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Sound : S.Chand
pg 185
Very Short Answer Type Questions :-
Q.1 Can sound travel through- (a) iron (b) water ? Sol. a)Yes b)
Yes.
Q.2 Can sound travel through vacuum ? Sol. No.
Q.3 Name the type of waves which are used by astronauts to
communicate
with one another on moon (or in outer space). Sol.
Electromagnetic waves (Radio waves).
Q.4 Name one solid, one liquid and one gas through which sound
can travel. Sol. Solid - iron, liquid - water, gas - oxygen.
Q.5 Which of the following cannot transmit sound ?
Water, Vacuum, Aluminium, Oxygen gas Sol. Vacuum.
Q.6 Name the physical quantity whose SI unit is 'Hertz'. Sol.
Frequency.
Q.7 What is the SI unit of frequency ? Sol. Hertz.
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Q.8 What type of wave is represented -
(a) by density-distance graph ?
(b) by displacement-distance graph ? Sol. (a) Longitudinal
wave
(b) Transverse wave.
Q.9 Is the speed of sound more in water or in steel ? Sol. In
steel.
Q.10 In which medium sound travels faster : air or iron ? Sol.
Iron.
Q.11 In which medium sound travels fastest : air, water or steel
? Sol. Steel.
Q.12 Out of solids, liquids and gases :
(a) in which medium sound travels slowest ?
(b) in which medium sound travels fastest ? Sol. (a) Gases
(b) Solids
Q.13 Which of the following is the speed of sound in copper and
which in
aluminium ?
(a) 5100 m/s
(b) 1500 m/s
(c) 3750 m/s Sol. Copper - 3750 m/s ; Aluminum - 5100 m/s.
Q.14 If you want to hear a train approaching from far away, why
is it more
convenient to put the ear to the track ? Sol. Since sound travel
much faster in solids, It will be heard much sooner through a
track
than through air.
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Q.15 What is the speed of sound :
(a) in air ?
(b) in water ?
(c) in iron ? Ans. (a) In air - 346 m/s
(b) In water - 1490 m/s
(c) In iron - 5950 m/s.
Q.16 What name is given to those aircrafts which fly at speeds
greater than
the speed of sound? Sol. Supersonic jets.
Q.17 A jet aircraft flies at a speed of 410 m/s. What is this
speed known as ? Sol. Supersonic speed.
Q.18 What is meant by supersonic speed ? Sol. When the speed of
an object is greater than the speed of sound, it is called
supersonic speed.
Q.19 State one observation from everyday life which shows that
sound
travels much more slow than light. Sol. Thunderstorm is heard
after lightning is seen.
Q.20 Name the two types of waves which can be generated in a
long flexible
spring (or slinky). Sol. Longitudinal and transverse waves can
be produced.
Q.21 A stone is dropped on the surface of water in a pond. Name
the type of
waves produced. Sol. Transverse waves.
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Q.22 Name the type of waves produced when a tuning fork is
struck in air. Sol. Longitudinal waves.
Q.23 What is the general name of the waves consisting of :
(a) compressions and rarefactions ?
(b) crests and troughs ? Sol. (a) Longitudinal waves
(b) Transverse waves.
Q.24 State the general name of the waves in which the particles
of the
medium vibrate:
(i) in the same direction as wave.
(ii) at right angles to the direction of wave. Sol. (a)
Longitudinal waves
(b)Transverse waves.
Q.25 What type of waves are illustrated by the movement of a
rope whose
one end is fixed to a pole and the other end is moved up and
down ? Sol. Transverse wave.
Q.26 What should an object do to produce sound ? Sol.
Vibrate.
Q.27 What is the name of the strings which vibrate in our voice
box when we
talk ? Sol. Vocal cords.
Q.28 Name the device which is used to produce sound in
laboratory
experiments. Sol. Tuning fork.
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Q.29 What is the nature of sound waves in air ? Sol.
Longitudinal Waves.
Q.30 What conclusion can be obtained from the observation that
when the
prongs of a sound making tuning fork touch the surface of water
in a beaker,
the water gets splashed? Sol. That the prongs of tuning fork are
vibrating.
Q.31 State whether the following statement is true or false
:-
Sound produced by a vibrating body travels to our ears by the
actual
movement of air. Sol. False.
Q.32 Which of the following travels slowest in air and which one
fastest ?
Supersonic aircraft, Light, Sound Sol. Slowest - Sound and
Fastest - Light.
Q.33 Which term is used to denote a speed greater than the speed
of sound ? Sol. Supersonic.
Q.34 In which medium sound travels faster : air or hydrogen ?
Sol. Hydrogen.
Q.35 A tuning fork has a number 256 marked on it. What does this
number
signify ? Sol. Frequency of the tuning fork .
Q.36 What is the time-period of a tuning fork whose frequency is
200 Hz ? Sol. =1T = 5103s.
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Q.37 Calculate the frequency of a wave whose time-period is 0.02
s. Sol. =1T
=10.02=50 Hz.
Q.38 What will be the change in the wavelength of a sound wave
in air if its
frequency is doubled ? Sol. Wavelength is halved.
Q.39 If 20 waves are produced per second, what is the frequency
in Hertz ? Sol. 20 Hz.
Q.40 Fill in the following blanks with suitable words :
(a) Sound is caused by.............
(b) A sound wave consists of places of higher pressure called
............. and
places of........ pressure called.......
(c) Wave speed in metres per second equals frequency
in........... multiplied
by.............in...............
(d) Sound cannot travel through..................
(e) The speed of sound in a solid is................... than the
speed of sound in air.
(f) When the frequency of the sound is increased, the
wavelength................
Sol.
(a) Vibrations
(b) Compressions : lower pressure ; rarefactions
(c) Hertz ; wavelength ; metres
(d) Vacuum
(e) Greater
(f) Decreases.
Short Answer Type Questions :-
Q.41 What is vacuum ? Explain why, sound cannot travel through
vacuum ? Sol. Region where there are no particles of matter is
called vacuum. Sound cannot travel
through vacuum because it needs a medium to travel.
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Q.42 Explain the term 'amplitude' of a wave. Draw the diagram of
a wave
and mark its amplitude on it. Sol. The maximum displacement of a
particle from its mean position is called amplitude.
In below figure A is amplitude.
Q.43 (a) Distinguish between longitudinal and transverse
waves.
(b) Are sound waves longitudinal or transverse ? Sol. (a)
Transverse wave
1. The particles move or vibrate about their mean position in a
direction
perpendicular to the direction of
propagation.
2. It travels in the form of crests and troughs.
3. Light waves are transverse waves and may travel in
vacuum.
Longitudinal waves
1. The particles vibrate about their mean position in a
direction parallel to the
direction of propagation.
2. It travels in the form of compression and rarefaction.
3. Sound waves are longitudinal waves and can be produced only
in a material
medium.
(b) Sound waves are longitudinal.
Q.44 A cricket ball is seen to hit the bat first and the sound
of hitting is heard
a little later. Why? Sol. Since light travels much faster than
sound, we see the ball hit the bat and hear the
sound later on.
Q.45 Explain why, the flash of lightning reaches us first and
the sound of
thunder is heard a little later. Sol. Since light travels at a
very great speed as compared to the speed of sound, we see
the flash before the sound.
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Q.46 Explain why, the flash of a gun shot reaches us before the
sound of the
gun shot. Sol. Since light travels at a very great speed as
compared to the speed of sound, we see
the flash before the sound.
Q.47 Which of the following terms apply to sound waves in air
and which to
water waves?
Transverse, Rarefaction, Trough, Crest, Compression,
Longitudinal Sol. Sound wave - rarefaction, compression,
longitudinal
Water wave - transverse, trough, crest.
Q.48 (a) Name four ways in which sound can be produced.
(b) Calculate the speed of a sound wave whose frequency is 2 kHz
and
wavelength 65 cm. Sol. (a) Sound can be produced by vibration of
vocal chords, blowing air in the flute,
striking a drum , explosion.
(b) = 2 KHz = 2000 Hz
= 65 cm = 0.65 m
V = = 2000 0.65 = 1300 m/s.
Q.49 If a ringing bicycle bell is held tightly by hand, it stops
producing
sound. Why ? Sol. Sound is produced due to vibration in the
bell. When the bell is held tightly in the
hand, the vibrations stop. Hence sound stops.
Q.50 Which object is vibrating when the following sounds are
produced ?
(i) The sound of a sitar
(ii) The sound of a tabla
(iii) The sound of a tuning fork
(iv) The buzzing of a bee or mosquito
(v) The sound of a flute Sol. (i) string
(ii) membrane of the tabla
(iii) prongs of the tuning fork
(iv) wings of the bee
(v) The air column inside the flute.
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Q.51 Give reason for the following :
In most of the cases, we cannot see the vibrations of a
sound
producing object with our eyes. Sol. The lowest frequency that
we can hear is 20 Hz. The time period of this vibration
will be 120s or 0.05s. Due to persistence of vision in our eyes
we cannot see vibration of less
than 0.1 s.Therefore we cannot see the vibrations of any sound
producing object.
Q.52 Describe a simple experiment to show that the prongs of a
sound
producing tuning fork are vibrating.
Sol. Suspend a tennis ball on a rigid support by means of a
string. Touch the vibrating
tuning fork. You will are that the tennis ball is set into
motion by the vibration.
Q.53 When we open a gas tap for a few seconds, the sound of
escaping gas is
heard first but the smell of gas comes later Why ? Sol. The
speed with which the gas moves is much lower than the speed of
sound. So we
can hear the sound of escaping gas much before we smell it.
Q.54 A sound signal of 128 vibrations per second has a
wavelength of 2.7 m.
Calculate the speed with which wave travels. Sol. = 128 Hz ; =
2.7 m
V = = 345.6 m/s
Q.55 A wave is moving in air with a velocity of 340 m/s.
Calculate the
wavelength if its frequency is 512 vibrations/sec ?
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Sol. V = 340 m/s ; = 512 Hz
=V=340512=0.67 m
Q.56 Define the 'frequency' and 'time-period' of a wave. What is
the relation
between the two? Sol. Frequency is the number of vibration
produced in one second ().
Time period is the time of one vibration,
Time period (T) =1
Q.57 Explain why, a ringing bell suspended in a vacuum chamber
cannot be
heard outside. Sol. As sound is a mechanical wave it cannot
travel in vacuum. So the bell ringing inside a
vacuum chamber cannot be heard.
Q.58 The frequency of the sound emitted by the loudspeaker is
1020 Hz.
Calculate the wavelength of the sound wave in air in cm where
its velocity is
340 m/s. Sol. = 1020 Hz,V = 340 m/s
=V=3401020=0.33 m
Q.59 What is the difference between a compression and a
rarefaction in a
sound wave ? Illustrate your answer with a sketch. Sol.
Compression : Particles of the medium are pushed close to each
other. It is a region
of high density and pressure.
Rarefaction : Particles of the medium move away. It is a region
of low density and
pressure.
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Long Answer Type Questions :-
Q.60 (a) What is sound ? What type of waves are sound waves in
air ?
(b) Describe an experiment to show that sound cannot pass
through
vacuum. Sol.
(a) Sound is a form of energy which gives the sensation of
hearing waves. Sound waves are
longitudinal waves.
(b) Take an electric bell and an airtight glass bell jar. The
electric bell is suspended inside the
airtight bell jar. The bell jar is connected to a vacuum pump,as
shown in Fig. If you press the
switch, you will be able to hear the bell. Now start the vacuum
pump. When the air in the jar
is pumped out gradually, the sound becomes fainter, although the
same current is passing
through the bell. After some time when less air is left inside
the bell jar you will hear a very
feeble sound. When all air is removed no sound is heard. This
shows that sound can not pass
through vacuum.
Q.61 (a) How is sound produced ? Explain with the help of an
example.
(b) How does sound from a sound producing body travel through
air
to reach our ears ? Illustrate your answer with the help of a
labelled diagram. Sol.
(a) Sound is produced by the vibrations of any object. These
vibrations are transferred to air
particles and reach to us. For example - In school bell, when
the gong strikes the bell,
vibrations are produced in the bell which are transmitted
through the air to our ears. These
vibration produce sensation of sound in our ears.
(b) When a body vibrates the air in its neighbourhood is
alternately compressed and rarefied.
The compressed air has higher pressure than surrounding air. It
therefore pushes the air
particles near it causing compression to move forward. A
rarefaction or low pressure is
created at the original place.
These compressions and rarefaction cause particles in the air to
vibrate about their mean
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position. The energy is carried forward in these vibration. This
is how sound travels.
Q.62 (a) An electric bell is suspended by thin wires in a glass
vessel and set
ringing. Describe and explain what happens if the air is
gradually pumped out
of the glass vessel.
(b) Why cannot a sound be heard on the moon ? How do
astronauts
talk to one another on the surface of moon ?
Sol.
(a) As the air inside the vacuum tube is pumped out, it becomes
fainter. When all air is
pumped out , no sound is heard. This is because air requires a
material medium to travel.
(b) There is no atmosphere in the moon, so sound cannot
propagate. Astronauts talk through
radio wave.
Q.63 (a) Define the terms 'frequency','wavelength' and
'velocity' of a sound
wave. What is the relation between them ?
(b) A body vibrating with a time-period of 1256s produces a
sound wave
which travels in air with a velocity of 350 m/s. Calculate the
wavelength.
Sol.
(a) Frequency : One compression and one rarefaction constitutes
one vibration. The number
of vibration in a second is called frequency. Its unit is
Hertz.
Wavelength : The distances between two consecutive compressions
or rarefaction of a wave.
Its S.I unit is meter.
Velocity : Velocity is defined as distance travelled by wave in
1 s. It is calculated by the
formula v=T [where is distance travelled by 1 wave, and T is the
time taken by 1 wave.]
(b) T = 1256 s , v = 350 m/s
V=T , therefore = v x t = 3501256 = 1.37 m.
Q.64 (a) What are longitudinal waves and transverse waves ?
Explain with
the help of labelled diagrams.
(b) Give two examples each of longitudinal waves and
transverse
waves.
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Sol. (a)Longitudinal waves
1. The particles vibrate about their mean position in a
direction parallel to the
direction of propagation.
2. It travels in the form of compression and rarefaction.
3. Sound waves are longitudinal waves and can be produced only
in a material
medium.
Transverse wave
1. The particles moves or vibrate about their mean position in a
direction
perpendicular to the direction of
propagation.
2. It travels in the form of crests and troughs.
3. Light waves are transverse waves and may travel in
vacuum.
(b) Longitudinal wave : Sound
waves, Compression of spring causes longitudinal waves.
Transverse wave : Light, waves on the surface of water.
Q.65 (a) Explain the terms 'compressions' and 'rarefactions' of
a wave.
What type of waves consist of compressions and rarefactions
?
(b) A worker lives at a distance of 1.32 km from the factory. If
the
speed of sound in air be 330 m/s, how much time will the sound
of factory
siren take to reach the worker ? Sol.
(a) Compression : Particles of the medium are pushed close to
each other. It is a region of
high density and pressure.
Rarefaction : Particles of the medium move away. It is a region
of low density and pressure.
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Longitudinal waves consist of compressions and rarefaction.
(b) Distance of factory = 1.32 km =1.32 1000 m
Speed of sound = 330 m/s
Time for sound to reach work =distancespeed=1.321000330s=4 s
Q.66 (a) Explain the terms 'crests' and 'troughs' of a wave ?
What type of
waves consist of crests and troughs?
(b) The flash of a gun is seen by a man 3 seconds before the
sound is
heard. Calculate the distance of the gun from the man. (Speed of
sound in air
is 332 m/s).
Sol.
(a) When a wave propagates , the particles of the medium vibrate
perpendicular to the
direction of the wave. The maximum displacement of particle on
one side of the wave (+ve
side) is called crest and on the other is called trough. Crests
and troughs occurs in transverse
waves.
(b) Time taken by flash to reach observer = 3 s
speed of sound in air = 332 m/s
So distance travelled = v t = 332 3 = 996 m
Q.67 (a) When we put our ear to a railway line, we can hear the
sound of an
approaching train even when the train is far off but its sound
cannot be heard
through the air. Why ?
(b) How could you convince a small child that when you speak, it
is not
necessary for air to travel from your mouth to the ear of a
listener ? Sol.
(a) Since sound travels faster in solids, we can hear the sound
of train by putting our ear on
the solid and only later we can hear though air.
(b) Take a slinky end tie one end to the wall and give the other
end a push or pull alternately.
Compressions and rarefactions move towards the wall i.e. wave
moves but the parts of the
slinky just vibrates about its mean position.
In the same way compressions and rarefactions are produced in
air and no net movement of
air particles is required for sound to travel.
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Multiple Choice Questions (MCQs) :-
Q.68 Which of the following statement best describes frequency
?
(a) the maximum disturbance caused by a wave
(b) the number of complete vibrations per second
(c) the distance between one crest of a wave and the next
one
(d) the distance travelled by a wave per second Sol. (b)
Q.69 Which of the following vibrates when a musical note is
produced by the
cymbals in an orchestra ?
(a) stretched strings (b) stretched membranes
(c) metal plates (d) air columns Sol. (c)
Q.70 If the speed of a wave is 340 m/s and its frequency is 1700
Hz, then
for this wave in cm will be :
(a) 2 (b) 0.2 (c) 20 (d) 200 Sol. (c)
Q.71 A musical instrument is producing a continuous note. This
note cannot
be heard by a person having a normal hearing range. This note
must then be
passing through :
(a) water (b) wax
(c) vacuum (d) empty vessel Sol. (c)
Q.72 Which one of the following does not consist of transverse
waves?
(a) light emitted by a CFL
(b) TV signals from a satellite
(c) ripples on the surface of a pond
(d) musical notes of an orchestra Sol. (d)
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Q.73 Sound travels in air :
(a) if particles of medium travel from one place to another
(b) if there is no moisture in the atmosphere
(c) if disturbance moves
(d) if both, particles as well as disturbance move from one
place to
another Sol. (c)
Q.74 In the sound wave produced by a vibrating turning fork
shown in the
diagram, half the wavelength is represented by :
(a) AB (b) BD (c) DE (d) AE
Sol. (b)
Q.75 The maximum speed of vibrations which produce audible sound
will be
in
(a) dry air (b) sea water
(c) ground glass (d) human blood Sol. (c)
Q.76 The sound waves travel fastest :
(a) in solids (b) in liquids
(c) in gases (d) in vacuum Sol. (a)
Q.77 The speeds of sound in four different media are given
below. Which of
the following is the most likely speed in m/s with which the two
under water
whales in a sea talk to each other when separated by a large
distance ?
(a) 340 (b) 5170 (c) 1280 (d) 1530 Sol. (d)
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Q.78 When the pitch of note produced by a harmonium is lowered,
then the
wavelength of the note :
(a) decreases
(b) first decreases and then increases
(c) increases
(d) remains the same Sol. (c)
Q.79 The velocities of sound waves in four media P, Q, R and S
are 18,000
km/h, 900 km/h, 0 km/h, andd 1200 km/h respectively. Which
medium could
be a liquid substance ?
(a) P (b) Q (c) R (d) S Sol. (d)
Q.80 Which of the following can produce longitudinal waves as
well as
transverse waves under different conditions?
(a) water (b) TV transmitter
(c) slinky (d) tuning fork Sol. (c)
Questions Based on High Order Thinking Skills (HOTS) :-
Q.81 Draw the sketches of two waves A and B such that wave A has
twice
the wavelength and half the amplitude of wave B.
Sol.
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Q.82 A device called oscillator is used to send waves along a
stretched string.
The string is 20 cm long, and 4 complete waves fit along its
length when the
oscillator vibrates 30 times per second. For the waves on the
string :
(a) what is their wavelength ?
(b) what is their frequency ?
(c) what is their speed ? Sol.
Length of string = 20 cm containing 4 waves
Therefore, length of one wave=204cm=5cm = wavelength
Frequency = 30/s
(a)wavelength, = 5 cm = 0.05 m
(b)frequency = 30 Hz
(c)speedV==.0530=1.5m/s
Q.83 Through which of the following materials can sound travel
?
Wood, air, water, steam, ice, hydrogen, steel, diamond. Sol. All
the materials.
Q.84 A sound producing body is at considerable distance from a
man. There
can be four different media W,X,Y and Z between the sound
producing body
and the man. The medium X brings the sound to man most quickly
whereas
medium Z takes the maximum time. The time taken by medium W
in
bringing sound to man is less than that of X but more than that
of Z. The
medium Y, however, fails to bring the sound from the sound
producing body
to the man. Which medium could be the one :
(a) having no fixed shape and no fixed volume ?
(b) having a fixed volume but no fixed shape ?
(c) having the same composition as that on the moon ?
(d) having a fixed shape and a fixed volume ? Sol.
(a) Z(gas)
(b) W (liquid)
(c) Y (vacuum)
(d) X (solid)
Q.85 The longitudinal waves travel in a coiled spring at a rate
of 4 m/s. The
distance between two consecutive compressions is 20 cm. Find
:
(i) Wavelength of the wave
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(ii) Frequency of the wave Sol. V = 4 m/s
(1) Wavelength = 20 cm = 0.20 m
(2) Frequency =v=4.2=20 Hz
Short Answer Type Questions Pg 206
Q.1 Which property of sound leads of the formation of echoes ?
Sol. Reflection of sound.
Q.2 What name is given to the repetition of sound caused by the
reflection of
sound waves ? Sol. Echo.
Q.3 What name is given to the persistence of sound in a big hall
or
auditorium ? Sol. Reverberation.
Q.4 Name three devices which work on the reflection of sound.
Sol. Loud hailers, stethoscopes, and sounding boards.
Q.5 What is the other name of a loud-hailer ? Sol.
Megaphone.
Q.6 Name the three characteristics of sound. Sol. Frequency,
Amplitude, Speed.
Q.7 Name the unit used to measure the loudness of sound. Also
write its
symbol. Sol. Decibels , its symbol is dB.
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Q.8 Name the characteristic which helps us distinguish between a
man's
voice and a woman's voice, even without seeing them. Sol.
Pitch
Q.9 How does the pitch of a sound depend on frequency ? Sol.
Higher the frequency , higher will be the pitch.
Q.10 Name the characteristic of sound which depends on (a)
amplitude (b)
frequency, and (c) waveform. Sol. (a) Loudness (b) pitch (c)
quality.
Q.11 Name the characteristic of sound which can distinguish
between the
'notes' (musical sounds) played on a flute and a sitar (both the
notes having
the same pitch and loudness). Sol. Quality or timbre.
Q.12 Name the organs of hearing in our body. Sol. Ears.
Q.13 Name that part of ear which vibrates when outside sound
falls on it. Sol. Tympanic membrane.
Q.14 Name the three tiny bones present in the middle part of
ear. Sol. Hammer , anvil , stirrups.
Q.15 There are three small bones in the middle ear : anvil,
hammer and
stirrup :
(a) Which of these bones is in touch with ear-drum ?
(b) Which of these bones is in touch with oval window ? Sol. (a)
hammer
(b) stirrup.
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Q.16 What is the function of three tiny bones in the ear ? Sol.
To amplify the vibrations of sound.
S.Chand pg 207.
Q.17 Name the tube which connects the middle ear to throat. Sol.
Eustachian tube.
Q.18 Name the nerve which carries electrical impulses from the
cochlea of
ear to the brain. Sol. Auditory nerve.
Q.19 What is the name of passage in outer ear which carries
sound waves to
the ear-drum? Sol. Auditory canal.
Q.20 Why should we not put a pin or pencil in our ears ? Sol. It
may damage the ear drum.
Q.21 What type of scans are used these days to monitor the
growth of
developing baby in the uterus of the mother ? Sol.
Ultrasound.
Q.22 How is an ultrasound scan for fetus (unborn baby) better
than X-ray ? Sol. X-rays can damage the body cells of an unborn
baby.
Q.23 What is the name of the device which is used to find the
depth of sea
(or ocean) by using ultrasonic sound waves ? Sol. SONAR.
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Q.24 Write the full name of 'SONAR'. Sol. Sound , Navigation and
Ranging.
Q.25 Name the principle on which a soundboard works. Sol.
Reflection of sound.
Q.26 Name the device which is used to address a small gathering
of people. Sol. Megaphone.
Q.27 Name the device used by doctors to listen to our
heartbeats. Sol. Stethoscope.
Q.28 What is the shape of a soundboard kept behind the speaker
on the stage
of a big hall ? Sol. Concave.
Q.29 Name two sound absorbing materials (or objects) which can
make our
big room less echoey. Sol. Curtain, Carpets.
Q.30 Can we hear (a) infrasonic waves (b) ultrasonic waves ?
Sol. No.
Q.31 What name is given to the sound waves of frequency too low
for
humans to hear ? Sol. Infra sound.
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Q.32 What name is given to the sound waves of frequency too high
for
humans to hear ? Sol. Ultrasound.
Q.33 What type of sound waves are produced by a vibrating
simple
pendulum ? Sol. Infra sound.
Q.34 What happens to the pitch of a sound if its frequency
increases ? Sol. Increases.
Q.35 What happens to the loudness of a sound if its amplitude
decreases ? Sol. Decreases.
Q.36 What name is given to sound waves of frequencies higher
than 20 kHz ? Sol. Ultrasound.
Q.37 Fill in the following blanks with suitable words :
(a) An echo is simply a.......sound.
(b) Pitch of sound depends on......
(c) Loudness of sound depends on.....
(d) Quality of sound depends on.....
(e) Echoes are caused by the........of sound. Sol. (a)
Reflected
(b) Frequency
(c) Amplitude
(d) Waveform
(e) Reflection
Q.38 On which day, a hot day or a cold day, an echo is heard
sooner ? Give
reason for your answer. Sol. On a hot day, echo is heard sooner
since velocity of sound increases with
temperature.
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Q.39 In which medium, air or water, an echo is heard much sooner
? Why ? Sol. In water , since speed of sound is greater in
liquids.
Q.40 What is reverberation ? What will happen if the
reverberation time in a
big hall is too long? Sol. The multiple reflection of sound from
the walls of a room or hall causing sound to
persist for some time is called reverberation. If the hall is
too long , the reflection will
become an echo.
Q.41 How can reverberations in a big hall or auditorium be
reduced ? Sol. Reverberation can be reduced by covering the walls
and ceiling with sound
absorbent materials like curtains and false ceilings.
Q.42 Why do we hear more clearly in a room with curtains than in
a room
without curtains ? Sol. There is less reverberation in rooms
with curtains so we can hear sound more clearly
as the sound is absorbed by the curtains.
Q.43 What is a megaphone ? Name the principle on which a
megaphone
works. Sol. Megaphone is a device to direct sound waves in one
direction so that the intensity of
sound in that direction increases. It s based on the principle
of reflection of sound.
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Q.44 What is a bulb horn ? Name the principle on which a bulb
horn works. Sol. A bulb horn is a tube followed by a conical
opening. As the bulb is pressed the
waves of air come out through the opening and creates sound. It
is based on the principle of
multiple reflection of sound.
Q.45 What is a stethoscope ? Name the principle on which a
stethoscope
works. Sol. Stethoscope is a medical instrument used for
listening to sounds produced within
the body, chiefly in the heart or lungs.
Principle - multiple reflection of sound.
Q.46 What is a soundboard ? Explain the working of a soundboard
with the
help of a labelled diagram. Sol. In an auditorium or cinema
halls the wall behind the speaker's platform is curved in
such a way that sound after reflection moves parallel in the
direction of the audience. Due to
which the loudness of sound is increased.
Q.47 (a) What is meant by the 'loudness' of sound ? On what
factor does the
loudness of a sound depend ?
(b) Draw labelled diagrams to represent
(i) soft sound, and
(ii) loud sound, of the same frequency. Sol. (a) Loudness is the
physiological response of our ears to sound. It depends on
amplitude and frequency and also age of the person. Human beings
can hear sound of
frequency between 20 Hz to 20,000 Hz.
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Q.48 (a) Explain the term 'pitch' of a sound. On what factor
does the 'pitch'
of a sound depend?
(b) Draw labelled diagrams to represent sound of
(i) low pitch, and
(ii) high pitch, of the same loudness. Sol. (a) Pitch is that
characteristic of sound that determines the shrillness of sound.
It
depends on frequency of sound.
(b)
Q.49 What is meant by the quality (or timbre) of sound ? On what
factor
does the quality (or timbre) of a sound depend? Sol. The quality
or timbre of sound is that characteristic which enables us to
distinguish
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one sound from another having the same pitch and loudness. The
sound which is more
pleasant is said to be of a rich quality.
Q.50 Explain why, if we strike a table lightly, we hear a soft
sound but if we
hit the table hard, a loud sound is heard. Sol. If we strike a
table lightly the vibrations produced have less amplitude than
when
we strike the table hard. Since loudness depends on amplitude,
soft sound is heard when we
strike lightly.
Q.51 Give one use of ultrasound in industry and one in
hospitals? Sol. Use of ultrasound
(1) To clean parts of machines hard to reach.
(2) Echocardiography is used to form image of heart.
Q.52 How is it that bats are able to fly at night without
colliding with other
objects? Sol. Bats are able to fly at night without colliding
with other objects as they use
reflection of ultrasound to know what object are around
them.
Q.53 Explain how, bats use ultrasound to catch the prey. Sol.
Bats emit ultrasound. These are reflected by various obstacles and
return to the bats
ear. The nature of reflection tells the bat where the obstacles
or prey is and accordingly the
bat is able to catch its prey.
Q.54 Explain how, flaws (or defects) in a metal block can be
detected by
using ultrasound. Sol. Ultrasound is passed through the metal
block which has to be tested. In case of flaws
ultrasound does not pass through it but is reflected back. A
detector on the other side of the
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block does not receive all the transmitted waves and hence the
flow is detected .
Q.55 Why are the ceilings of concert halls made curved ? Draw a
labelled
diagram to illustrate your answer. Sol. The walls and ceilings
are made curved so that sound reaches all parts of the hall
after
reflection.
Q.56 Draw a labelled diagram to show the multiple reflections of
sound in a
part of the stethoscope tube.
Sol.
Multiple reflection within a stethoscope
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Q.57 What is the range of frequencies associated with
(a) infra sound
(b) audible sound, and
(c) ultrasound ? Sol. (a) less than 20 Hz
(b) 20 Hz to 20,000 Hz
(c) Above 20,000 Hz
Q.58 (a) What is the difference between infrasonic waves and
ultrasonic
waves ?
(b) Choose the infrasonic waves and ultrasonic waves from
the
following frequencies :
(i) 10,000 Hz (ii) 30,000 Hz (iii) 18 Hz
(iv) 50,000 Hz (v) 10 Hz Sol. (a) Infrasonic - less then 20
Hz
Ultrasonics- more than 20,000 Hz
(b) Infrasonic - 10 Hz, 18 Hz
Ultrasonic - 30,000 Hz, 50,000 Hz
Q.59 (a) What is the frequency range of hearing in humans ?
(b) Which of the following sound frequencies cannot be heard by
a
human ear ?
(i) 10 Hz (ii) 100 Hz (iii) 10,000 Hz
(iv) 15 Hz (v) 40,000 Hz Sol. (a) 20 Hz - 20,000 Hz
(b) (i), (iv) and (v)
Q.60 The echo of a sound is heard after 5 seconds. If the speed
of sound in
air be 342 m/s, calculate the distance of the reflecting
surface. Sol. Echo time = 5 s
Speed of sound = 342 m/s
Distancevt2=34252=855m
Q.61 The speed of sound in water is 1500 meters per second. How
far away
from an under-sea rock should a deep sea diver be so that he can
hear his own
echo ? Sol. Speed of sound in water = 1500 m/s
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Minimum time of sound for the echo to be heard = 0.1 s
Minimum distance =VT2=15000.12=75m
Long Answer Type Questions
Q.62 (a) What is meant by 'reflection of sound' ? What type of
surfaces are
the best for reflecting sound ?
(b) Name any two objects which are good reflectors of sound.
(c) State the laws of reflection of sound. Sol. (a) The bouncing
back of sound wave is called reflection . Hard surfaces are the
best
reflection surfaces
(b) Walls, cliffs
(c) The laws of reflection of sound are -
(1) The incident wave, reflected wave and the normal all lie in
the same plane.
(2) The angle of incidence = angle of reflection
Q.63 (a) What is an echo ? How is echo formed ?
(b) What is the minimum distance in air required from a
sound
reflecting surface to hear an echo (at 20C)
(c) A man standing 825 meters away from a cliff (steep rock)
fires a
gun. After how long will he hear echo? Speed of sound in air is
330 m/s.
Sol. (a) The repetition of sound due to reflection is called
echo. It is formed if reflection
of sound is heard after 0.1 s.
(b) At 20o , speed of sound = 344 m/s
Time for echo to reach = 0.1 s
Minimum distance = v t2 = 344 0.12 = 17.2m
(c) distance d = 825m
speed V = 330 m/s
t=2dv=2825330=5s
Q.64 (a) What is ultrasound ? What is the difference between
ordinary sound
and ultrasound ?
(b) Write any three applications (or uses) of ultrasound. Sol.
(a) Sound having frequency greater then 20,000 hz is called
ultrasound. Ultrasound
cannot be heard by human beings like ordinary sound.
(b) Uses of ultrasound -
(1) To check flaws and cracks in metal
(2) In ultrasonography
(3) In echocardiography
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Q.65 (a) What are infrasonic waves ? Name two animals which
produce
infrasonic waves.
(b) What are ultrasonic waves ? Name two animals which can
produce
ultrasonic waves
(c) The audible range of frequencies of an average human ear is
from
20 Hz to 20 kHz. Calculate corresponding wavelengths. (Speed of
sound in air
is 344ms1 ). Sol. (a) Infrasonic are vibration of frequency less
then 20 Hz. Whales and elephants
produced infrasonic.
(b) Sound waves of frequencies greater then 20,000 Hz are called
ultrasonic waves.
Porpoises and bats produce ultrasonic waves.
(c) 1=v=34420=17.2m
2=34420,000=0.0172m
Q.66 (a) Define the following terms :
(1) Echolocation (2) Echocardiography (3) Ultrasonography.
(b) Name an animal which navigates and finds food by
Echolocation.
(c) Which of the two produces ultrasonic waves : porpoise or
whale ? Sol. (a)
(1) Echolocation is the technique by which objects are located
by sensing time of
echo of the sound emitted
(2) Formation of image of the heart by reflection of ultrasound
from heart is called
echocardiography.
(3) Creating images of different parts of the body by means of
reflection of
ultrasound is called Ultrasonography.
(b) Bats
(c) Porpoises
Q.67 (a) What is sonar ? Explain its use.
(b) A sonar station picks up a return signal after 3 seconds.
How far
away is the object ? (Speed of sound in water = 1440 m/s). Sol.
(a) SONAR stand for Sound, Navigation And Ranging. It is a device
used to
measure distance, direction and speed of underwater objects. It
has a transmitter and detector
near its base. The transmitter transmits ultrasonic signals
which get reflected by various
underwater objects. These are received by the detector which can
convert these waves into
appropriate electrical signals and give us the required
information.
(b) Time of echo = 3 s
speed of sound = 1440 m/s
Distance = =vt2=144032=2160m
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Q.68 Draw a neat and labelled diagram of the human ear. With the
help of
this diagram, explain the construction and working of the human'
ear? Sol. The human ear consists of three parts the outer ear,
middle ear and inner ear.
Outer ear : This is also called pinna. It collects the sound
from the surrounding
and directs it towards auditory canal.
Middle ear : The sound reaches the end of the auditory canal
where there is a thin
membrane called eardrum or tympanic membrane. The sound waves
set this membrane to
vibrate. These vibrations are amplified by three small bones-
hammer, anvil and stirrup.
Inner ear : These vibration reach the cochlea in the inner ear
and are converted into
electrical signals which are sent to the brain by the auditory
nerve, and the brain interprets
them as sound.
Multiple Choice Questions (MCQs)
Q.69 In SONAR we use :
(a) ultrasonic waves (b) infrasonic waves
(c) radio waves (d) audible sound waves Sol. (a)
Q.70 When we change a feeble sound to a loud sound, we increase
its :
(a) frequency (b) amplitude
(c) velocity (d) wavelength Sol. (b)
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Q.71 Which kind of sound is produced in an earthquake before the
main
shock wave begins?
(a) ultrasound (b) infra sound
(c) audible sound (d) none of the above Sol. (b)
Q.72 Before playing the orchestra in a musical concert, a
sitarist tries to
adjust the tension and pluck the strings suitably. By doing so
he is adjusting
(a) intensity of sound only
(b) amplitude of sound only
(c) frequency of the sitar string with the frequency of other
musical
instruments
(d) loudness of sound Sol. (c)
Q.73 'Note' is a sound :
(a) of a mixture of several frequencies
(b) of mixture of only two frequencies
(c) of a single frequency
(d) always unpleasant to listen to Sol. (c)
Q.74 A key of mechanical piano is first struck gently and then
struck again
but much harder this time. In the second case :
(a) sound will be louder but pitch will not be different
(b) sound will be louder and the pitch will also be higher
(c) sound will be louder but pitch will be lower
(d) both loudness and pitch will remain unaffected Sol. (a)
Q.75 One of the following can hear infra sound. This one is
:
(a) dog (b) bat
(c) rhinoceros (d) humans Sol. (c)
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Q.76 An echo-sounder in a trawler (fishing boat) receives an
echo from a
shoal of fish 0.4 s after it was sent. If the speed of sound in
water is 1500 m/s,
how deep is the shoal ?
(a) 150 m (b) 300 m (c) 600 m (d) 7500 m Sol. (b)
Q.77 The speed of highly penetrating ultrasonic waves is :
(a) lower than those of audible sound waves
(b) higher than those of audible sound wave
(c) much higher than those of audible sound waves
(d) same as those of audible sound waves Sol. (d)
Q.78 The ultrasound waves can penetrate into matter to a large
extent
because they have :
(a) very high speed
(b) very high frequency
(c) very high wavelength
(d) very high amplitude Sol. (b)
Q.79 The frequencies of four sound waves are given below. Which
of these
sound waves can be used to measure the depth of sea by the echo
method ?
(a) 15,000 Hz (b) 10 kHz
(c) 50 kHz (d) 10,000 Hz Sol. (c)
Q.80 Which of the following frequency of sound can be generated
by a
vibrating simple pendulum as well as by the vibrating vocal
cords of a
rhinoceros?
(a) 5 kHz (b) 25 Hz
(c) 10 Hz (d) 15,000 Hz Sol. (c)
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Q.81 Which of the following device does not work on the multiple
reflections
of sound waves ?
(a) stethoscope
(b) hydrophone
(c) soundboard
(d) megaphone Sol. (b)
Q.82 What type of waves are generated by SONAR device fixed to a
fishing
ship ?
(a) water waves
(b) radio waves
(c) sound waves
(d) infrared waves Sol. (c)
Q.83 We can distinguish between the musical sounds produced by
different
singers on the basis of the characteristic of sound called :
(a) frequency (b) timbre
(c) pitch (d) loudness Sol. (b)
Q.84 At 20C, the minimum distance of a person from a sound
reflecting
surface to hear an echo is :
(a) 12.2 m (b) 17.2 m (c) 15.2 m (d) 34.4 m Sol. (b)
Questions Based on High Order Thinking Skills (HOTS)
Q.85 The drawing shows a ship 800 in from a cliff. A gun is
fired on the ship.
After 5 seconds the people at the front of the ship hear the
sound of the gun
again.
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(a) What is the name of this effect ?
(b) What happens to the sound at the cliff ?
(c) How far does the sound travel in 5 seconds ?
(d) Calculate the speed of sound ?
Sol. (a) Echo
(b) The sound gets reflected
(c) 1600 m
(d) speed of sound =distancetime=16005=320m/s
Q.86 Consider the following sound waves marked A, B, C and
D:
(a) Which two waves represent sounds of the same loudness
but
different pitch?
(b) Which two waves represent sounds of the same frequency
but
different loudness?
(c) State whether all these sound waves have been produced by
the
same vibrating body or different vibrating bodies ?
(d) Which vibrating body/bodies could have generated the sound
waves
shown here?
Sol. (a) A and D
(b) B and D
(c) Different vibrating bodies
(d) Tuning forks of different frequencies.
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Q.87 In an experiment, Anhad studies sound waves. He sets up
a
loudspeaker to produce sound as shown below :
Anhad adjusts the signal to the loudspeaker to give a sound of
frequency
200 Hz.
(a) What happens to the air in-between Anhad and the loudspeaker
?
(b) Explain how Anhad receives sound in both ears. Sol. (a) The
air starts vibrating at 200 Hz.
(b) Anhad receives sound by his right ear through loudspeaker
and by his left
ear by the reflected sound.
Q.88 Figure X shows a trace of a sound wave produced by a
particular
tuning fork :
(a) On the graph paper given
in Figure Y, draw a trace of the sound wave which has a higher
frequency
than that shown in Figure X.
(b) On the graph paper shown in Figure Z, draw a trace of the
sound
wave which has a larger amplitude than that shown in Figure
X.
Sol.
Q.89 Three different vibrating objects produce three types of
sounds X, Y
and Z. Sounds X and Y cannot be heared by a man having normal
range of
hearing but sound Z can be heard easily. The sound X is used in
hospital to
break kidney stones of a patient into fine grains which then
get:flushed out
with Urine. The sound similar to that which is produced during
an
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earthquake before the main shock wave is generated.
(a) What type of sounds are (i) X (ii) Y, and (iii) Z ?
(b) Name one device which can produce sound like X.
(c) Name one device in a science laboratory which can produce
sound
like Y.
(d) Name one device in our homes which can produce sound like
Z.
(e) What is the frequency range of sounds like Z? Sol. (a)
Ultrasound (ii) Infra sound (iii) Audible sound
(b) SONAR
(c) Simple pendulum.
(d) Doorbell.
(e) 20 - 20,000 Hz
Q.90 A man is kidnapped, blindfolded and imprisoned in a big
room. How
could the man tell if he was in :
(a) a city (b) a village (c) a bare room (d) a furnished room?
Sol. (a) In a city lot of sound from traffic would be heard.
(b) In a village much less noise.
(c) In a bare room if too much reverberation is produced.
(d) In a furnished room if sound of a person speaking is
clear
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