NCERT Class 8 Science Chapter 13 - Sound

Chapter 13 - Sound

Question 1: Define the following terms:
a) Amplitude -
It is the maximum displacement of an object from its mean position.

b) Audible - The sound of frequencies that are more than 20 Hz and less than 20000 Hz (20 kHz) can be heard by humans. Such sound are called audible sounds.

c) Oscillation - It is the movement of objects or particles.

d) Pitch - It is the highest or lowest sound, an object makes. 

e) Shrillness - The sound produced by a vibrating object with a high frequency is called shrillness.

f) Time period - Time period is the time taken to complete one full oscillation or vibration.

g) Vibration - The to and fro motion of movement of object or particle is called vibration.

Question 2: Choose the correct answer.
Sound can travel through
a) gases only
b) solids only
c) liquids only
d) solids, liquids and gases.
Answer:
d) solids, liquids and gases.

Question 3: Voice of which of the following is likely to have minimum frequency?
a) Baby girl
b) Baby boy
c) A man
d) A woman
Answer:
c) A man

Question 4: 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)
Answer:

a) True; Sound needs medium to travel.
b) False; Frequency is the number of times the object vibrates per second. Time period is the time taken to complete one full vibration or oscillation.
c) False; The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.
d) True; The sound of frequencies that are more than 20 Hz and less than 20000 Hz (20 kHz) can be heard by humans. Such sound are audible sounds for humans.
e) False; The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher, the sound is shrill and has a higher pitch. If the frequency of vibration is lower, the sound has a lower pitch.
f) False; Music is sound that is pleasant to ears while the noise is sound that is unpleasant or unwanted and irritating to ears.
g) True; Noises are unwanted and unpleasant sounds, which may cause temporary hearing impairment when heard continuously for a long time.

Question 5: 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.
Answer:

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.

Question 6: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
Time taken to complete one full vibration or oscillation is termed as time period. Number of times the object vibrates per second is termed as frequency.
Frequency = 40/4 = 10 Hz
Time period = 1/10 = 0.1 seconds
Therefore, the frequency is 10 Hz while the time period is 0.1 seconds.

Question 7: 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:
Time taken to complete one full vibration or oscillation is termed as time period.
Time period = 1/Frequency
Frequency of sound produced by mosquito wings = 500 Hz
= 1/500
= 1/500 × 2/2
= 2/1000
= 0.002 seconds
Therefore, the time period is 0.002 seconds.

Question 8: Identify the part which vibrates to produce sound in the following instruments.
a) Dholak
b) Sitar
c) Flute
Answer:

a) It consists of a head which is a stretched membrane. Vibrations are set into these stretched strings when the head is beaten gently, these vibrations produce sound. Therefore, in dholak the stretched membrane when vibrated produces sound.

b) It is a musical instrument. Stretched strings are part of it. Vibrations are produced when the string is plucked when played. These vibrations produce sound. Therefore, in sitar, the stretched string when vibrated produces sound.

c) It is an instrument which has holes in it. It is a hollow pipe. The air inside the pipe is set into vibration when air is blown over its mouth and this produces a pleasant sound. Therefore, in flute, the air column when vibrated produces sound.

Question 9: What is the difference between noise and music? Can music become noise sometimes?
Answer:
Music is a sound that is pleasant to hear. Example: Sounds from piano, flute etc are pleasant to hear. Noise is a sound that are unpleasant to hear. Example: Sounds from car horns, construction are unpleasant to hear. 

Question 10: List sources of noise pollution in your surroundings.
Answer:
Sources of noise pollution in the surroundings are
→ Car horns
→ TV at loud volume
→ Firecracker or Fireworks bursting
→ Transistors

Question 11: Explain in what way noise pollution is harmful to human.
Answer:
Noise pollution is harmful to humans in following ways:
→ It can cause headache.
→ Lack of sleep
→ Hypertension (high blood-pressure)
→ Anxiety
→ It can also cause temporary or permanent impairment of hearing
→ Stress

Question 12: 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.
Answer:
I would suggest my parents to buy a house that is three lane away from the roadside because there will be less noise as compared to the house on the main road. As the distance between the source and listener increases the intensity of noise decreases.

Question 13: Sketch larynx and explain its function in your own words.
Answer:
In human, the sound is produced by the voice box or the larynx. It is located at the upper end of the wind pipe. Two vocal cords, 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. The air passes through a small gap which is present in between them. The lungs force the air into the gap when we speak and this vibrates the vocal cord, due to which sound is produced.

Larynx

Question 14: 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?
Answer:
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 light travels faster than the sound so we see the lightning first and then the sound.

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