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Sound : S.Chand
pg 185Very 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 gasSol. 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.
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/sSol.
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.
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.
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, SoundSol. 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.
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.
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.
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, LongitudinalSol. 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 fluteSol. (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
?
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.
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
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/sV=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.
Longitudinal waves consist of compressions and rarefaction.
(b) Distance of factory = 1.32 km =1.32 1000 mSpeed of sound =
330 m/sTime 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 sspeed of sound in
air = 332 m/sSo 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.
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 secondSol. (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 columnsSol. (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)
200Sol. (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 vesselSol. (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
orchestraSol. (d)
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 anotherSol. (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
bloodSol. (c)
Q.76 The sound waves travel fastest : (a) in solids (b) in
liquids (c) in gases (d) in vacuumSol. (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)
1530Sol. (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 sameSol. (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)
SSol. (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 forkSol. (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.
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 wavesTherefore, 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
(ii) Frequency of the waveSol. V = 4 m/s (1) Wavelength = 20 cm
= 0.20 m (2) Frequency =v=4.2=20 Hz
Short Answer Type Questions Pg 206Q.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.
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.
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.
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.
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.
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
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
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 HzSol. (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
HzSol. (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
Minimum time of sound for the echo to be heard = 0.1 s Minimum
distance =VT2=15000.12=75m
Long Answer Type QuestionsQ.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
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
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 wavesSol. (a)
Q.70 When we change a feeble sound to a loud sound, we increase
its : (a) frequency (b) amplitude (c) velocity (d) wavelengthSol.
(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 aboveSol. (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
soundSol. (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 toSol. (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 unaffectedSol. (a)
Q.75 One of the following can hear infra sound. This one is :
(a) dog (b) bat (c) rhinoceros (d) humansSol. (c)
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 mSol. (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 wavesSol. (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 amplitudeSol.
(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 HzSol.
(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
HzSol. (c)
Q.81 Which of the following device does not work on the multiple
reflections of sound waves ? (a) stethoscope (b) hydrophone (c)
soundboard (d) megaphoneSol. (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 wavesSol. (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) loudnessSol.
(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 mSol. (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.
(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
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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