Class 8thA (Science)

Chapter 13- Sound

NCERT Textbook Questions

1. Choose the correct answer.

Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids, and gases
Answer:(d) solids, liquids, and gases.

2. Voice of which of the following is likely to have a minimum frequency?

(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Answer: (c) A man

3. In the following statements, tick ‘T’ against those which are true and
‘F’ against those which are false.

1. Sound cannot travel in a vacuum. (T)

2. The number of oscillations per second of a vibrating object is called its
time period. (F)
3. If the amplitude of vibration is large, the sound is feeble. (F)
4. For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
5. The lower the frequency of vibration, the higher is the pitch. (F)
6. Unwanted or unpleasant sound is termed as music. (F)
7. Noise pollution may cause partial hearing impairment. (T)

4. Fill in the blanks with suitable words.

1. Time taken by an object to complete one oscillation is called Time period.

2. Loudness is determined by the Amplitude of vibration.
3. The unit of frequency is Hertz (Hz).
4. Unwanted sound is called Noise.
5. The shrillness of a sound is determined by the Frequency of vibration.

5. A pendulum oscillates 40 times in 4 seconds. Find its time period and

frequency.
A. No. of oscillation = 40
Total time is taken = 4 seconds

6. The sound from a mosquito is produced when it vibrates its wings at an

average rate of 500 vibrations per second. What is the time period of the
vibration?

A. Number of vibrations per second = 500

7. Identify the part which vibrates to produce sound in the following

instruments.

1. Dholak- Stretched membrane

2. Sitar- String of sitar
3. Flute - Air column

8. What is the difference between noise and music? Can music become
noise sometimes?
A. The type of sound which are unpleasant to listen is known as noise whereas
music is a pleasant sound, which produces a sensation.
Yes, music can become noise when it’s too loud.

9. List the sources of noise pollution in your surroundings.

A. Following are the major sources of noise pollution:

 Sound of vehicles
 Sound of kitchen appliances
 Sound of bursting crackers
  Sound of loudspeakers, TV, transistors

10. Explain in what way noise pollution is harmful to humans.

A. Noise pollution causes:
(a) Lack of sleep
(b) Anxiety
(c) Hypertension
(d) A person who is exposed to a loud sound continuously may get
temporary or even permanent impairment of hearing.
and thus it is harmful to health.
11. Your parents are going to buy a house. They have been offered one on
the roadside and another three lanes away from the roadside. Which
house would you suggest your parents should buy? Explain your answer.
Answer:
I would suggest my parents buy a house three lanes away from the roadside
because house on the roadside would be much noisy in both days and night
due to running vehicles. Whereas, a house three lanes away would be
comparatively quieter as the intensity of noise decreases with the distance
between the source and the listener.

12. Sketch larynx and explain its function in your own words.
Answer:
Larynx is also known as 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 passage of air (Fig. 13.12). When lung
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
from that when they are loose and thick.
13. Lightning and thunder take place in the sky at the same time and at
the same distance from us. Lightning is seen earlier and thunder is heard
later. Can you explain why?
Answer:
The speed of light is more than that of the speed of sound. Thus, due to more
speed of light it reaches us before sound. So, lightning is seen earlier and
thunder is heard later.

Q. How do we hear?
The shape of the outer part of the ear is like a funnel. When sound enters in it,
it travels down a canal at the end of which a thin membrane is stretched
tightly. It is called the eardrum. The eardrum is like a stretched rubber
sheet. Sound vibrations make the eardrum vibrate. The eardrum sends
vibrations to the inner ear. From there, the signal goes to the brain. That is
how we hear.

Q. What is inaudible sound?

A. Sounds of frequencies less than about 20 vibrations per second (20 Hz) and
higher than about 20,000 vibrations per second (20 kHz) cannot be detected by the
human ear. Such sounds are called inaudible.

Q. What is the range of audible frequencies in humans?

A. Thus, for human ear, the range of audible frequencies is roughly from 20
to 20,000 Hz.

Q. How can the noise pollution be controlled in a residential area?

A. The noisy operations must be conducted away from any residential
area. Noise producing industries should be set up away from such
areas. Use of automobile horns should be minimized. TV and music
systems should be run at low volumes. Trees must be planted along the
roads and around buildings to cut down on the sounds reaching the
residents, thus reducing the harmful effects of noise pollution.
Q. Define a vibration.
A. Vibration is the to and fro or back and forth motion of an object.

Q. Which part of the human body is responsible for producing sound?

A. In humans, the sound is produced by the voice box or larynx

Q. What is the length of vocal cords in men?

A. The vocal cords in men are about 20 mm long.

Q. Can sound travel in vacuum?

A. No, sound cannot travel in vacuum.

Q. What is meant by oscillatory motion?

A. The to and fro motion of an object is known as oscillatory motion.

Q. Define frequency.
A. The number of oscillations per second is called the frequency of oscillation.

Q. Define 1 hertz.
A. A frequency of 1 hertz means one oscillation per second.

Q. How is frequency of a sound and pitch related?

A. If the frequency of vibration is higher than the sound has a higher pitch.

Q. Whose voice has a higher frequency – man or woman?

A. The voice of woman has higher frequency.

Q. What is range of audible sound?

A. Sound of frequency 20 Hz to 20,000 Hz is the audible range.

Q. Which animal can hear sounds of frequencies higher than 20,000 Hz?
A. Dogs can hear frequencies higher than 20,000 Hz.

Q. What is meant by noise pollution?

A. Presence of excessive or unwanted sound in the atmosphere is called noise
pollution.

Q. If the frequency of a sound is below 20 Hz, will it be audible to human

beings?
A. No, it will not be audible.

Q. Why do we hear the sound of the horn of an approaching car before the
car reaches us?
A. This happens because the speed of sound is much greater than the speed of
the car.

Q. How can the noise pollution be controlled in residential area?

Answer:

1. The noisy operations must be conducted away from residential areas.

2. Noise producing industries should be set away from such areas.
3. Use of automobile horns be minimized.
4. TV and music systems should be run at lower volumes.

NCERT In text Activities Solved

Activity 1 (NCERT Textbook, Page 158)

Take a metal plate (or a shallow 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. 13.1). Touch
the plate or pan gently with your finger. Do you feel the vibrations? 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 vibrations now?

Solution:
When we touch the pan gently with our finger after striking we feel the
vibration. When we hold the pan tightly after striking it, we do not hear the
sound. When the pan stops producing sound it also stops vibrating. Thus, we
can conclude that vibrating body produces sound.

Activity 2 (NCERT Textbook, Page 758)

Jake a rubber band. Put it around the, longer side of a pencil box (Fig. 13.2).
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?

Solution:
Yes, we hear the sound on plucking the rubber band. Also, we find that the
band is vibrating. Thus, all vibrating bodies produce sound.

Activity 3 (NCERT Textbook, Page 758-759)

Take a metal dish. Pour water in it. Strike it at its edge with a spoon (Fig. 13.3).
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?

Solution:
On striking the metal dish we hear sound and on touching it we feel the dish
vibrating. Striking the dish with water we see circular wave are produced. Thus
vibrating object produces sound.

Activity 4 (NCERT Textbook, Page 159)

Take a hollow coconut shell and make a musical instrument ektara. You can
also make it with the help of an earthen pot (Fig. 13.4). Play this instrument
and identify its vibrating part.

Solution:
We observed that the vibrating part of the musical instrument ektara is
stretched string.

Activity 5 (NCERT Textbook, Page 160)

Take 6-8 bowls or tumblers. Fill them with water upto 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 Jaltarang (Fig. 13.5).
Solution:
We can hear a pleasant sound. This is due to different levels of water in the
bowls.
Thus, we find that shorter the length of the vibrating air column, higher is the
pitch of the sound produced.

Activity 6 (NCERT Textbook, Page 161)

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.
13.6(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. 13.6(b)]. Now blow through the slit and
listen to the sound.

Solution:
This activity shows that vocal cords also produce sound in a similar manner
when they vibrate.

Activity 7 (NCERT Textbook, Page 161)

Take a metal or glass tumbler. Make sure that it is dry. Place a cell phone in it.
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. 13.7). Put your
mouth on the opening between your hands. Indicate to your friend to give a
ring again. Listen to the ring while sucking air from the tumbler. Does the
sound become fainter as you suck air?
Remove the tumbler from your mouth. Does the sound become loud again?
Solution:
We observed that sound becomes fainter than earlier when we try to suck air.
But when we remove tumbler from our mouth the sound again becomes loud.
Thus, sound needs a medium to travel.

Activity 8 (NCERT Textbook, Page 162)

Take a bucket or a bathtub. Fill it with clean water.
Take a small bell in one hand. Shake this bell inside the water to produce
sound. Make sure that the bell does not touch the body of the bucket or the
tub. Place your ear gently on the water surface (Fig. 13.8). Can you hear the
sound of the bell? Does it indicate that sound can travel through liquids?

Solution:
We can hear the sound of the bell which indicates that sound can travel
through liquids.

Activity 9 (NCERT Textbook, Page 162)

Take a metre scale or a long metal rod and hold its one end to your ear. Ask
your friend to gently scratch or tap at the other end of the scale (Fig. 13.9).
Can you hear the sound of the scratching? Ask your friends around you if they
were able to hear the same sound?
Solution:
Yes, we find that we can hear the sound of the scratch. But, the people
standing around us cannot hear the same sound or we can say that it is
limping not audible to them.

Activity 10 (NCERT Textbook, Page 163)

Take a plastic or tin can. Cut its ends. Stretch a piece of rubber balloon across
one end of the can and fasten it with a rubber band. Put four or five grains of
dry cereal on the stretched rubber. Now ask your friend to speak ”Hurrey,
Hurrey” from the open end (Fig. 13.10). Observe what happens to the grain.
Why do the grain jump up and down?

Solution:
The grain jump up and down because of the vibration caused underneath the
stretched rubber. Thus when sound waves fall on the eardrum, it starts
vibrating back and forth rapidly.

Activity 11 (NCERT Textbook, Page 164-165)

Take a metallic tumbler and a tablespoon. Strike the tablespoon gently at the
brim of the tumbler. Hear the sound produced. Now bang the spoon on the
tumbler and hear the sound produced again. Is the sound louder when the
tumbler is struck hard?
Now suspend a small thermocol ball touching the rim of the tumbler (Fig.
13.11). Vibrate the tumbler by striking it. See how far the ball is displaced. The
displacement of the ball is a measure of the amplitude of vibration of the
tumbler.
Now, strike the tumbler gently and then with some force. Compare the
amplitudes of vibrations of the tumbler in the two cases. In which case is the
amplitude larger?
Solution:
The sound produced is louder when the tumbler is struck hard. The amplitude
of vibration of the tumbler is larger when the glass is struck hard. Thus the
loudness of sound depends upon the amplitude of vibration.