Class 8 Science Textbook Solution

# Sound – Class 8 Science Textbook Solution

Chapter 9 Sound

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

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

3. In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false:
(а) Sound cannot travel in a 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, the 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) Sound cannot travel in a 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, the 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)

4. Fill in the blanks with suitable words:
1.    Time taken by an object to complete one oscillation is called _________
2.    Loudness is determined by the _________ of vibration.
3.    The unit of frequency is _________
4.    Unwanted sound is called _________
5.    The shrillness of a sound is determined by the _________ of vibration.

1.    Time taken by an object to complete one oscillation is called a time period.
2.    Loudness is determined by the amplitude of vibration.
3.    The unit of frequency is hertz.
4.    An unwanted sound is called noise.
5.    The 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.
Time taken by pendulum to complete  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?

For making 500 vibrations time taken is 1 second.
For making 1 vibration time taken is = 1500 second = 0.002 second.
Hence time period of the vibration is 0.002 seconds.

7. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute
8. What is the difference between noise and music? Can music become noise sometimes?
Noise: Unpleasant sounds are called noise. Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.
Music: Pleasant sound is called music. Music brings about a soothing effect.
Yes, music can become noise sometimes when the musical instruments produce very high volume sounds.
40 oscillations = 4 seconds
Time taken by pendulum to complete 1 oscillation = 440 = 110 = 0.1 sec.
Time period of pendulum = 0.1 sec.
Frequency of pendulum =  oscillation  time  = 404 = 10Hz.
Hence the frequency of the pendulum is 10 Hz.

9. List sources of noise pollution in your surroundings.
Sources of noise pollution in our surroundings are:
•    Noise of machines
•    Noise of Musical bands
•    Sound produced by loudspeakers
•    Loud sound of T.V and Radio
•    Loud music in parties and marriages
•    Noise of crakers
•    Loud sounds of domestic electronic appliances such as washing machines, cooler, A.C, mixer grinder, etc.
•    The noise of vehicles.
•    The honking of horns.

10. Explain in what way noise pollution is harmful to humans.
Noise pollution is harmful to humans because:
•    Noise pollution can cause temporary or permanent deafening.
•    It can cause many health-related problems like high blood pressure
•    It can cause mental illness due to lack of sleep.

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.

I would suggest my parents buy the house which is three lanes away from the roadside. This would protect us from noise pollution which is maximum at the roadside building. This would safeguard our health and peace of mind.

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

The other name of the larynx is the voice box. It is present at the upper end of the windpipe. The function of the larynx is to produce sound.

13. Lightning and thunder take place in the sky and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
The light travels at the speed of 3 × 108 ms-1 which is very large in comparison to the speed of sound which travels at the rate of 330 ms-1 in the air. That is why lightning is seen earlier and thunder is heard later.

1. Touch the school bell when not in use. what do you feel?
There is nothing happening in the bell.

2. Again touch it (bell) when producing sound. Can you feel it vibrating?
Yes, the bell is vibrating this time.

3. How is the sound produced?
Sound is produced by vibrations in objects.

4. What is vibration?
The to and fro or back and forth motion of an object is called vibration.

5. Describe the organ in humans that produces sound.
In humans, the sound-producing organ is the voice box. It is also called the larynx. In the larynx there are two vocal cords stretched across, leaving a slit for the air to pass through them.

6. How does the larynx produce sound?
The larynx has two vocal cords stretched across it. They are stretched in a way that they leave a slit for the passage of air. When air is pumped in slit by the lung, the vocal cords start vibrating, thus producing the sound.

7. Does sound need a medium for propagation?
Yes, sound needs a medium for propagation.

8. Sound travels fastest in which state of matter?
Sound travels fastest in solids.

9. Will the voice come from the telephone you made with paper cups if somebody holds the thread tied in between the cups?
The voice will stop coming because vibrations produced by sound in the thread cannot travel further.

10. What is oscillatory motion?
Oscillatory motion is to and fro or back and forth motion of any object.

11. What is the frequency of oscillation?
The number of oscillations per second is called the frequency of oscillation.

12. What is the unit of frequency?
Hertz (Hz).

13. What is the range of audible sounds for the human ear?
From 20 to 20,000 Hz.

14. What are the sounds above 20,000 Hz called?