NCERT Solutions for Class 8 Science Chapter 11 - Sound

Q1. Choose the correct answer:
Sound can travel through
(a) Gases only
(b) Solids only
(c) Liquids only
(d) Solids, Liquids, and Gases.
Ans: (d) Solids, Liquids, and Gases.
Sound speed in various medium

Q2. Which of the following voices is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Ans: (c) A man

Q3. 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)
Ans: True

(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
Ans: False

(c) If the amplitude of vibration is large, sound is feeble. (T/F) 
Ans: False

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F) 
Ans: True

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
Ans: False

(f) Unwanted or unpleasant sound is termed as music. (T/F)
Ans: False

(g) Noise pollution may cause partial hearing impairment. (T/F)
Ans: True

Q4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called _____________
Ans: Time period

(b) Loudness is determined by the _____________ of vibration.
Ans: Amplitude

(c) The unit of frequency is _____________
Ans: Hertz

(d) Unwanted sound is called _____________
Ans: Noise

(e) Shrillness of a sound is determined by the _____________ of vibration.
Ans: Frequency and Amplitude

Q5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Ans: Frequency is defined as the number of oscillations per second.

Since the time period is the time taken to complete one oscillation. It is the inverse of the frequency.

Q6. 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?
Ans: Since the time period is the time taken to complete one oscillation.

Q7. Identify the part which vibrates to produce sound in the following instruments:
(a) Dholak
(b) Sitar
(c) FluteMusical InstrumentsAns: (a) In Dholak, there is a stretched membrane at the top known as the head. When the head is beaten, the stretched membrane vibrates and thus the sound is produced.
(b) In Sitar, there are many stretched strings. When these strings are plucked, the string vibrates and thus the sound is produced.
(c) In Flute, there is a hollow pipe. When air is blown into it, the air inside it starts vibrating, and thus the sound is produced.

Q8. What is the difference between noise and music? Can music become noise sometimes?
Ans: 

• The unwanted sound that is unpleasant to the ear is called noise. A pleasant sound is called music.

• Music can become noise in many instances. When someone tries to enjoy very loud music, it can be noise for someone else. When loud music is played during religious celebrations or marriages, it can be annoying for many people.

Q9. List sources of noise pollution in your surroundings.
Ans: Following is the list of sources of noise pollution in my surroundings:

• Horns of vehicles.
• Bursting of crackers.
• Machines
• Loudspeakers
• Television and radio at high volumes.
• Kitchen appliances like mixer, etc.
• Air conditioners

Q10. Explain in what way noise pollution is harmful to humans.
Ans: There are many harmful effects of noise pollution on humans like:

• Lack of sleep.
• High blood pressure (Hypertension).
• Temporary or permanent impairment of hearing.
• Headache
• Stress

Q11. 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.
Ans: I will suggest my parent buy a house which is away from the roadside because the house at the roadside would be noisy because of running vehicles. On the other hand, the house which is three lanes away from the roadside would be quieter.

Q12. Sketch the larynx and explain its function in your own words.
Ans: 

• The larynx is the part of the throat that is responsible for producing sound. 
• Inside the 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, the 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.

Q13. 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?
Ans: The speed of sound is 330 m/s while that of light is 3 x 108 m/s.
Thus, the speed of light is much higher than the speed of sound.
This is the reason light reaches us before sound during lightning.