NCERT Solutions for Science Class 8 Chapter 13 – Sound

4 .solids, liquids and gases.

Sound can travel in any medium.

3 . A man

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

 

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.

 

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.

 

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�

 

Time period is the inverse of frequency of an object.

Therefore,

T=

1

frequency

�=1���������

T=

1

500

�=1500

∴T=0.002s

∴�=0.002�

 

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.

 

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.

 

 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.

 

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.

 

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.

 

 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

 

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 because speed of light is greater than the speed of sound and therefore, light travels faster than thunder.