NCERT SOLUTIONS FOR CLASS 8 SCIENCE SOUND CHAPTER 13

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

(d) solids, liquids, and gases.

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

(c) A man

Question 3.
In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.

  1. Sound cannot travel in a vacuum.
  2. The number of oscillations per second of a vibrating object is called its time period.
  3. If the amplitude of vibration is large, the sound is feeble.
  4. For human ears, the audible range is 20 Hz to 20,000 Hz.
  5. The lower the frequency of vibration, the higher is the pitch.
  6. Unwanted or unpleasant sound is termed as music.
  7. Noise pollution may cause partial hearing impairment.
  1. True
  2. False
  3. False
  4. True
  5. False
  6. False
  7. True

Question 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 period
  2. Amplitude
  3. Hertz (Hz)
  4. Noise
  5. Frequency

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

No. of oscillation = 40
Total time is taken = 4 seconds
NCERT Solutions for Class 8 Science Chapter 13 Sound Q5

Question 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?

Number of vibrations per second = 500
NCERT Solutions for Class 8 Science Chapter 13 Sound Q6

Question 7.
Identify the part which vibrates to produce sound in the following instruments.

  1. Dholak
  2. Sitar
  3. Flute
  1. Stretched membrane
  2. String of sitar
  3. Air column

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

The type of sound which are unpleasant to listen is known as noise whereas music is a pleasant sound, which produces a sensation.
Yes, music can become noise when it’s too loud.

Question 9.
List the sources of noise pollution in your surroundings.

Following are the major sources of noise pollution:

  • Sound of vehicles
  • Sound of kitchen appliances
  • Sound of bursting crackers
  • Sound of loudspeakers, TV, transistors

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

Noise pollution causes:
(a) Lack of sleep
(b) Anxiety
(c) Hypertension
and these are harmful to health.

Question 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 a house three lanes away from the roadside because house on the roadside would be much noisy in both days and night due to running vehicles. Whereas, a house three lanes away would be comparatively quieter as the intensity of noise decreases with the distance between the source and the listener.

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

Larynx is also known as voice box. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for passage of air (Fig. 13.12). When lung force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose.
NCERT Solutions for Class 8 Science Chapter 13 Sound Q12
When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.

Question 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 why?

The speed of light is more than that of the speed of sound. Thus, due to more speed of light it reaches us before sound. So, lightning is seen earlier and thunder is heard later.
Take a metal plate (or a shallow pan). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick (Fig. 13.1). Touch the plate or pan gently with your finger. Do you feel the vibrations? Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Do you still hear the sound? Touch the plate after it stops producing sound. Can you feel vibrations now?
NCERT Solutions for Class 8 Science Chapter 13 Sound Activity 1
Solution:
When we touch the pan gently with our finger after striking we feel the vibration. When we hold the pan tightly after striking it, we do not hear the sound. When the pan stops producing sound it also stops vibrating. Thus, we can conclude that vibrating body produces sound.
Jake a rubber band. Put it around the, longer side of a pencil box (Fig. 13.2). Insert two pencils between the box and the stretched rubber. Now, pluck the rubber band somewhere in the middle. Do you hear any sound? Does the band vibrate?
NCERT Solutions for Class 8 Science Chapter 13 Sound Activity 2
Solution:
Yes, we hear the sound on plucking the rubber band. Also, we find that the band is vibrating. Thus, all vibrating bodies produce sound.

Question 13.
Choose the correct answer. Sound can travel through

  • Gases only
  • Solids only
  • Liquids only
  • Solids, liquids and gases.

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Question 14.
Voice of which of the following is likely to have minimum frequency ?

  • Baby girl
  • Baby boy
  • A man
  • A woman

Aman

Question 15.
Identify the part which vibrates to produce sound in the following instruments.

  • Dholak
  • Sitar
  • Flute
  • Dholak – stretched membrane
  • Sitar – stretched string
  • Flute – air column

Question 16.
In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.

  1. Sound cannot travel in vacuum. (T/F)
  2. The number of oscillations per second of a vibrating object is called its time period. (T/F)
  3. If the amplitude of vibration is large, sound is feeble. (T/F)
  4. For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
  5. The lower the frequency of vibration, the higher is the pitch. (T/F)
  6. Unwanted or unpleasant sound is termed as music. (T/F)
  7. Noise pollution may cause partial hearing impairment. (T/F)
  1. T
  2. F
  3. F
  4. T
  5. F
  6. F
  7. T

Question 17.
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. Shrillness of a sound is determined by the …………….. of vibration.
  1. time period
  2. amplitude
  3. Hertz
  4. noise
  5. frequency

Question 18.
Define vibration.

Vibration is the to and fro or back and forth motion of an object.

Question 19.
Which part of the human body is responsible for producing sound ?

The vocal cords in men are about 20 mm long.

Question 20.
Can sound travel in vacuum ?

No, sound cannot travel in vacuum.

Question 21.
What is meant by oscillatory motion ?

The to and fro motion of an object is known as oscillatory motion.

Question 22.
Define frequency.

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

Question 23.
Define 1 hertz.

A frequency of 1 hertz means one oscillation per second.

Question 24.
How are frequency of a sound and pitch related ?

If the frequency of vibration is higher then the sound has a higher pitch.

Question 25.
Whose voice has a higher frequency – man or woman ?

The voice of woman has higher frequency.

Question 26.
What is range of audible sound ?

Sound of frequency 20 Hz to 20,000 Hz is the audible range.

Question 27.
Which animal can hear sounds of frequencies higher than 20,000 Hz ?

Dogs can hear frequencies higher than 20,000 Hz.

Question 28.
What is meant by base loudness level ?

The base loudness level is defined as that loudness of sound that the human ear can just perceive.

Question 29.
What is meant by noise pollution ?

Presence of excessive or unwanted sound in the atmosphere is called noise pollutipn.

Question 30.
If the frequency of a sound is below 20 Hz, will it be audible to human beings ?

No, it will not be audible.

Question 31.
In which state of matter does sound travel the

  • slowest
  • fastest ?
  • Air.
  • Solids.

Question 32.
What happens to sound when it strikes a surface ?

Sound gets reflected on striking a surface.

Question 33.
Why do we hear the sound of the hom of an approaching car before the car reaches us ?

This happens because the speed of sound is much greater than the speed of the car.

Question 34.
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 ? 

Time taken for 500 vibrations = 1 second
Time taken for 1 vibration = 1/500 second.
∴ Time period = 1/500 second.

Question 35.
How do plants help in reducing noise pollution ?

Plants absorb sound and so help us in minimizing noise pollution.

Question 36.
How can we control the sources of noise pollution ?

We can control noise pollution by designing and installing silencing devices in machines.

Question 37.
How can a hearing impaired child communicate ?

A hearing impaired child can communicate effectively by using sign language.

Question 38.
If the amplitude increases 3 times, by how much will the loudness increase ?

If the amplitude increases three times, the loudness will increase by a factor of 9.

Question 39.
The frequency of a given sound is 1.5 kHz. How many vibrations is it completing in one second ?

Frequency = No.of vibrations/time
∴ No. of vibrations = Frequency x time = 1.5 x 1000 x 1 = 1500 vibrations

Question 40.
Which characteristic of a vibrating body determines

  1. loudness
  2. pitch of the sound produced by it ?
  1. Amplitude.
  2. Frequency.

Question 41.
Why do we not hear echoes in our ordinary surroundings ?

We do not hear echoes in our ordinary surroundings because the distance to hear echo should be more than 17 m.

Question 42.
We cannot hear the sound of the exploding meteors in the sky, though we can see them. Why ?

Sound cannot travel through vacuum. In space there is vacuum. Light can travel through vacuum, so we can see the exploding meteor but cannot hear the explosion.

Question 43.
We can hear the supersonic jet planes flying. How ?

The supersonic jet planes fly in the air. Since sound can travel through air, we can hear then flying

Question 44.
What are vocal cords? What is their function?

The larynx has a pair of membranes known as vocal cords stretched across their length. The vocal cords vibrate and produce sound.

Question 45.
When does a thud become music ?

When thuds are repeated at’regular intervals, it becomes music, e.g., beating of drums or wood.

Question 46.
How do birds and insects produce sound ?

Birds chirp with the help syrinx in their wind pipe. Insects produce sound by flapping their wings.

Question 47.
What is the function of eusfachian tube in human ear ?

The vibrations of the spoken words reach our ears through eustachian tubes.

Question 48.

  1. In our body which part of the ear receives sound waves ?
  2. What may happen if the eardrum is absent from our ear ?
  1. Pinna helps in receiving sound waves.
  2. If the eardrum is absent we would not be able to hear.

Question 49.
Can a hearing impaired child speak ? If not why ?

A child having hearing impairment can not speak because if he is able to hear, he will leam to speak.

Question 50.
Give an example each of:

  1. stringed instrument
  2. percussion instrument
  3. wind instrument
  4. striking instrument
  1. Violin
  2. Drums
  3. Flute
  4. Jal Tarang
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