CBSE Class 8 Science Sound VBQs

What is the MAIN advantage of having two ear rather than only one to hear with?
(a) The clarity of the sound heard increases with two ears.
(b) The second ear is a backup in case the first ear fails
(c) The direction of sounds can be judged with two ears.
(d) The symmetry of the human face is maintained. 

Answer : C

On a rainy night, Mohan observed that every time there was lightning and thunder, he would usually SEE the lightning a few seconds BEFORE he HEARD the thunder. What could be the reason for this?
(a) Thunder travels faster than lightning.
(b) Sound waves travel faster than light in air.
(c) Light waves travel faster than sound waves.
(d) Light waves travel through a less dense medium than sound waves.

Answer : C

A ship has anchored in an ocean. The ocean is 10,500 m deep. An engineer from the ship sent a signal straight down into the water. How long will it take for the echo to reach him? (velocity of sound in water  1.5 km/s)
(a) 16.5 sec
(b) 15 sec
(c) 14 sec
(d) 7 sec

Answer : C

The unit used for measuring intensity of sound is DECIBEL. The faintest sound which the human ear can detect is assigned a value of 0 decibel. Study this table and answer the question.Psychologists believe that if one sound source has 10 TIMES the power of another sound source, we hear the first about TWICE as loud - thus its decibel value will be double. If the decibel value of the sound from a 50W source is 60 dB, what would be the approximate decibel value of the sound from a loud 2500W source?

(a) 30 dB
(b) 120 dB
(c) 160 dB
(d) 240 dB

Answer : C

Speech is much better developed in humans than in other animals. This is MAINLY due to which organ being better developed in humans?
(a) Mouth
(b) Tongue
(c) Heart
(d) Brain

Answer : D

An astronaut is moving in space when a big explosion occurs about 50 metres BEHIND him. How will the astronaut come to know about the explosion?

(a) From the sound of the explosion
(b) From the light due to the explosion
(c) From light and sound due to the explosion
(d) From the vibrations due to the explosion

Answer : B

Each of the four glasses below contains different quantities of water. A musical instrument is made by tapping the top of each glass with a metal spoon. Which glass would have the lowest pitch?

Answer : A

What does a tabla player do to increase the pitch of his instrument?
(a) He tightens the skin across the tabla.
(b) He strikes the tabla harder.
(c) He changes the skin of the tabla.
(d) He cannot increase the pitch of the tabla.

Answer : A

The figure shows a person holding a polythene strip after it has been rubbed with a duster cloth. The strip acquires a negative charge. Yet, after a few seconds, the duster cloth does not show any charge. Which of the following could be the reason for this?

(a) The negative charges flow from the Earth
(b) The positive charges flow to the strip
(c) The negative charges flow from the strip
(d) The positive charges flow to the Earth

Answer : A

Given below is a diagrammatic representation of an electromagnet. The strength of the magnetic field between the poles of the electromagnet would be UNCHANGED if the

(a) material of the core was changed
(b) number of turns of the coils was halved
(c) distance between the poles was doubled
(d) direction of current in the electromagnet was reversed

Answer : D

It is likely that when a person sweats, the electrical resistance of the surface of his skin
(a) stays the same.
(b) increases
(c) decreases
(d) becomes zero.

Answer : C

Your task is to generate electricity through a chemical reaction. Which among the following groups would you use?
(a) Magnet, Copper Wire, HCl, Test Tube
(b) Beaker, HCl, Zinc, Copper Strips
(c) Silk Material, Rubber Rod, Electroscope
(d) Graphite, Silicon, Graduated Cylinder

Answer : B

A coil is wrapped loosely around a cylindrical wooden piece and its ends are connected to a battery as shown. In which case will the compass needle placed near the coil deflect when the circuit is closed?

(a)  if the cylindrical wooden piece is replaced by an iron piece
(b) if the cylindrical wooden piece is replaced by a magnet.
(c) if the cylindrical wooden piece is replaced by a magnet which is moved in and out of the coil.
(d) (The needle will deflect even with the wooden piece alone.)

Answer : D

What is the normal AC voltage found in Indian homes?
(a) 110 V
(b) 200 V
(c) 220 V
(d) 440 V

Answer : C

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

Answer : D

Question. 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

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

(a) Sound cannot travel in vacuum.
Answer : True

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

(c) If the amplitude of vibration is large, sound is feeble. 
Answer : False

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. 
Answer : True

(e) The lower the frequency of vibration, the higher is the pitch. 
Answer : False

(f) Unwanted or unpleasant sound is termed as music.
Answer :

(g) Noise pollution may cause partial hearing impairment.
Answer : True

Fill in the blanks with suitable words.

(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.

(d) Unwanted sound is called Noise.

(e) Shrillness of a sound is determined by the frequency of vibration

Question. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer : Number of oscillations in 4 seconds = 40
Number of oscillations in 1 seconds = 40/4 = 10
Therefore, frequency of pendulum = f = 10 Hz
Now Time period = 1/f = 1/10 = 0.1 sec.

Question. Explain in what way noise pollution is harmful to humans.
Answer : Noise pollution causes: (a) Lack of sleep, (b) Anxiety, (c) Hypertension.

Question. 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 : Number of vibrations in 1 seconds = 500
Therefore, frequency of vibration = f = 500 Hz
Now Time period = 1/f = 1/500 = 0.002 sec.

Question. Sketch larynx and explain its function in your own words.
Answer : The other name of larynx is voice box. It is present at the upper end of the windpipe. The function of larynx is to produce sound with the help of two vocal cords on passing air.

Question. List the sources of noise pollution in your surroundings.
Answer : Noise pollution sources are:
(a) Bus and car horns.
(b) Firecrackers and loudspeakers.
(c) High volumes in televisions and DJ etc.
(d) Sound of kitchen appliances.
(e) Sirens from factories

Question. 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 the house which is three lanes away from the roadside. This would protect us from noise pollution which is maximum at roadside building.

Question. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute
Answer : (a) Dholak - Stretched membrane.
(b) Sitar - Stretched string.
(c) Flute - Air column.

Question. 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 : Speed of the light is much more than the speed of sound. Thus, lightning is seen earlier than thunder.

Question. What is the difference between noise and music? Can music become noise sometimes?
Answer : The type of sound which is 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.