is closest to her?
Sound plays an important role in our
lives. It helps us to communicate with
one another. We hear a variety of sounds
in our surroundings.
Make a list of sounds you hear in
your surroundings.
In the music room of your school you
hear the sounds produced by musical
instruments like flute, tabla,
harmonium, etc. (Fig. 10.1).
How is sound produced? How does it
travel from one place to another? How
do we hear sound? Why are some sounds
louder than others? We shall discuss
such questions in this chapter.
Fig. 10.1 : Some musical instruments
Tabla
Harmonium
Sitar
Flute
2024-25
SCIENCE 124
10.1 Sound is Produced by a
Vibrating Body
Touch the school bell when not in use.
What do you feel? Again touch it when
producing sound. Can you feel it
vibrating?
Activity 10.1
Take a metal plate (or a pan). Hang
it at a convenient place in such a
way that it does not touch any wall.
Now strike it with a stick (Fig.10.2).
Do you hear a sound? Touch the
plate or pan gently with your finger.
Do you feel the vibrations?
Fig. 10.2 : Striking a pan
Again strike the plate with the stick
and hold it tightly with your hands
immediately after striking. Do you
still hear the sound? Touch the
plate after it stops producing sound.
Can you feel the vibrations now?
As you learnt in Class VII the to and
fro or back and forth motion of an
object is called vibration. When a
tightly stretched band is plucked, it
vibrates and produces sound. When it
stops vibrating, it does not produce
any sound.
Activity 10.3
Take a metal dish. Pour water in it.
Strike it at its edge with a spoon
(Fig. 13.4). Do you hear a sound?
Again strike the dish and then
touch it. Can you feel the dish
vibrating? Strike the dish again.
Look at the surface of water. Do you
see any waves there? Now hold the
dish. What change do you observe
on the surface of water? Can you
explain the change? Is there a hint
to connect sound with the
vibrations of a body?
Activity 10.2
Take a rubber band. Put it around
the longer side of a pencil box
(Fig. 10.3). Insert two pencils
between the box and the stretched
rubber. Now, pluck the rubber band
somewhere in the middle. Do you
hear any sound? Does the band
vibrate?
Fig. 10.3 : Plucking the rubber band
2024-25
SOUND 125
We see that a vibrating object
produces sound. In some cases, the
vibrations are easily visible to us. But
in most cases, their amplitude is so small
that we cannot see them. However, we
can feel them.
Activity 10.4
Take a hollow coconut shell and
make a musical instrument ektara.
You can also make it with the help
of an earthen pot (Fig. 10.5). Play
this instrument and identify its
vibrating part.
Table 10.1 : Musical Instruments and
their Vibrating Parts
S.No. Musical Vibrating Part
Instrument Producing Sound
1. Veena Stretched string
2. Tabla Stretched
membrane
3.
4.
5.
6.
7.
Many of you might have seen the
manjira (cymbals), the ghatam, and the
noot  (mudpots) and the kartal. These
musical instruments are commonly
used in many parts of our country. These
instruments are simply beaten or struck
(Fig. 10.6). Can you name a few other
musical instruments of this type?
You too can make a musical
instrument.
Fig. 10.4 : Vibrating dish produces waves
in water
Fig. 10.6 : A few more musical instruments
Ghatam
Make a list of familiar musical
instruments and identify their vibrating
parts. A few examples are given in
Table 10.1. Complete rest of the Table.
Fig. 10.5 : Ektara
Manjira
2024-25
Page 4


SOUND
SOUND
H
ow do you come to know that a
‘period’ is over in your school?
You come to know easily that
someone is at your door when he knocks
or you hear the sound of the doorbell.
Most of the time you can make out that
someone is approaching you by just
hearing the foot steps.
You might have played a game called
hide and seek. In this game a person is
blind-folded and has to catch the
remaining players. How is the blind-
folded person able to guess which player
is closest to her?
Sound plays an important role in our
lives. It helps us to communicate with
one another. We hear a variety of sounds
in our surroundings.
Make a list of sounds you hear in
your surroundings.
In the music room of your school you
hear the sounds produced by musical
instruments like flute, tabla,
harmonium, etc. (Fig. 10.1).
How is sound produced? How does it
travel from one place to another? How
do we hear sound? Why are some sounds
louder than others? We shall discuss
such questions in this chapter.
Fig. 10.1 : Some musical instruments
Tabla
Harmonium
Sitar
Flute
2024-25
SCIENCE 124
10.1 Sound is Produced by a
Vibrating Body
Touch the school bell when not in use.
What do you feel? Again touch it when
producing sound. Can you feel it
vibrating?
Activity 10.1
Take a metal plate (or a pan). Hang
it at a convenient place in such a
way that it does not touch any wall.
Now strike it with a stick (Fig.10.2).
Do you hear a sound? Touch the
plate or pan gently with your finger.
Do you feel the vibrations?
Fig. 10.2 : Striking a pan
Again strike the plate with the stick
and hold it tightly with your hands
immediately after striking. Do you
still hear the sound? Touch the
plate after it stops producing sound.
Can you feel the vibrations now?
As you learnt in Class VII the to and
fro or back and forth motion of an
object is called vibration. When a
tightly stretched band is plucked, it
vibrates and produces sound. When it
stops vibrating, it does not produce
any sound.
Activity 10.3
Take a metal dish. Pour water in it.
Strike it at its edge with a spoon
(Fig. 13.4). Do you hear a sound?
Again strike the dish and then
touch it. Can you feel the dish
vibrating? Strike the dish again.
Look at the surface of water. Do you
see any waves there? Now hold the
dish. What change do you observe
on the surface of water? Can you
explain the change? Is there a hint
to connect sound with the
vibrations of a body?
Activity 10.2
Take a rubber band. Put it around
the longer side of a pencil box
(Fig. 10.3). Insert two pencils
between the box and the stretched
rubber. Now, pluck the rubber band
somewhere in the middle. Do you
hear any sound? Does the band
vibrate?
Fig. 10.3 : Plucking the rubber band
2024-25
SOUND 125
We see that a vibrating object
produces sound. In some cases, the
vibrations are easily visible to us. But
in most cases, their amplitude is so small
that we cannot see them. However, we
can feel them.
Activity 10.4
Take a hollow coconut shell and
make a musical instrument ektara.
You can also make it with the help
of an earthen pot (Fig. 10.5). Play
this instrument and identify its
vibrating part.
Table 10.1 : Musical Instruments and
their Vibrating Parts
S.No. Musical Vibrating Part
Instrument Producing Sound
1. Veena Stretched string
2. Tabla Stretched
membrane
3.
4.
5.
6.
7.
Many of you might have seen the
manjira (cymbals), the ghatam, and the
noot  (mudpots) and the kartal. These
musical instruments are commonly
used in many parts of our country. These
instruments are simply beaten or struck
(Fig. 10.6). Can you name a few other
musical instruments of this type?
You too can make a musical
instrument.
Fig. 10.4 : Vibrating dish produces waves
in water
Fig. 10.6 : A few more musical instruments
Ghatam
Make a list of familiar musical
instruments and identify their vibrating
parts. A few examples are given in
Table 10.1. Complete rest of the Table.
Fig. 10.5 : Ektara
Manjira
2024-25
SCIENCE 126
10.2 Sound Produced by
Humans
Speak loudly for a while or sing a song,
or buzz like a bee. Put your hand on
your throat as shown in Fig. 10.8. Do
you feel any vibrations?
In humans, the sound is produced
by the voice box or the larynx. Put your
fingers on the throat and find a hard
bump that seems to move when you
swallow. This part of the body is known
as the voice box. It is at the upper end
of the windpipe. Two vocal cords, are
stretched across the voice box or larynx
in such a way that it leaves a narrow
slit between them for the passage of air
(Fig.10.8).
Fig. 10.7 : Jaltrang
When we pluck the string of an
instrument, like the sitar, the sound
that we hear is not only that of the
string. The whole instrument is forced
to vibrate, and it is the sound of the
vibration of the instrument that we
hear. Similarly, when we strike the
membrane of a mridangam, the sound
that we hear is not only that of the
membrane but of the whole body of
the instrument.
When we speak, does
any part of our body
vibrate?
Fig.10.8 : Voice box in humans
When the lungs force air through the
slit, the vocal cords vibrate, producing
sound. Muscles attached to the vocal
cords can make the cords tight or loose.
When the vocal cords are tight and thin,
the type or quality of voice is different
Activity 10.5
Take 6-8 bowls or tumblers. Fill
them with water up to  different
levels, increasing gradually from
one end to the other. Now take
a  pencil and strike the bowls gently.
Strike all of them in succession. You
will hear pleasant sounds. This is
your jaltrang (Fig.10.7).
2024-25
Page 5


SOUND
SOUND
H
ow do you come to know that a
‘period’ is over in your school?
You come to know easily that
someone is at your door when he knocks
or you hear the sound of the doorbell.
Most of the time you can make out that
someone is approaching you by just
hearing the foot steps.
You might have played a game called
hide and seek. In this game a person is
blind-folded and has to catch the
remaining players. How is the blind-
folded person able to guess which player
is closest to her?
Sound plays an important role in our
lives. It helps us to communicate with
one another. We hear a variety of sounds
in our surroundings.
Make a list of sounds you hear in
your surroundings.
In the music room of your school you
hear the sounds produced by musical
instruments like flute, tabla,
harmonium, etc. (Fig. 10.1).
How is sound produced? How does it
travel from one place to another? How
do we hear sound? Why are some sounds
louder than others? We shall discuss
such questions in this chapter.
Fig. 10.1 : Some musical instruments
Tabla
Harmonium
Sitar
Flute
2024-25
SCIENCE 124
10.1 Sound is Produced by a
Vibrating Body
Touch the school bell when not in use.
What do you feel? Again touch it when
producing sound. Can you feel it
vibrating?
Activity 10.1
Take a metal plate (or a pan). Hang
it at a convenient place in such a
way that it does not touch any wall.
Now strike it with a stick (Fig.10.2).
Do you hear a sound? Touch the
plate or pan gently with your finger.
Do you feel the vibrations?
Fig. 10.2 : Striking a pan
Again strike the plate with the stick
and hold it tightly with your hands
immediately after striking. Do you
still hear the sound? Touch the
plate after it stops producing sound.
Can you feel the vibrations now?
As you learnt in Class VII the to and
fro or back and forth motion of an
object is called vibration. When a
tightly stretched band is plucked, it
vibrates and produces sound. When it
stops vibrating, it does not produce
any sound.
Activity 10.3
Take a metal dish. Pour water in it.
Strike it at its edge with a spoon
(Fig. 13.4). Do you hear a sound?
Again strike the dish and then
touch it. Can you feel the dish
vibrating? Strike the dish again.
Look at the surface of water. Do you
see any waves there? Now hold the
dish. What change do you observe
on the surface of water? Can you
explain the change? Is there a hint
to connect sound with the
vibrations of a body?
Activity 10.2
Take a rubber band. Put it around
the longer side of a pencil box
(Fig. 10.3). Insert two pencils
between the box and the stretched
rubber. Now, pluck the rubber band
somewhere in the middle. Do you
hear any sound? Does the band
vibrate?
Fig. 10.3 : Plucking the rubber band
2024-25
SOUND 125
We see that a vibrating object
produces sound. In some cases, the
vibrations are easily visible to us. But
in most cases, their amplitude is so small
that we cannot see them. However, we
can feel them.
Activity 10.4
Take a hollow coconut shell and
make a musical instrument ektara.
You can also make it with the help
of an earthen pot (Fig. 10.5). Play
this instrument and identify its
vibrating part.
Table 10.1 : Musical Instruments and
their Vibrating Parts
S.No. Musical Vibrating Part
Instrument Producing Sound
1. Veena Stretched string
2. Tabla Stretched
membrane
3.
4.
5.
6.
7.
Many of you might have seen the
manjira (cymbals), the ghatam, and the
noot  (mudpots) and the kartal. These
musical instruments are commonly
used in many parts of our country. These
instruments are simply beaten or struck
(Fig. 10.6). Can you name a few other
musical instruments of this type?
You too can make a musical
instrument.
Fig. 10.4 : Vibrating dish produces waves
in water
Fig. 10.6 : A few more musical instruments
Ghatam
Make a list of familiar musical
instruments and identify their vibrating
parts. A few examples are given in
Table 10.1. Complete rest of the Table.
Fig. 10.5 : Ektara
Manjira
2024-25
SCIENCE 126
10.2 Sound Produced by
Humans
Speak loudly for a while or sing a song,
or buzz like a bee. Put your hand on
your throat as shown in Fig. 10.8. Do
you feel any vibrations?
In humans, the sound is produced
by the voice box or the larynx. Put your
fingers on the throat and find a hard
bump that seems to move when you
swallow. This part of the body is known
as the voice box. It is at the upper end
of the windpipe. Two vocal cords, are
stretched across the voice box or larynx
in such a way that it leaves a narrow
slit between them for the passage of air
(Fig.10.8).
Fig. 10.7 : Jaltrang
When we pluck the string of an
instrument, like the sitar, the sound
that we hear is not only that of the
string. The whole instrument is forced
to vibrate, and it is the sound of the
vibration of the instrument that we
hear. Similarly, when we strike the
membrane of a mridangam, the sound
that we hear is not only that of the
membrane but of the whole body of
the instrument.
When we speak, does
any part of our body
vibrate?
Fig.10.8 : Voice box in humans
When the lungs force air through the
slit, the vocal cords vibrate, producing
sound. Muscles attached to the vocal
cords can make the cords tight or loose.
When the vocal cords are tight and thin,
the type or quality of voice is different
Activity 10.5
Take 6-8 bowls or tumblers. Fill
them with water up to  different
levels, increasing gradually from
one end to the other. Now take
a  pencil and strike the bowls gently.
Strike all of them in succession. You
will hear pleasant sounds. This is
your jaltrang (Fig.10.7).
2024-25
SOUND 127
from that when they are loose and
thick. Let us see how the vocal cords
function.
Activity 10.6
Take two rubber strips of the same
size. Place these two pieces one
above the other and stretch them
tight. Now blow air through the gap
between them [Fig. 10.9(a)]. As the
air blows through the stretched
rubber strips, a sound is produced.
You can also take a piece of paper
with a narrow slit and hold it
between your fingers as shown in
Fig. 10.9 (b). Now blow through the
slit and listen to the sound. Our
vocal cords produce sound in a
similar manner.
(a)
(b)
Fig. 10.9 (a), (b) : Working of vocal cords
The vocal cords in men are about
20 mm long. In women these are about
15mm long. Children have very short
vocal cords. This is the reason why
the voices of men, women and
children are different.
10.3 Sound Needs a Medium
for Propagation
When you call up your friend who is
standing at a distance, your friend is
able to hear your voice. How does the
sound propagate or  travel to her?
Activity 10.7
Take a metal  or glass tumbler. Make
sure that it is dry. Place a cell phone
in it. (Remember that the cell phone
must not be kept in water .) Ask your
friend to give a ring on this cell
phone from another cell phone.
Listen to the ring carefully.
Now, surround the rim of the
tumbler with your hands
(Fig. 10.10). Put your mouth on the
Fig. 10.10 : Sound needs a medium to travel
2024-25