Sound - Class 8 : NCERT Exercise Questions



Q.1 Choose the correct answer:
Sound can travel through
(a) gases only             (b) solids only
(c) liquids only            (d) solids, liquids and gases.
Sol. (d) solids, liquids and gases.

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

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)
Sol. (a) Sound cannot travel in vacuum.                                                                                   (True)
(b) The number of oscillations per second of a vibrating object is called its time period.       (False)
(c) If the amplitude of vibration is large, sound is feeble.                                                        (False)
(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)
(f) Unwanted or unpleasant sound is termed as music.                                                            (False)
(g) Noise pollution may cause partial hearing impairment.                                                     (True)

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) Shrillness of a sound is determined by the ................. of vibration.
Sol. (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.

Q.5 A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Sol. Time period is defined as the time required to complete one oscillation. It is the inverse of the frequency.
                    Time period = Time taken/Number of oscillations = 4/40 = 1/10 = 0.1s
Frequency is defined as the number of oscillations per second.
                    Frequency = Number of oscillations/Time taken = 40/4 = 10 Hz.

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?
Sol. Time period is defined as the time required to complete one oscillation. Hence, time period of vibration of mosquito will be
                  Time period = Time taken/Number of oscillations = 1/500 = 0.002 s.

Q.7 Identify the part which vibrates to produce sound in the following instruments:
(a) Dholak       (b) Sitar           (c) Flute
Sol. (a) Dholak:
In Dholak, there is a stretched membrane at the top known as head. When the head is beaten, the stretched membrane vibrates and thus the sound is produced.
(b) Sitar:
In Sitar, there are many stretched strings. When these strings are plucked, the string vibrates and thus the sound is produced.
(c) Flute:
In Flute, there is hollow pipe. When air is blown into it, the air inside it starts vibrating and thus the sound is produced.

Q.8 What is the difference between noise and music? Can music become noise sometimes?
Sol. The sound which produces a pleasing sensation on hearing is called music. For example, the sounds produced by musical instruments like flute, sitar, etc. On the other hand, the sound which produces unpleasing sensation on hearing is called noise. For example, the sound produced by horns of vehicles, machines, etc.
Yes, music can become noise sometimes like if the music is played at very high volume or different types of music are played at the same time.

Q.9 List sources of noise pollution in your surroundings.
Sol. Following is the list of sources of noise pollution in my surroundings:
(i) Horns of vehicles.
(ii) Bursting of crackers.
(iii) Machines.
(iv) Loudspeakers
(v) Television and radio at high volumes.
(vi) Kitchen appliances like mixer, etc.
(vii) Air conditioners.

Q.10 Explain in what way noise pollution is harmful to humans.
Sol. There are many harmful effects of noise pollution on humans like:
(i) Lack of sleep.
(ii) High blood pressure (Hypertension).
(iii) Temporary or permanent impairment of hearing.
(iv) Headache
(v) Stress

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 answer.
Sol. I would suggest my parents to buy a house three lanes away from the roadside because there would be more noise along the roadside produced by the vehicles. And the amount of noise will decrease as one move away from the roadside.

larynx

Q.12 Sketch larynx and explain its function in your own words.
Sol. A sketch of larynx is shown below:
Larynx is that part of throat which is responsible for producing sound. Inside larynx, there are two vocal cords, which 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 we speak, air is forced into this small gap by the lungs which prompt vocal cords to vibrate. When the lungs force air through the slit, the vocal cords vibrate, producing sound.

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?
Sol. The speed of sound is 330 m/s while that of light is 3 x 108 m/s. Thus, speed of light is much higher than the speed of sound. This is the reason light reaches us before sound during a lightning.



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