Class 8 - Physics - Sound

Question1.

Sound can travel through

• gases only (b) solids only  (c) liquids only (d) solids, liquids and gases

Correct option is: (d) solids, liquids and gases.

Sound requires a medium to travel. This medium can be solid, liquid or gas.

Question2.

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

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

Correct option is :( d) A woman.

Frequency is related to the pitch of the sound. So higher is the frequency, higher will be the pitch of the sound. Pitch means shrillness in the voice.

In case of a female pitch is always higher as compared to a male.

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. (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 vibration is large, 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)

• Sound cannot travel in vacuum. (True)

Explanation: -

Sound needs a medium to travel.

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

Explanation: -

The number of oscillations is frequency. Time period is the time taken to complete one oscillation.

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

Explanation: -

Loudness is directly proportional to the square of the amplitude. If amplitude is large then the sound will also be loud.

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

Explanation: -

The range below 20Hz is infrasound and range above 20,000 is ultrasound.

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

Explanation: -

Pitch is directly proportional to the frequency.

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

Explanation: -

Unwanted or unpleasant sound is noise.

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

Explanation: - Lot of noise pollution can affect the ear drum which will result in 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 times in 4 seconds. Find its time period and frequency.

Given Frequency = 40 Hz. Frequency is defined as the number of oscillations per unit time.

Time = 4 sec.

Time Period = (Time taken)/ (Number of Oscillations)

= (4/40) = 0.1 sec.

Frequency = (Number of Oscillations)/ (Time taken)

= (40/4) =10Hz.

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?

Frequency = 500 vibrations/second.

Time Period = (Time Taken)/ (Number of Oscillations)

= (1/500) = 0.002 sec.

Question 7.

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

(a) Dholak                (b) Sitar                         (c) Flute

(a) Dholak: Dholak has stretched membranes which vibrate to produce sound.

(b) Sitar: In sitar, the stretched strings, when vibrate, they produce sound.

(c) Flute: Flute is a wind musical instrument which uses air column to produce sound.

Question 8.

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

Music is a sound which produces a pleasing sensation.

Noise is an unwanted and unpleasant sound.

Music is produced by nature, musical instruments etc.

Noise is produced by horns of vehicles, by machines etc.

Music becomes noise when played at very large volumes.

Question 9.

List sources of noise pollution in your surroundings.

Major causes of noise pollution are sounds of vehicles, explosions including bursting of crackers, machines, loudspeakers etc.

Sources in the home may lead to noise pollution. Television and transistor radio at high volumes,

some kitchen appliances, desert coolers, air conditioners, all contribute to noise pollution.

Question 10.

Explain in what way noise pollution is harmful to humans.

Lack of sleep, hypertension (high blood pressure), anxiety and many more health disorders may be caused by noise pollution.

A person who is exposed to a loud sound continuously may get temporary or even 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.

House which is near to roadside will have more noise pollution .I would suggest the house which is three lanes away from the roadside,

because noise pollution will be less.

Question 12.

Sketch larynx and explain its function in your own words.

Larynx is known as voice - box as it is responsible for the voice produced. Two vocal cords are stretched across the voice box or larynx in

such a way that it leaves a narrow slit between them for the passage of air. When the lungs 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.

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?

Speed of sound is 330 m/s and speed of light is 300,000,000 m/s. Light travels much faster than sound that is why

lightning is seen earlier and thunder is heard.