# TEXTBOOK ANSWERS AND SOLUTIONS OF CBSE CLASS VIII SCIENCE Chapter 13 Sounds

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

ANSWER :-( d) Sound can travel through solids, liquids, and gases.

2) Which of the following voices is likely to have minimum frequency?
(a) Baby girl (b) Baby boy
(c) A man (d) A woman

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)

a)True
b) False
c) False
d)True
e) False
f) False
g)True

4) Fill in the blanks with suitable words.
(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.

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

ANSWER:-Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by
Frequency = (Number of Oscillations)/Time = 40/4 = 10 Hz
The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Time period = 1/ (Frequency of Oscillation) = 1/10 = 0.1 sec.

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:-The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.
Time period = 1/ (Frequency of Oscillation)
Frequency of oscillations = 500 Hz
Time period = 1/500 = .002 s

7) Identify the part which vibrates to produce sound in the following instruments. (a) Dholak (b) Sitar (c) Flute

ANSWER:-(a) Dholak is a musical instrument. It consists of a stretched membrane called its head. When the head is beaten gently, the stretched membrane sets into vibration. Since sound is produced when an object vibrates, the dholak produces a sound.
(b) Sitar is a musical instrument. It consists of stretched strings. When a string is plucked, it sets into vibration. Since sound is produced when an object vibrates, the sitar produces a sound.
(c) Flute is a hollow pipe. When air is blown over its mouth, the air inside the pipe is set into vibration. As a result, a pleasant sound is produced.

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

ANSWER:-The sound that is pleasing to the ear is called music. For example, the sound produced by violins, pianos, flutes, pangs, etc. The sound that is unpleasing to the ear is called noise

9) List sources of noise pollution in your surroundings.

ANSWER:-Some sources of noise pollution are as follows:
(i) Televisions and transistors running at high volumes
(ii) Loudspeakers and crackers
(iii) Horns of buses, cars and trucks

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

ANSWER:-Noise pollution can lead to a number of health-related problems like Hearing loss, insomnia, stress etc.

ANSWER:-There will be more noise in the house which is along the roadside. This is because noise produced by transportation vehicles may cause trouble to the residents. The intensity of noise decreases with the distance between the source and the listener. Hence, it is better to take the house that is three lanes away from the roadside

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