3
I.
SOUND
Tick (✔
✔ ) the most appropriate answer.
1. Rapid back and forth motion of a particle about its mean position is
called
(a) displacement (b) acceleration (c) vibration (d) friction
2. In which state of matter does sound travel the fastest?
(a) gases
(b) solids
(c) liquids (d) none of these
3. Sound cannot travel through
(a) solids
(b) liquids
(c) vacuum (d) none of these
4. The intensity of sound is measured in the units of
(a) joule
(b) decibel
(c) watts
(d) none of these
5. The speed of sound in air is
(a) 3000 m/s (b) 340 m/s
(c) 400 m/s (d) 1000 m/s
6. The instrument used in the laboratory to produce sound of a fixed
frequenncy is
(a) simple pendulum
(b) tuning fork
(c) thermometer
(d) metre scale
7. Violin is a
(a) stringed musical instrument (b) percussion instrument
(c) reed instrument
(d) wind instrument
8. An instrument based on the principle of echo is
(a) sonar
(b) metre scale (c) radar
(d) barometer
9. An echo of the sound produced can be heard only if it reahes our
ear after
(a) 1/15th of a seond
(b) 1/13th of a seond
(c) 1/10th of a seond
(d) 1/5th of a seond
10. The characteristic of sound which distinguishes a feeble sound from
a loud sound of the same frequeny is
(a) loudness (b) pitch
(c) music
(d) noise
11. To and fro motion of a particle about its mean position is called
Class-VII Physics
1
Question Bank
(a) frequency (b) amplitude (c) vibration (d) acceleration
12. The approximate speed of sound in air is
(a) 33.2 ms–1 (b) 3320 ms–1 (c) 332 ms–1 (d) 3.32 ms–1
13. A simple instrument used to produce pure sound of fixed frequency
is
(a) tuning fork
(b) fire tongs
(c) simple pendulum
(d) metre scale
14. The number of vibrations made by a vibrating body in one second
is called
(a) wavelength (b) time period (c) amplitude (d) frequency
15. The frequency of ultrasonic vibrations is
(a) less than 20 Hz
(b) between 20 and 10000 Hz
(c) between 20 and 20000 Hz (d) more than 20000 Hz
16. The loudness of sound is normal between
(a) 10 dB – 25 dB
(b) 50 dB – 60 dB
(c) 90 dB – 105 dB
(d) 120 dB – 130 dB
17. The frequency of sound increases when the closed length of an air
column
(a) increases
(b) decreases
(c) remain same
(d) none of these
18. Pitch of sound does not depend upon the
(a) thickness of vibrating wire
(b) area of cross-section of vibrating wire
(c) amplitude of vibrating wire
(d) length of wire
Ans. 1. (c)
2. (b)
3. (c)
4. (b)
5. (b)
6. (b)
7. (a)
8. (a)
9. (c)
10. (b)
11. (c)
12. (c)
13. (a) 14. (d)
15. (d)
16. (b)
17. (b)
18. (c)
II (A). Fill in the blanks.
1. Every source of sound is a ______________ body.
2. Sound travels faster in ______________ than in gases.
3. Sound requires a ______________ for propagation.
4. The reflected sound is called ______________.
Class-VII Physics
2
Question Bank
5. The sound produced by an object vibrating with a high frequency is
called ______________ sound.
6. ______________ produces sensation in the ears.
7. Musical instruments in which leather membrane is made to vibrate
are known as ______________ instruments.
8. The maximum displacement of the particles of a medium in which a
wave is travelling is called ______________.
9. The ______________ of a sound is that characteristic by virtue of
which we can distinguish between two sounds of the same pitch
and loudness produced by two different instruments.
10. Pitch increases with the increase in the ______________ of a
vibrating body.
11. Only vibrating bodies produce ______________.
12. The pitch of a sound depends upon the ______________ of a
vibrating string.
Ans. 1. vibrating
2. solids
3. medium
4. echo
5. shrill
6. sound
7. percussion
8. amplitude
9. quality
10. frequency
11. sound
12. thickness
II (B). Fill in the blank spaces by choosing correct words from the list
given below.
List : hertz, echo, frequency, twenty, solids, later, depth, vibration,
sound, ultrasonic.
1. Only vibrating bodies produce ______________.
2. The range of subsonic vibrations is less than _____________ hertz.
3. The unit of frequency is ______________.
4. The repetition of reflected sound from a distant object is called an
______________.
5. The sound travels faster in ______________ as compared to gases.
6. The ______________ of the sound produced by a vibrating wire
decreases, if its length is increased.
7. The thunder of a cloud is heard ____________, compared to flash
of lightning.
Class-VII Physics
3
Question Bank
8. The range of ______________ vibrations is above 20,000 Hz.
9. Sonar is an instrument used for finding the ____________ of sea.
10. The to and fro motion of a vibrating body about its mean position is
called ______________.
Ans. 1. sound
2. twenty
3. hertz
4. echo
5. solids
6. frequency
7. later
8. ultrasonic
9. depth
10. vibration
III (A). Statements given below are incorrect. Write the correct statements.
1. The sounds produced by different whistles are the same.
Ans. Sounds produced by different whistles are different.
2. We hear echoes only in the mountains.
Ans. We hear echoes in the mountains or in the valleys.
3. The ticking of a watch sounds louder when heard through water
than through a metal.
Ans. The ticking of a watch sound is louder when heard through a metal
than through water.
4. Small vibrations produce loud sounds.
Ans. Large vibrations produce loud sounds.
5. All kinds of vibrations in the material bodies produce sensation of
sound.
Ans. The frequency of the vibrations between 20 Hz to 20,000 Hz in the
material bodies produce sensation of sound.
6. The time taken by a vibrating body to complete one vibration is
called frequency.
Ans. The time taken by a vibrating body to complete one vibration is
called time period.
7. The maximum displacement of a vibrating body from one extreme
position to another extreme position is called amplitude.
Ans. The maximum displacement of a vibrating body from its mean
position is called amplitude.
8. Louder the sound, less is the amplitude of vibrating body.
Class-VII Physics
4
Question Bank
Ans.
9.
Ans.
10.
Ans.
11.
Ans.
12.
Louder the sound, more is the amplitude of vibrating body.
Galton whistle is used to train dolphins.
Galton whistle is used to train dogs.
Sound travels slower in water as compared to air.
Sound travels faster in water as compared to air.
Material medium is not necessary for the propagation of sound.
Material medium is necessary for the propagation of sound.
In order to produce an echo, the minimum distance between source
of sound and reflecting body should be 14 m.
Ans. In order to produce an echo, the minimum distance between source
of sound and reflecting body should be 17 m.
13. The characteristics of sound which distinguishes between a sharp
sound and dull sound is called loudness.
Ans. The characteristics of sound which distinguishes between a sharp
sound and dull sound is called pitch.
III (B). Write true or false for each statement given below.
1. The range of sonic vibrations is between 20 Hz to 20,000 Hz.
2. The sound can travel through vacuum.
3. Amplitude is the maximum displacement of a particle from its mean
position.
4. The speed of sound in air is 5100 m/s.
5. The number of vibrations produced by a vibrating particle in one
second is called frequency.
6. The more the amplitude of a vibrating body, the more is the loudness.
7. The minimum distance between source of sound and reflecting body
should be 17 m, for the formation of an echo.
8. With the increase in frequency of a vibrating body, the sound
becomes shrill.
9. Subsonic vibrations have a range above 20,000 Hz.
Ans. 1. True 2. False 3. True 4. False 5. True 6. True 7. True 8. True 9. False
IV. Find the odd-one out. Give reasons for your choice.
1. Loudness, reflection, pitch, quality
Class-VII Physics
5
Question Bank
Ans. Reflection — because it is not the characteristics of sound while
loudness, pitch and quality are characteristics of sound.
2. 40 decibels, 70 decibels, 150 decibels, 90 decibels
Ans. 150 decibels — because it is undesirable, very loud and painful
sound and thus considered as noise.
3. Soft horns, ear plugs, silencers, loudspeakers
Ans. Loudspeakers — because it causes noise pollution while other devices
are used to prevent the noise pollution.
4. Loudness, reed instrument, pitch, timbre
Ans. Reed instrument — because it is a type of musical instrument while
other terms are the characteristics of sound.
5. Flute, dholak, tabla, mridangam
Ans. Flute — because it is a wind instrument while others are Percussion
instruments.
6. Sitar, guitar, mouth organ, violin
Ans. Mouth organ — because it is a reed instrument while others are
stringed musical instruments.
7. Loudness, pitch, echo, quality
Ans. Echo — because it is the reflection of sound while other terms are
characteristics of sound.
8. Loudspeakers, aeroplanes, vacuum cleaners, ear plugs
Ans. Ear plugs — because they are used to avoid noise pollution while
others are the source to cause noise pollution.
V. Match the following.
1. reflection of sound
(a) sound of high-pitch
2. a shriller sound
(b) echo
3. 120 decibels
(c) tuning fork
4. a source producing sounds of (d) noise
fixed frequency
Ans. 1. (b), 2. (a), 3. (d), 4. (c)
VI. Give reasons for the following.
1. Sound cannot reach us from space.
Ans. Sound requires a medium to propagate. It cannot travel in the
Class-VII Physics
6
Question Bank
2.
Ans.
3.
Ans.
4.
Ans.
5.
Ans.
6.
Ans.
7.
Ans.
vacuum. Since there is a vacuum in the space, therefore, sound
cannot reach us from space.
A fighter aeroplane is seen before hearing its sound.
A fighter aeroplane is seen before hearing its sound because light
travels at a very high speed of 3 × 108 m/s whereas the sound travels
in air at 330 m/s. Thus as soon as a fighter aeroplane appears it is
seen at once whereas its sound take some time to reach us.
Bats can locate objects in the dark.
Bats can hear ultrasonic sounds having frequencies upto 1,20,000 Hz.
Infact bats can also produce ultrasonic sounds while screaming.
Ultrasonic sounds produced by bats are reflected back by other
objects in their path and the echoes thus formed are detected by
bats. It is due to this echo location of objects around them, the bats
being nearly blind can locate objects in the dark.
A large drum can produce a louder sound than a small drum.
Loudness depends on the area of the vibrating body. Thus, loudness
increases with an increase in the area of the vibrating body. As a
large drum occupies more area than a small drum. Therefore, a large
drum can produce a louder sound than a small drum.
Noise pollution can lead to health hazards.
Noise pollution can lead to health hazards because it may cause
partial or permanent hearing loss. It increases nervous tension,
irritation and high blood pressure.
Echoes can be produced in mountains or valleys.
The repetition of sound when it is reflected from a distant body is
called an echo. Echo can be heard if the minimum distance between
source of sound and reflecting body is 17 metre. Because there is
enough distance in mountains or valleys,. So, echoes can be
produced in such places.
We cannot talk to one another on the surface of moon.
Sound requires a medium to propagate. It cannot travel in the
vacuum. Since there is a vacuum on the moon therefore the sound
of other person does not reach to our ear, so we cannot talk to
Class-VII Physics
7
Question Bank
another on the surface of moon.
8. A supersonic plane disappears from the line of sight when we hear
its sound.
Ans. A supersonic plane disappears from the line of sight when we hear
its sound, because light travels at a very high speed of 3 × 108 m/s
whereas sound travels in air at 330 m/s. That is why its sound takes
some time to reach us mean while the plane disappears.
VII. Differentiate between the following.
1. Wind instrument and percussion instrument
Ans. Wind instruments : These musical instruments are generally made
in the form of pipes. When we blow air into them the column of air
inside vibrates producing a note. Whistle, flute and shehnai are some
common wind instruments.
Percussion instruments : In these instruments, we make a leather
membrane vibrate by striking it with a hammer or with our hands.
Drum, tabla and dholak are the commonly used Percussion musical
instruments.
2. High-pitch sound and low-pitch sound
Ans. High-pitch sound : The sound produced by an object vibrating
with a high frequency is called shrill or high-pitched sound.
Low-pitch sound : The sound produced by an object vibrating with
a low frequency is described as flat or low-pitched sound.
3. Fundamental and harmonics
Ans. Fundamental : When a note is played on a sitar, it produces the
basic sound of a certain frequency called the fundamental.
Harmonics : Along with the fundamental a note also produces a
mixture of sound of other frequencies each of which differs in
loudness. These are called harmonics.
4. Sound produced by guitar and sound produced by a loudspeaker
Ans. Sound produced by guitar is a musical sound. It has pleasant effect
on the ears. On the other hand sound produced by a loudspeaker is
a noise. It does not have a pleasant effect on the ears.
VIII. Define these terms.
Class-VII Physics
8
Question Bank
1. Vibration
Ans. A vibration is a rapid back and forth motion of a particle about its
mean position.
2. Echo
Ans. The repetition of sound, when it is reflected from a distant body is
called an echo.
3. Shrill
Ans. The sound produced by an object vibrating with a high frequency is
called shrill.
4. Sound quality
Ans. The quality of a musical sound is that characteristics by virtue of
which we can distinguish between two sounds of the same pitch
and loudness produced by two different musical instruments or by
different voices.
5. Ultrasonic sound
Ans. Sounds having frequencies higher than 20,000 Hz are known as
ultrasonic sounds. Some animals like bats, dogs and dolphins can
hear ultrasonic sounds.
6. Infrasonic sound
Ans. Sounds having frequencies lower than 20 Hz are known as infrasonic
sounds. Earthquakes, volcanic eruptions, simple pendulum and some
animals like whales and elephants produce infrasonic sounds.
IX. Answer these questions.
1. Describe an experiment to prove that
(a) sound travels faster in solids than in liquids.
(b) sound travels faster in solids than in gaseous medium.
Ans. (a) Experiment to prove that sound travels faster in solids than in
liquids.
Take a squeky toy and put it inside a plastic bag. Seal the bag with
the help of a candle or with a thread. Fill a bucket with water and
place the bag in the water squeeze the toy. You will hear a low
sound. Now place your ear against the side of the bucket and squeeze
the toy again. You will hear the sound better.
Class-VII Physics
9
Question Bank
2.
Ans.
3.
Ans.
From the above experiment we conclude that sound travels through
a solid medium faster than in liquid medium.
(b) Experiment to prove that sound travels faster in solids than in
gaseous medium.
Palce a wristwatch at one end of a long wooden table stand at the
other end of the table. Try to hear the ticking of the watch. Place
your ear on the table and listen. You will observe that the ticking of
the watch sound is louder when heard through wood.
From the above experiment we conclude that sound travels through
a solid medium faster than through a gaseous medium.
What do you mean by reflection of sound? Describe an experiment
to explain it.
The phenomenon of sound waves that first strike some solid objects
like walls of a room than get reflected and reach our ears is called
reflection of sound.
Experiment.
Take a clock and place it at the bottom
of a glass container. You cannot hear
the sound of the clock clearly. Hold a
smooth wooden sheet inclined at 45°
to the glass container. You can now
hear the sound better. This is because
the sound starts from the clock and
reaches the sheet, on striking the sheet,
the direction of the sound waves
change and you hear the sound.
Why is a woman’s voice different from a man’s voice?
In human beings vocal cords are responsible for the difference of
woman’s voice from man’s voice. When the cords are tight and
thin, the sound is of higher pitch or frequency. On the other hand,
when the cords are loose and thick the sound is of low pitch or
frequency. Men have longer vocal cords than women. That is why
the frequency of a sound produced by women is of a higher
Class-VII Physics
10
Question Bank
4.
Ans.
5.
Ans.
6.
Ans.
7.
Ans.
8.
Ans.
9.
Ans.
frequency than the sound produced by men.
Why is the sound produced from a streched string different when it
is plucked lightly than when plucked strongly?
When a stretched string is plucked lightly it vibrates and produces a
soft sound. If we pluck it strongly it vibrates with a greater amplitude
and a louder sound is produced.
What are musical instruments? What are their main types?
Musical instruments : Music is a sound which has pleasant
sensation on our ears. A musical sound is produced by regular
vibrations and the instruments used to produce music are called
musical instruments.
Depending upon the way the air vibrates they are of four main types:
(i) Stringed musical instruments
(ii) Percussion instruments
(iii) Wind instruments
(iv) Reed instruments
What is the source of sound in reed instruments? Give examples.
Reed instruments contain metal reeds which produce sound when
air is blown through them. Instruments like harmonium and mouth
organ are examples of reed instruments.
How is sound produced in wind instruments? Give examples.
Wind instruments are generally made in the form of pipes. When
we blow air into them the column of air inside vibrates and produce
a note. Whistle, flute and shehnai are some common wind instruments.
How does the pitch of a sound depend on its frequency?
The pitch of sound depends on the frequency of vibration. The higher
the frequency the more shrill is the sound and vice versa.
How are stringed musical instruments made to produce sounds?
Give two examples.
Stringed instruments have taut strings mounted over specially
designed wooden frames. When the strings are plucked or struck or
played with bow they vibrate to produce a musical sound of some
particular frequency. The pitch of the sound of a musical instrument
Class-VII Physics
11
Question Bank
10.
Ans.
11.
Ans.
12.
Ans.
can be changed by altering its length. Guitar, sitar and violin are
examples of stringed musical instruments.
How do percussion instruments produce sounds? Give two
examples.
In percussion instruments, we make a leather membrane vibrate by
striking it with a hammer or with our hands. Drum, tabla and dholak
are the commonly used percussion musical instruments.
What steps can be taken to prevent noise pollution?
Steps to prevent noise pollution are as follows :
(i) Automobiles should be fitted with silencers and soft horns.
(ii) We should not play radio, television and stereo system too
loudly.
(iii) The horns of motor vehicles should not be blown unnecessarily.
(iv) Ear plugs shold be used by bus or train drivers, mechanics,
workers in a factory, etc.
(v) Airports and noise-making factories should be shifted away
from the residential areas of the city.
Describe an experiment to prove that sound cannot travel in vacuum.
To prove that sound cannot travel in vacuum. We can perform a
simple experiment to demonstrate this fact.
Experiment
Connect the bell to the battery so that
it starts ringing. Arrange the bell jar
around the bell and connect it to a
vacuum pump. Start the vacuum pump
so that it starts evacuating air from the
bell jar. Keep on observing the sound
of the ringing bell all the time.
We observe that we keep on hearing
the sound of the bell after the bell jar
is kept over it. As the air inside the bell
jar is slowly evacuated, the sound
becomes dimmer and dimmer and after
Class-VII Physics
12
Question Bank
13.
Ans.
14.
Ans.
sometime we cannot hear it at all. We
can however still see the bell vibrating as before.
We conclude that the sound of the bell reaches us through the air in
the bell jar. When the vacuum pump removes the air from the bell
jar, the sound does not reach us even though the bell is still vibrating.
It follows that sound cannot travel through vacuum.
What is the condition required for a body to produce sound?
Describe an experiment to prove it.
Vibration is a necessary condition for a body to produce sound.
Experiment
Take a metallic ruler and press
its one end on a table with the
left hand and pull down the free
end of the ruler with the right
hand and let it go, what do we
observe?
The metallic ruler is seen vibrating and a humming sound is heard.
The humming sound is produced by the to-and-fro motion of the
metallic ruler; the vibrations produced by the vibrating body.
(a) What is sound and how is it produced?
(b) Name some pleasant and unpleasant sound?
(c) How many vocal cords are there in our voice boxes? Where is
the voice box situated?
(d) What makes our vocal cords vibrate?
(e) Do bees have a voice box? How do they produce sound?
(a) Sound is an energy which can be produced by the vibrations in
a material body.
(b) Pleasant sound : Sound produced by musical instruments like
sitar, flute, tabla, etc.
Unpleasant sound : Sound produced by various machines,
the bark of a dog, the sound of a supersonic aircraft etc.
(c) There are two vocal cords in our voice box. Voice box is
situated in our throat.
Class-VII Physics
13
Question Bank
(d) A narrow slit between the vocal cords makes our vocal cord
vibrate.
(e) Bees have no voice boxes. They make sound by moving their
wings up and down very fast.
15. Taking the example of tuning fork, explain how a vibrating body
produces longitudinal waves in air.
Ans. When a tuning fork is put into vibrations, its prongs move
alternatively inwards and outwards between the positions A and B.
As it moves from A to B it compresses the layer of air in front of it.
This layer because of elasticity, tries to regain its original size. For
this it has to expand. When it does that, it compresses the next layer
and so on. This in turn, compresses the next layer and the process
is repeated. The compression therefore advances to the right from
layer to layer.
Now consider the instant when the prong moves from B to A. As it
moves this way it creates a partial vacuum behind it so that the
layer of air in contact with it expands. We call this a rarefaction.
Again because of elasticity, it tries to regain its original size, it
therefore creates a partial vacuum behind it. The expansion of one
layer thus allows the next layer to expand and so on. In this way a
rarefaction moves to the right. When the prong vibrates, the
compressions and rarefactions follow each other and we have the
longitudinal waves travelling through air. Other vibrating bodies,
like a ringing bell also produce similar compressions and rarefactions
in air before their sound reaches us.
Class-VII Physics
14
Question Bank
16. Define the following :
(i) Frequency, (ii) Time period, (iii) Amplitude and,
(iv) Wavelength
Ans. (i) Frequency : The number of complete oscillations made in
one second is known as the frequency of the oscillating body.
It is measured in units of hertz (Hz).
(ii) Time period : The time taken to complete one oscillation is
Class-VII Physics
15
Question Bank
17.
Ans.
18.
Ans.
known as the time period.
(iii) Amplitude : The amplitude of an oscillating body is the
maximum displacement from its mean or normal position.
(iv) Wavelength : The linear distance between two consecutive
particles of a vibrating medium in the same phase is called its
wavelength. It is denoted by Greek letter λ (lambda).
Name and explain the main parts of human ear.
The Human Ear : A vibrating body produces sound that needs a
medium (solid, liquid or gas) for its propagation. The medium is
usually air. When the sound propagating through air reaches our
ears we are able to hear it.
Our ears are made up of three parts : (i) the outer ear, (ii) the middle
ear, and (iii) the inner ear. Now as we know, sounds produced make
air vibrate. These vibrations in the air make our ear drum vibrate
when they strike against it. This in turn makes some tiny delicate
bones in our middle ear vibrate with the same frequency, as the
source of sound. The vibrations of these tiny bones are passed on
to a liquid present in our inner ear. Here there are several nerves
which send the message to our brain. When these messages reach
our brain we are able to hear and recognise the sound.
(a) What do you understand by the term ultrasonic vibrations?
(b) State three uses of ultrasonic vibrations.
(c) Name three animals which can hear ultrasonic vibrations.
(a) Ultrasonic vibrations : The vibrations whose frequencies are
greater than 20,000 Hz are called ultrasonic vibrations.
(b) Uses of ultrasonic vibrations.
(i) Ultrasonic vibrations are used in SONAR to measure the
depth of sea (or ocean) and to locate under water objects
like submarines, sea-rocks and shipwrecks.
(ii) It is used for scanning and imaging the position and
growth of a foetus and presence of stones in the gall
bladder and kidney.
(iii) It is used for homogenising milk in milk plants where fresh
Class-VII Physics
16
Question Bank
19.
Ans.
20.
Ans.
21.
Ans.
22.
Ans.
milk is agitated with desired quantity of fat and powdered
milk to obtain toned milk.
(c) Bats can hear ultrasonic sounds having frequencies upto
1,20,000 Hz. Some animals like dogs and dolphins can hear
sounds having frequencies upto 40,000 Hz.
Why is flash of lightning seen first but thunder is heard sometime
later?
Flash of lightning seen first but thunder is heard sometime later
this is because, light travels at a very high speed of 3 × 108 m/s
whereas the sound travels in air at 330 m/s. Thus, whenever lightning
takes place it is seen at once whereas sound will take some time to
reach us.
(a) What is an echo?
(b) State two conditions necessary for hearing an echo.
(a) Echo : The repetition of the sound which is reflected from a
high building or any such object is called an echo.
(b) Conditions necessary for hearing an echo.
(i) There should be a high rise building or hill or wall or
well.
(ii) Minimum distance between source of sound and reflecting
body should be 17 metre.
Why is sound of a sitar different from sound of a drum?
Sound of a sitar is different from sound of a drum because of the
quality of musical sound by virtue of which we can distinguish
between two sounds of the same pitch and loudness produced by
two different musical instruments.
What do you understand by the following terms:
(a) Pitch,
(b) Loudness (c) Timbre
(a) Pitch : Pitch is the effect or sensation produced in the brain
due to the sound of some particular frequency. Pitch depends
upon the frequency of a vibrating body. It means that higher
the pitch more is the frequency and lower the pitch less is the
frequency.
Class-VII Physics
17
Question Bank
23.
Ans.
24.
Ans.
25.
Ans.
(b) Loudness : Loudness is the characteristic property of a sound
which distinguishes a feeble sound from a loud sound of the
same frequency. The loudness of sound depends upon the
amplitude of the vibrating body. It means more the amplitude ,
more is the loudness.
(c) Timbre : Timbre or sound quality is that characteristic by virtue
of which we can distingish between two sounds of the same
pitch and loudness produced by two different musical
instruments or by different voices. For example when two
musician produce the same note on two different instruments
like piano and harmonium we can easily distinguish between the
two notes because of their sound quality.
How can we measure the depth of a sea by using “Sonar”?
Sonar is an instrument based on the reflection of sound. The Sonar
sends out ultrasonic (high frequency) sound towards the bottom of
the sea. The sound reflected from the sea-bed is received by it. By
measuring the time taken by the sound to return to the ship and
knowing the speed of sound in sea-water, we can find the depth of
the sea or ocean at that place.
What are (i) infrasonic vibrations, (ii) audible vibrations? Write their
range.
(i) Infrasonic vibrations : The vibrations whose frequencies are
less than 20 Hz are called infrasonic vibrations. Human ear
cannot hear these vibrations.
(ii) Audible vibrations : The vibrations whose frequencies are
from 20 Hz to 20,000 Hz constitute the audible vibrations.
Human ear can hear only these vibrations.
Which part of a musical instrument vibrates to produce sound in (i)
drum, (ii) flute, (iii) violin, (iv) tuning fork?
(i) Drum : In drum we make a leather membrane vibrate by
striking it with a hammer or with our hands to produce sound.
(ii) Flute : In flute we blow air into it, the column of air inside the
flute vibrates and produce sound.
Class-VII Physics
18
Question Bank
(iii) Violin : In violin we draw a bow across the strings to put them
into vibrations to produce sound.
(iv) Tuning fork : We hit the tuning fork hard against a rubber
pad and prongs of the tuning fork begins to vibrate to produce
the sound.
26. (a) Name the unit for the measurement of loudness of sound?
(b) What do you understand by the term noise pollution?
(c) Suggest two ways of minimising noise pollution in
(i) home, (ii) surrounding, (iii) automobile
(d) State four harmful effects of noise pollution on the workers
working in factories.
Ans. (a) Loudness of sound is measured in decibels (dB). A sound of
about 50 to 80 decibels is considered as audible sound.
(b) Unwanted, unpleasant and very loud sounds are called noise.
The running of automobiles, loudspeakers and music systems
cause noise pollution.
(c) Ways of minimising noise pollution.
(i) Home
(A) Television, radio and power music system should be
played at low volume.
(B) Electric generator should be provided with improved
and modified silencers.
(ii) Surrounding
(A) Exploding crackers on various functions should be
avoided.
(B) Loudspeakers used in marriages and religious places
should be played at low volume.
(iii) Automobile
(A) Horns should not be blown unnecessarily.
(B) Automobiles should be provided with improved and
modified silencers.
(d) Harmful effects of noise pollution in factories
(i) Noise in the surroundings interfaces with speech and talk
Class-VII Physics
19
Question Bank
with another person.
(ii) A long exposure to noise pollution may result in the loss
of hearing to deafness.
(iii) Noise pollution reduces concentration and results in the
loss of work efficiency.
(iv) Noise produces headaches, irritability and nervous
tension.
X. Practice for numerical problems.
1. The thunder of cloud is heard 9.8 s later than flash of lightning. If
the speed of sound in air is 330 m/s. Find the height of cloud.
Ans.
Time = 9.8 s
Speed of sound = 330 m/s
Height of cloud = Distance travelled by sound
= Speed of sound × Time
= 330 m/s × 9.8 s
= 3234 m
2. Two observers are stationed in two boats, 4.5 km apart. A sound
signal sent by one under water reaches the other after 3 s. What is
the speed of sound in water?
Ans. Distance between two observers = 4.5 km
Time = 3 s
Speed of sound in water =
=
Distance between two observers
Time
4.5 km
= 1.5 km/s = 1500 m/s
3s
3. An echo is heard from a distant cliff after an interval of 0.8 s. If the
speed of sound in air is 340 m/s, how far away is the cliff?
0.8 s
= 0.4 s
Ans. Time taken by the sound to reach the cliff =
2
Speed of sound in air = 340 m/s
Distance of the cliff = Speed × Time
Class-VII Physics
20
Question Bank
4.
Ans.
5.
Ans.
= 340 m/s × 0.4 s
= 136 m
A strong sound signal is sent from a ship towards the bottom of the
sea. It is received back after 1 s. What is the depth of the sea?
Given that the speed of sound in water 1450 m/s?
1
Time taken by the signal to reach the bottom of the sea = s
2
Speed of sound in water = 1450 m/s
Depth of the sea = Distance travelled by the signal
= Speed × Time
1
= 1450 m/s × s
2
= 725 m
A boy stands at a distance of 175 metres from a high wall. He claps
his hands and hears the echo after one second. Calculate the speed
of sound in air.
Distance of the boy from the wall = 175 m
1
s
Time taken by the clap sound to reach the wall =
2
Distance
Speed of sound in air =
Time
175 m
1
=
s = 350 m/s
2
6. During a thunderstorm, the thunder is heard 2 seconds after the
flash of lightning is seen. If the speed of sound is 300 m/s, calculate
the distance at which the lightning takes place.
Ans.
Time = 2 s
Speed of sound = 300 m/s
Distance = Speed × Time
= 300 m/s × 2 s
= 600 m
Class-VII Physics
21
Question Bank