NCERT Solutions for Science Class 8 Chapter 13 – Sound

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Access Answers to NCERT Solutions for Science Class 8 Chapter 13 – Sound

Sound

Question 1 :

Sound can travel through

1. gases only

2. solids only

3. liquids only

4. solids, liquids and gases.

4 .solids, liquids and gases.

Sound can travel in any medium.

Question 2 :

Which of the following voices is likely to have minimum frequency?

1. Baby girl

2. Baby boy

3. A man

4. A woman

3 . A man

Frequency of a sound is directly proportional to pitch. Here, the pitch of a man is minimum.

Question 3 :

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

a) Sound cannot travel in vacuum.

b) The number of oscillations per second of a vibrating object is called its time period.

c) If the amplitude of vibration is large, sound is feeble.

d) For human ears, the audible range is

20

20

Hz to

20,000

20,000

Hz.

e) The lower the frequency of vibration, the higher is the pitch.

f) Unwanted or unpleasant sound is termed as music.

g) Noise pollution may cause partial hearing impairment.

a) . T

Vacuum is an enclosed place in which there are no molecules or matter. Therefore, sound cannot travel through a vacuum where there is no molecule or atom to induce vibrations.

b) .F

The number of oscillations per second of a vibrating object is known as its frequency.

c) .  F

Sound is feeble for small amplitudes.

d) . T

For human ears, the audible range is

20

20

Hz to

20,000

20,000

Hz.

e) . F

The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases and vice versa.

g) . T

If one is subjected to loud unpleasant sound continuously for a long time, then it may cause temporary hearing impairment.

Question 4 :

Fill in the blanks with suitable words.

a) Time taken by an object to complete one oscillation is called _________.

b) Loudness is determined by the ________ of vibration.

c) The unit of frequency is _________.

d) Unwanted sound is called _________.

e) Shrillness of a sound is determined by the _________ of vibration.

a) . Time taken by an object to complete one oscillation is called time period.

b) . Loudness is determined by the amplitude of vibration.

c) .The unit of frequency is hertz (Hz).

d) . Unwanted sound is called noise.

e) . Shrillness of a sound is determined by the frequency of vibration.

Question 5 :

A pendulum oscillates

40

40

times in

4

4

seconds. Find its time period and frequency.

Time period is defined as the time required to complete one oscillation.

Frequency is defined as the number of oscillations per unit time.

Here,

Time

t=4s

�=4�

Number of oscillations

n=40

�=40

Frequency

f=

n

t

�=��

Thus,

f=

40

4

�=404

∴f=10

∴�=10

Hz

Time period is the inverse of frequency of an object.

Therefore,

T=

1

f

�=1�

T=

1

10

�=110

∴T=0.1s

∴�=0.1�

Question 6 :

The sound from a mosquito is produced when it vibrates its wings at an average rate of

500

500

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

Time period is the inverse of frequency of an object.

Therefore,

T=

1

frequency

�=1���������

T=

1

500

�=1500

∴T=0.002s

∴�=0.002�

Question 7 :

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

a) Dholak

b) Sitar

c) Flute

a) . Dholak has stretched membranes called heads that vibrate to produce sound.

b) .In sitar, the stretched strings are plucked and when they vibrate, they produce sound.

c) . In flute, the air column inside it vibrates when blown which produces sound.

Question 8 :

What is the difference between noise and music? Can music become noise sometimes?

Noise is an unpleasant sound which is a result of irregular vibrations. Music is a sound which is pleasant to hear. Yes, music can sometimes become noise when it is played at very high volumes on loudspeakers.

Question 9 :

List sources of noise pollution in your surroundings.

Following are the major sources of noise pollution:

1. Sounds of vehicle horns.

2. Explosions that include bursting of crackers and explosives.

3. Sounds of machines and loudspeakers.

4. Sounds of television and transistor radio at high volumes.

5. Sounds of kitchen appliances.

Question 10 :

Explain in what way noise pollution is harmful to humans:

Noise pollution is harmful to humans as it may cause many health-related problems; like insomnia, hypertension, headache, stress and may even lead to loss of hearing.

Question 11 :

I would suggest that the house be three lanes away from the roadside because the house next to the roadside will be more noisy compared to the house three lanes away from the road.

Question 12 :

Sketch the larynx and explain its function in your own words.

Larynx is a part of the throat. It moves when we swallow something. There are two vocal cords inside the Larynx. The air passes through a small gap between them. The lungs force the air into the space when we speak and this vibrates the vocal cord, resulting in the production of sound. That is why the larynx is also known as a sound box.

Side and Top view of Larynx new

Question 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?