Sound

Exercises

Question 1

Sound can travel through

1. gases only
2. solids only
3. liquids only
4. solids, liquids and gases

Answer

solids, liquids and gases

Reason — Sounds needs a medium to travel. Solids, liquids and gases provide a medium through which sound can travel.

Question 2

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

1. Baby girl
2. Baby boy
3. A man
4. A woman

Answer

A man

Reason — The frequency of a sound is proportional to its pitch. An adult man's voice has a lower pitch than the voices of a baby boy, a baby girl, and a woman.

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 a vacuum. (T/F)

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

(c) If the amplitude of the vibration is large, the sound is feeble. (T/F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)

(f) Unwanted or unpleasant sound is termed as music. (T/F)

(g) Noise pollution may cause partial hearing impairment. (T/F)

Answer

(a) Sound cannot travel in a vacuum. — True

(b) The number of oscillations per second of a vibrating object is called its time period. — False
Reason — The number of oscillations per second of a vibrating object is called its frequency.

(c) If the amplitude of the vibration is large, the sound is feeble. — False
Reason — If the amplitude of the vibration is large, the sound is loud.

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. — True

(e) The lower the frequency of vibration, the higher is the pitch. — False
Reason — The lower the frequency of vibration, the lower is the pitch.

(f) Unwanted or unpleasant sound is termed as music. — False
Reason — Unwanted or unpleasant sound is termed as noise.

(g) Noise pollution may cause partial hearing impairment. — True

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.

Answer

(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.

(d) Unwanted sound is called noise.

(e) The shrillness of a sound is determined by the frequency of vibration.

Question 5

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

Answer

Given,

Total number of oscillations = 40

Total time taken = 4 s

We know,

Frequency = $\dfrac{\text{Total number of oscillations}}{\text{Total time taken}}$

Substituting we get,

Frequency = $\dfrac{40}{4}$ = 10 Hz

Time period = $\dfrac{1}{\text{Frequency}}$

Substituting we get,

Time period = $\dfrac{1}{10}$ = 0.1 s

Hence, frequency = 10 Hz and time period = 0.1 s.

Question 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?

Answer

Given,

Total number of vibrations = 500

Total time taken = 1 s

We know,

Time period = $\dfrac{1}{\text{Frequency}}$ and

Frequency = $\dfrac{\text{Total number of oscillations}}{\text{Total time taken}}$

Hence,

Time period = $\dfrac{\text{Total time taken}}{\text{Total number of oscillations }}$

Substituting we get,

Time period = $\dfrac{1}{500}$ = 0.002 s

So, time period = 0.002 s.

Question 7

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

(a) Dholak

(b) Sitar

(c) Flute

Answer

(a) Dholak — Stretched membrane.

(b) Sitar — Stretched strings

(c) Flute — Air column

Question 8

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

Answer

Yes, when music is played at very high volume, it becomes noise.

Question 9

List sources of noise pollution in your surroundings.

Answer

Sources of noise pollution are:

1. Horn of vehicles
2. Explosions including bursting of crackers
3. Sounds from machines, loudspeakers
4. Television, transistor and radio at high volumes
5. Desert coolers, air conditioners

Question 10

Explain in what way noise pollution is harmful to humans.

Answer

Presence of excessive noise in the surrounding may cause many health related problems like:

1. Lack of sleep
2. Hypertension
3. High blood pressure
4. Anxiety
5. Temporary or permanent impairment of hearing.

Question 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'd suggest that they go for the house that's three lanes away from the roadside. Being further from the roadside reduces the level of noise pollution from passing vehicles, making it a quieter and more peaceful living environment.

Question 12

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

Answer

The diagram of larynx is shown below:

Larynx is known as the voice box in the human body. There are two vocal cords across the larynx. The air passes through a small gap which is present in between them. The lungs force the air into the gap when we speak, and this vibrates the vocal cord, due to which sound is produced.

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?

Answer

Thunder is heard a few seconds after lightning is seen because sound travels much slower than light.

Light takes almost negligible time in comparison to sound in reaching us from the place of thunder because speed of light is much more (3 x 10⁸ ms^-1) than the speed of sound (= 330 ms^-1).