# NCERT Solutions for Class 8 Science Chapter 13 Sound

Written by Team Trustudies
Updated at 2021-05-07

## NCERT solutions for class 8 Science Chapter 13 Sound Exercise

Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases

NCERT Solutions for Class 8 Science Chapter 13 Sound

(d) solids, liquids and gases

EXPLANATION

Sound requires a medium to travel from one point to another. The medium can have particles of any state i.e., solid , liquid , gases which can transmit the vibrations of a particular sound wave.

Q.2 Voice of which of the following is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman

NCERT Solutions for Class 8 Science Chapter 13 Sound

(c) A man

EXPLANATION

The voice of an adult man has a lower pitch because he has lower frequency as compared to the voices of the baby boy, baby girl and the woman.

Q.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)

NCERT Solutions for Class 8 Science Chapter 13 Sound

(a) Sound cannot travel in vacuum. ( T )

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

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

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

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

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

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

EXPLANATION

Sound waves travel from one point to another through vibrations of particles in the medium. As vacuum doesn't have any medium, so there are no atoms or molecules to vibrate and thus sound doesn't travel in vacuum.

The number of oscillations per second of a vibrating object is called as frequency and time period is the time taken by the vibrating object to complete one oscillation.

If the amplitude of vibration is large, then the sound is very strong and large. And when the amplitude of vibration is small, then the sound is feeble.

For human ears, the audible range is 20 Hz to 20,000 Hz. The sound below 20 Hz is called as infrasonic sound and the sound above 20,000 Hz is called as ultrasonic sound. Both infrasonic sound and ultrasonic sound is not audible to human ears.

The lower the frequency of vibration, the lower is the pitch and the higher the frequency of vibration, the higher is the pitch.

Unwanted or unpleasant sound is termed as noise while the pleasant and soothing sound is considered as music.

Noise pollution may cause partial hearing impairment and also many mental health problems.

Q.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) The shrillness of a sound is determined by the __________ of vibration.

NCERT Solutions for Class 8 Science Chapter 13 Sound

(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.

EXPLANATION

The time period is the time taken by a wave to complete one oscillation. It is the reciprocal of frequency.

Amplitude is the maximum displacement or distance moved by a wave measured from its equilibrium position.

Hertz is the unit of frequency which is the measure of 1 cycle in 1 second.

Noise is the unpleasant sound that is loud and disruptive and affects the mental health adversely.

Frequency is the measure of the number of oscillations of a repeating event per unit of time.

Q.5 A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

NCERT Solutions for Class 8 Science Chapter 13 Sound

Given,
Number of oscillations = 40 times

Time taken by the oscillations = 4 seconds

Frequency of oscillation = Number of oscillations / Time taken by the oscillations

= 40 / 4

= 10 Hertz

Time period of oscillation = 1 / Frequency of oscillation

= 1 / 10

= 0.1 seconds

Q.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?

NCERT Solutions for Class 8 Science Chapter 13 Sound

Frequency of vibration = 500 vibrations per second

Time period = 1 / Frequency

= 1 / 500

= 0.002 second

Q.7 Identify the part which vibrates to produce sound in the following instruments.
1. Dholak
2. Sitar
3. Flute

NCERT Solutions for Class 8 Science Chapter 13 Sound

1. Dholak - Stretched membrane

2. Sitar - String of sitar

3. Flute - Air column

Q.8 What is the difference between noise and music? Can music become noise sometimes?

NCERT Solutions for Class 8 Science Chapter 13 Sound

Noise

Noise is unwanted sound which is considered unpleasant, loud and disruptive to hearing. Exposure to prolonged or excessive noise causes a wide range of health problems such as stress, poor concentration, productivity loss during any work, communication difficulties and fatigue from lack of sleep etc.

Music

Music is the arrangement of sounds in time to produce a composition through the elements of melody, harmony, rhythm, and timbre which is considered as pleasant sound. It calms our mind with a soothing effect on the brain.

Yes sometimes music can become noise when it crosses the bearable range of sound for our ears.

Q.9 List sources of noise pollution in your surroundings.

NCERT Solutions for Class 8 Science Chapter 13 Sound

Sources of noise pollution are :

(a) Noise arising from firecrackers and loudspeakers.

(b) Noises from the nearby construction works.

(c) Noises arising from the vehicular horns and their engines.

(d) Noises from home appliances like mixer and electronic appliances like TV.

(e) Noises created by nearby cattle and street animals like barking of dogs etc.

Q.10 Explain in what way noise pollution is harmful to humans.

NCERT Solutions for Class 8 Science Chapter 13 Sound

Noise pollution causes:

(a) Lack of sleep which causes fatigue and this results in tiredness and depression.

(b) Anxiety which is the result of the body's natural response to stress.

(c) Hypertension which occurs due to high blood pressure. Blood pressure is the result of blood exerting pressure against the walls of the blood vessels.

(d) Hearing loss which is the loss of hearing in one or both ears, ranging from mild to profound.

(e) Insomnia which is a sleep disorder in which there is trouble in falling asleep.

Q.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

NCERT Solutions for Class 8 Science Chapter 13 Sound

I would suggest my parents to buy the house which is three lanes away from the road side. Because the house that is three lanes away from the roadside would receive less vehicular noise as compared to the house that is present on the roadside.

Because exposure to prolonged or excessive noise cause a wide range of health problems such as stress, poor concentration, productivity loss during any work, communication difficulties and fatigue from lack of sleep, to more serious issues such as cardiovascular disease, cognitive impairment etc.

Q.12 Sketch larynx and explain its function in your own words.

NCERT Solutions for Class 8 Science Chapter 13 Sound

The other name of larynx is voice box. It is present at the upper end of the wind pipe.

It is involved in breathing, producing sound and protecting the trachea from food transmission into it. The larynx manipulates the pitch and volume which is essential for phonation.

Q.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?

NCERT Solutions for Class 8 Science Chapter 13 Sound

Lightning and thunder take place in the sky at the same time and at the same distance from us.

But lightning is seen earlier and thunder is heard later because the speed of light is more than that of the speed of sound. So due to more speed of light, it reaches us before sound.

The speed of light in air is 3 x 108 m/s2 while the speed of sound in air is 343 m/s2.

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