Sound

solids, liquids and gases

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

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.

(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

(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.

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.

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.

(a) Dholak — Stretched membrane.

(b) Sitar — Stretched strings

(c) Flute — Air column

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

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

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.

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.

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.

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).