Chapter 12 : Sound
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1. Birds have a ring of cartilage, called ________ in their wind pipes.
ANS: syrinx
2. The SI unit of 'loudness' of sound, is ________.
ANS: decibel (dB)
3. The pitch of a sound is determined by its ________.
ANS: frequency
4. Sounds, of frequency more than 20 kHz, are known as ________.
ANS: ultrasonics
5. An irregular, irritating sound is referred to as a ________.
ANS: noise
1. Human beings, as well as all animals, have a voice box for producing their characteristic sounds.
ANS: False
2. Of the three mediums, solids, liquids and gases, sound propagates fastest in liquids.
ANS: False
3. Sound cannot propagate through vacuum.
ANS: True
4. Light propagates, through vacuum, with a speed that is nearly a million times more than the speed of sound in air.
ANS: True
5. The frequency of a given sound determines its 'shrillness' or 'flatness'.
ANS: True
6. Irregular non-periodic vibrations generally produce musical sounds.
ANS: False
1. The maximum displacement of a vibrating body, on
either side of its mean position, is known as its—
•
frequency
•
loudness
•
amplitude
•
pitch
ANS: amplitude
2. The frequency of a given sound is 1.5 kHz. The
vibrating body is then—
•
completing
1,500 vibrations in one second.
•
taking 1,500
seconds to complete one vibration.
•
taking 1.5
seconds to complete one vibration.
•
completing 1.5
vibrations in one second.
ANS: completing 1,500 vibrations in one second.
3. Sound can propagate through—
•
vacuum as well
as gases.
•
only through
gases and liquids.
•
only through
gases and solids.
•
all the three
states of matter.
ANS: all the three states of matter.
4. When lightning and thunder take place, they occur—
•
together and
are also observed together.
•
one after the
other but are observed together.
•
together but
the thunder is observed a little after the lightning.
•
together but
the thunder is observed a little before the lightning.
ANS: together but the thunder is observed a little
after the lightning.
5. A given sound is inaudible to the human ear. We can
then say that this is so—
•
only because
its amplitude is too small.
•
only because
its frequency is below 20 Hz.
•
only because
its frequency is above 20 kHz.
•
because it has
any one, or more, of the three characteristics listed above.
ANS: because it has any one, or more, of the three
characteristics listed above.
6. The following are some statements about sound:
(i) Sound propagates faster in steel than in air.
(ii) Sounds are produced through vibrations.
(iii) Sound propagates faster than light.
(iv) Sound does not require a medium for its
propagation.
•
(i) and (ii)
•
(ii) and (iii)
•
(iii) and (iv)
•
(i), (ii) and
(iv)
ANS: (i) and (ii)
1. Explain the production of sound in the human
beings.
ANS: In human beings, sound is produced by the voice
box or larynx present at the upper end of the windpipe. It contains two vocal
cords stretched across a slit. When air from the lungs passes through this
slit, the vocal cords vibrate and produce sound.
2. State the role of the following in the production
of voice in birds:
(a) windpipe
ANS: The windpipe carries air into and out of the
lungs. The air passing through it helps in producing sound.
(b) syrinx
ANS: Syrinx is the voice-producing organ in birds
present at the base of the windpipe. It contains vibrating membranes that
produce sound when air passes through them.
3. Explain how sound produced by a school bell reaches
your ears.
ANS: When the school bell is struck, it starts
vibrating. These vibrations produce compressions and rarefactions in the
surrounding air. The disturbance travels through air as sound waves and reaches
our ears. The eardrum vibrates, and we hear the sound.
4. What is meant by the following terms?
(a) amplitude of vibration
ANS: The maximum displacement of a vibrating body on
either side of its mean position is called amplitude of vibration.
(b) time period of vibration
ANS: The time taken by a vibrating body to complete
one vibration is called time period.
(c) frequency of vibration
ANS: The number of vibrations completed in one second
is called frequency of vibration.
5. State one condition under which—
(a) the pitch of a sound becomes higher.
ANS: The pitch of a sound becomes higher when its
frequency increases.
(b) the loudness of a sound becomes lower.
ANS: The loudness of a sound becomes lower when its
amplitude decreases.
6. Why are sounds produced by the following generally
unpleasant to hear?
(a) moving heavy vehicles in traffic
ANS: They produce irregular, harsh and loud sounds
which are unpleasant and irritating.
(b) bursting crackers
ANS: They produce sudden, very loud and unpleasant
sounds that cause discomfort and irritation.
7. State one harmful effect of each of the following
on us:
(a) high pitched sound
ANS: High pitched sound may cause irritation and
headache.
(b) loud sound
ANS: Loud sound may damage hearing and may even cause
temporary or permanent deafness.
8. State one use of sounds of frequency more than 20
kHz.
ANS: Sounds of frequency more than 20 kHz (ultrasonic
sounds) are used in medical ultrasonography.
1. Distinguish between music and noise.
ANS:
Music
Music is a pleasant and soothing sound.
It is produced by regular periodic vibrations.
It is melodious and agreeable to hear.
Noise
Noise is an unpleasant and irritating sound.
It is produced by irregular non-periodic vibrations.
It is harsh and unpleasant to hear.
2. State the factors on which the following
characteristics of sound depend:
(a) loudness
ANS: Loudness of a sound depends on the amplitude of
vibration of the source.
(b) pitch
ANS: Pitch of a sound depends on the frequency of
vibration of the source.
3. State the speed of sound in
(a) air
ANS: The speed of sound in air is about 340 m/s.
(b) water
ANS: The speed of sound in water is about 1500 m/s.
(c) steel
ANS: The speed of sound in steel is about 5000 m/s.
Give one reason to justify the above sequence.
ANS: Sound travels faster in media whose particles are
more closely packed. Since particles are closest in steel, less close in water
and far apart in air, sound travels fastest in steel and slowest in air.
4. Describe, with the help of a simple activity, to
show that sound cannot propagate through vacuum.
ANS: Take an electric bell in a glass jar connected to
a battery. When the bell is switched on, its sound is heard clearly. Now remove
the air from the jar gradually using a vacuum pump. As air is removed, the
sound becomes fainter and finally cannot be heard, though the hammer is still
seen striking the bell. This shows that sound cannot travel through vacuum.
5. Explain why:
(a) We can hear better at night than in day.
ANS: At night, the surrounding atmosphere is quieter
and there is less noise. Therefore, sounds can be heard more clearly.
(b) We hear thunder a little after seeing the
lightning.
ANS: Light travels much faster than sound, so the
light from lightning reaches us first and the sound of thunder is heard later.
6. State any four harmful effects of noise pollution.
ANS:
a) It may cause
irritation and stress.
b) It may cause
headache and lack of concentration.
c) It may disturb
sleep.
d) It may damage
hearing and cause deafness.
7. State four ways to control noise pollution.
ANS:
a) Avoid
unnecessary honking of vehicles.
b) Keep the
volume of TV, radio and music systems low.
c) Use silencers
in vehicles and machines.
d) Avoid bursting
loud crackers.
8. State the common causes of hearing impairment. How
is hearing impairment assessed?
ANS:
Common causes of hearing impairment are ear infection,
injury to the ear, old age, very loud sound and congenital defects. Hearing
impairment is assessed by hearing tests using an audiometer.
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