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SOUND
Q. What is the sound?
Ans. Sound is a form of energy which produces sensation in our ears.
Q. How does sound produce?
Ans. Sound is produced by any vibrating object.
Q. What does vibration mean?
Ans. Vibration means a kind of rapid to and fro motion of an object.
Q. Which part of humanโs body vibrates to produce the human voice?
Ans. The sound of the human voice is produced due to vibrations in the vocal cords.
Q What does Propagation of Sound mean?
Ans. Sound propagation is the process of sound waves traveling through a medium,
such as air or water, to transmit energy.
Q. What does medium mean?
Ans. The matter or substance through which sound is transmitted is called a medium. It
may be solid, liquid or gas.
Q. Through which sound cannot propagate?
Ans. Through vacuum.
Q. How does sound travel through a medium?
Ans. When an object vibrates, it sets the particles of the medium around it vibrating and
produces a series of pressure waves that travel through a medium by alternately
expanding and contracting the medium's parts.
Q. What does a wave mean?
Ans. A wave is a disturbance that moves through a medium when the particles of the
medium set neighbouring particles into motion.
Q. What does mechanical wave mean?
Ans. The waves which need medium to travel through, are called mechanical waves.
Thus the sound wave is a mechanical wave.
Q. What does compression mean?
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Ans. When a vibrating object moves forward, it pushes and compresses the air in front
of it creating a region of high pressure. This region is called a compression (C).
Q. What does rarefaction mean?
Ans. When the vibrating object moves backwards, it creates a region of low pressure
called rarefaction (R).
Q. How does the sound produced by a vibrating object in a medium reach your ear?
Ans. When an object vibrates, it sets the particles of the medium around it vibrating and
produces a series of pressure waves that travel through a medium by alternately
expanding and contracting the medium's parts and reach our ear.
Q. Explain how sound is produced by your school bell.
Ans. When the school bell is struck by any hard substance it starts vibrating and
produces a sound.
Q. Why are sound waves called mechanical waves?
Ans. The waves which need medium to travel through, are called mechanical waves
and sound waves also need medium to travel through so the sound wave is a
mechanical wave.
Q. Suppose you and your friend are on the moon. Will you be able to hear any sound
produced by your friend?
Ans. No, I will not be able to hear any sound produced by my friend because there is no
medium on the moon and sound cannot travel through vacuum.
Q. What does longitudinal wave mean?
Ans. The waves in which the individual particles of the medium move in a direction
parallel to the direction of propagation of the disturbance are called longitudinal waves.
The sound wave is a longitudinal wave.
Q. What does transverse wave mean?
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Ans. The waves in which the individual particles of the medium move about their mean
positions in a direction perpendicular to the direction of wave propagation, are called
transverse waves. The light wave is a transverse wave.
Q. Represent graphically the density or pressure variations when the sound wave
moves in the medium.
Ans.
In the graph, compression is represented by โCrestโ and the rarefaction is
represented by โTroughโ.
Q. What does wavelength mean?
Ans. The distance between two consecutive compressions (C) or two consecutive
rarefactions (R) is called the wavelength. The wavelength is usually represented by ฮป
(Greek letter, lambda). Its SI unit is metre (m).
Q. What does one oscillation mean?
Ans. The change in density from the maximum value(C) to the minimum value(R), then
again to the maximum value(C), makes one complete oscillation.
Q. What does frequency of the sound wave mean?
Ans. The number of oscillations per unit time is the frequency of the sound wave. It is
usually represented by ฮฝ (Greek letter, nu) or f. Its SI unit is hertz (symbol, Hz) named
after a scientist Heinrich Rudolph Hertz.
Q. What does time period mean?
Ans. The time taken for one complete oscillation is called the time period of the sound
wave. It is represented by the symbol T. Its SI unit is second (s).
Q. How is frequency and time period of the sound wave related?
Ans. Frequency and time period are related as follows:
ฮฝ( ๐ )=
1
๐
Q. What does the pitch of a sound mean?
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Ans. How the brain interprets the frequency of an emitted sound is called its pitch. The
faster the vibration of the source, the higher is the frequency and the higher is the pitch.
Q. What does amplitude of the sound wave mean?
Ans. The magnitude of the maximum disturbance in the medium on either side of the
mean value is called the amplitude of the wave. It is usually represented by the letter A.
Its SI unit is m.
The loudness or softness of a sound is determined basically by its amplitude.
The amplitude of the sound wave depends upon the force with which an object is made
to vibrate.
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Q. What does timber or quality of a sound mean?
Ans. The quality or timber 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. What does tone mean?
Ans. A sound of a single frequency is called a tone.
Q. What does note mean?
Ans. The sound which is produced due to a mixture of several frequencies is called a
note and is pleasant to listen to.
Q. What does a noise mean?
Ans. The sound which is unpleasant to hear is called a noise.
Q.b What does tone mean?
Ans. The sound which is unpleasant to hear is called music and is of rich quality.
Q. Which wave property determines (a) loudness, (b) pitch?
Ans. Loudness is determined by amplitude of the sound wave and pitch is determined
by the frequency of the sound wave.
Q. Guess which sound has a higher pitch: guitar or car horn?
Ans. A guitar has a higher pitch than a car horn.
Q. What does the speed of a sound wave mean?
Ans. The speed of sound is defined as the distance which a point on a wave, such as a
compression or a rarefaction, travels per unit time.
๐๐๐๐๐ =
๐ท๐๐ ๐ก๐๐๐๐
๐๐๐๐
๐ฃ =
ฮป
๐
๐ฃ =ฮป ๐
Q. A sound wave has a frequency of 2 kHz and wavelength 35 cm. How long will it take
to travel 1.5 km?
Ans.
๐ = 2 ๐๐ป๐ง = 2000 ๐ป๐ง
ฮป= 35 ๐๐ = 0 . 35 ๐
๐๐๐ก๐๐ ๐๐๐ ๐ก๐๐๐๐ = 1 . 5 ๐๐ = 1500 ๐
๐๐๐ก๐๐ ๐ก๐๐๐ = ?
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๐ฃ =ฮป ๐
๐ฃ = 0 . 35 ร 2000
๐ฃ = 700 ๐ / ๐
๐๐๐๐ =
๐ท๐๐ ๐ก๐๐๐๐
๐ ๐๐๐๐
=
1500
700
= 2 . 1 ๐
Q. How are the wavelength and frequency of a sound wave related to its speed?
Ans.
๐๐๐๐๐ = ๐ค๐๐ฃ๐๐๐๐๐๐กโ ร ๐๐๐๐๐ข๐๐๐๐ฆ
Q. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is
440 m/s in a given medium.
Ans.
ฮป = ?
๐ = 220 ๐ป๐ง
๐ฃ = 440 ๐ / ๐
๐ฃ =ฮป ๐
440 =ฮปร 220
440
220
=ฮป
ฮป= 2 ๐
Q. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the
source of the sound. What is the time interval between successive compressions from
the source?
Ans.
๐ = 500 ๐ป๐ง
๐๐๐ ๐ก๐๐๐๐ = 450 ๐
๐ =?
๐ =
1
๐
500 =
1
๐
1
500
= ๐
๐ = 0 . 002 ๐
Q. What does intensity of sound mean?
Ans. The amount of sound energy passing each second through the unit area is called
the intensity of sound.
Q. Distinguish between loudness and intensity of sound.
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Ans.
Loudness
Intensity
Loudness is a measure of the response
of the ear to the sound.
The amount of sound energy passing
each second through the unit area is
called the intensity of sound.
Its SI unit is db.
Its SI unit is W/m
2
.
Loudness of a sound may be different
from person to person.
It does not vary person to person.
It depends on the sensation of the ear.
It does not depend on the sensation of
the ear.
Q. What are the factors on which speed of a sound in a medium depends?
Ans. Speed of a sound in a medium depends on :-
1. Density of the medium : Density and speed of a sound are directly proportional. A
solid has high density so the speed of a sound is maximum in solid and minimum
in gas state.
2. Temperature of the medium : Temperature and speed of a sound are directly
proportional. In any medium as we increase the temperature, the speed of sound
increases.
Q. In which of the three media, air, water or iron, does sound travel the fastest at a
particular temperature?
Ans. In iron, does sound travel the fastest at a particular temperature?
Q. Explain the reflection of sound.
Ans. Like light, sound gets reflected at the surface of a solid or liquid and follows the
same laws of reflection.
Q. What does echo mean?
Ans. The reflected sound which we hear after the reflection, is called echo.
Q. What are the conditions for echo to be heard?
Ans. The sensation of sound persists in our brain for about 0.1 s. To hear a distinct
echo the time interval between the original sound and the reflected one must be at least
0.1s and for this the obstacle minimum distance from the source of sound is 17.2 m.
Q. What does reverberation mean?
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Ans. Reverberation is the phenomenon of the persistence of sound after it is produced
because of repeated reflection.
Q. How is unwanted reverberation reduced?
Ans. To reduce reverberation, the roof and walls of the auditorium are generally
covered with sound-absorbent materials like compressed fibreboard, rough plaster or
draperies. The seat materials are also selected on the basis of their sound absorbing
properties.
Q. A person clapped his hands near a cliff and heard the echo after 2 s. What is the
distance of the cliff from the person if the speed of the sound, v is taken as 346 m sโ1?
Ans.
๐๐๐๐ = 2 ๐
๐ ๐๐๐๐ = 346 ๐ / ๐
๐ท๐๐ ๐ก๐๐๐๐ ๐๐ ๐กโ๐ ๐๐๐๐๐ ๐๐๐๐ ๐กโ๐ ๐๐๐๐ ๐๐ =?
๐ท๐๐ ๐ก๐๐๐๐ = ๐๐๐๐๐ ร ๐ก๐๐๐
๐ท๐๐ ๐ก๐๐๐๐ = 346 ร 2
๐ท๐๐ ๐ก๐๐๐๐ = 692 ๐
692 m is twice the distance of the cliff from the person. So the distance of the cliff from
the person =
692
2
๐ = 346 ๐
Q. An echo is heard in 3 s. What is the distance of the reflecting surface from the
source, given that the speed of sound is 342 m sโ1?
Ans.
๐๐๐๐ = 3 ๐
๐ ๐๐๐๐ = 342 ๐ / ๐
๐ท๐๐ ๐ก๐๐๐๐ ๐๐ ๐กโ๐ ๐๐๐๐๐๐๐ก๐๐๐ ๐ ๐ข๐๐๐๐๐ ๐๐๐๐ ๐กโ๐ ๐ ๐๐ข๐๐๐ =?
๐ท๐๐ ๐ก๐๐๐๐ = ๐๐๐๐๐ ร ๐ก๐๐๐
๐ท๐๐ ๐ก๐๐๐๐ = 342 ร 3
๐ท๐๐ ๐ก๐๐๐๐ = 1026 ๐
692 m is twice the distance of the reflecting surface from the source. So the distance of
the reflecting surface from the source =
1026
2
๐ = 513 ๐
Q. Write the uses of multiple reflections.
Ans. Following are the uses of multiple reflections:-
1. Used in designing of Megaphones or loudhailers, horns, musical instruments to
send sound in a particular direction without spreading it in all directions.
2. Used in designing of stethoscopes.
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3. Ceilings of concert halls, conference halls and cinema halls are made curved to
use the multiple reflections of sound so that it reaches all corners of the hall.
Q. Why are the ceilings of concert halls curved?
Ans. Ceilings of concert halls, conference halls and cinema halls are made curved to
use the multiple reflections of sound so that sound reaches all corners of the hall.
Q. What is the audible range of the average human ear?
Ans. The audible range of sound for human beings extends from about 20 Hz to 20000
Hz (one Hz = one cycle/s).
Q. What does infrasound mean?
Ans. Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound.
Q. Name the animals who can produce infrasound?
Ans. Rhinoceroses, Whales, and elephants are some animals who can produce
infrasound.
Q. Name the animals who can produce ultrasound?
Ans. Bat, rat, dolphins and porpoises are some animals who can produce ultrasound.
Q. What does ultrasound mean?
Ans. Sounds of frequencies above 20000 Hz are called ultrasonic sound or ultrasound.
Q. What are the applications of ultrasound?
Ans. The applications of ultrasound are as follows:-
1. Ultrasound is generally used to clean parts located in hard-to-reach places, for
example, spiral tubes, odd shaped parts, electronic components, etc.
2. Ultrasounds can be used to detect cracks and flaws in metal blocks.
3. Ultrasonic waves are made to reflect from various parts of the heart and form the
image of the heart. This technique is called โechocardiographyโ.
4. Ultrasound scanner is an instrument which uses ultrasonic waves for getting
images of internal organs of the human body. This technique is called
โultrasonographyโ.
5. Ultrasonography is also used in the examination of the fetus during pregnancy to
detect congenital defects and growth abnormalities.
6. Ultrasound may be employed to break small โstonesโ formed in the kidneys into
fine grains.
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Exercises
1. What is sound and how is it produced?
Ans. Sound is a form of energy which produces sensation in our ears. It is
produced by vibrating objects.
2. Describe with the help of a diagram, how compressions and rarefactions are
produced in air near a source of sound.
Ans. When a vibrating object moves forward, it pushes and compresses the air
in front of it creating a region of high pressure. This region is called a
compression (C). When the vibrating object moves backwards, it creates a region
of low pressure called rarefaction (R).
3. Why is a sound wave called a longitudinal wave?
Ans. The waves in which the individual particles of the medium move in a
direction parallel to the direction of propagation of the disturbance are called
longitudinal waves. The same happens in the case of sound wave, so a sound
wave is called a longitudinal wave
4. Which characteristic of the sound helps you to identify your friend by his voice
while sitting with others in a dark room?
Ans. It is the frequency and timber of sound which helps us to identify anyoneโs
voice.
5. Flash and thunder are produced simultaneously. But thunder is heard a few
seconds after the flash is seen, why?
Ans. It is because the speed of light is much faster than speed of sound so
thunder is heard a few seconds after the flash is seen.
6. A person has a hearing range from 20 Hz to 20 kHz. What are the typical
wavelengths of sound waves in air corresponding to these two frequencies? Take
the speed of sound in air as 344 m s
โ1
.
Ans.
๐ = 20 ๐ป๐ง ๐ = 20 ๐๐ป๐ง
= 20000 ๐ป๐ง
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๐ฃ = 344 ๐ / ๐ ๐ฃ = 344 ๐ / ๐
๐ = ? ๐ = ?
๐ฃ = ๐ ร ๐ ๐ฃ = ๐ ร ๐
๐ =
๐ฃ
๐
๐ =
๐ฃ
๐
๐ =
344
20
๐ =
344
20000
๐ = 17 . 2 ๐ ๐ = 0 . 0172 ๐
7. Two children are at opposite ends of an aluminium rod. One strikes the end of
the rod with a stone. Find the ratio of times taken by the sound wave in air and in
aluminium to reach the second child.
Ans.
๐๐๐๐๐ ๐๐ ๐ ๐๐ข๐๐ ๐๐ ๐๐๐ = 340 ๐ / ๐ ๐๐๐ ๐๐ ๐๐๐ข๐๐๐๐๐ข๐ = 6420 ๐ / ๐
๐๐๐ก ๐๐๐ ๐ก๐๐๐๐ = ๐ฅ ๐
๐๐๐๐ ๐ก๐๐๐๐ ๐กโ๐๐๐ข๐โ ๐๐๐ =
๐ท๐๐
๐ ๐๐๐๐
=
๐ฅ
340
๐๐๐๐ ๐ก๐๐๐๐ ๐กโ๐๐๐ข๐โ ๐๐๐ข๐๐๐๐๐ข๐ =
๐ท๐๐
๐ ๐๐๐๐
=
๐ฅ
6420
๐๐ , ๐๐๐ก๐๐ ๐๐ ๐ก๐๐๐๐ ๐ก๐๐๐๐ ๐๐ฆ ๐กโ๐ ๐ ๐๐ข๐๐ ๐ค๐๐ฃ๐ ๐๐ ๐๐๐ ๐๐๐ ๐๐ ๐๐๐ข๐๐๐๐๐ข๐ =
๐ฅ
340
รท
๐ฅ
6420
=
๐ฅ
340
ร
6420
๐ฅ
=
321
17
= 321 : 17
8. The frequency of a source of sound is 100 Hz. How many times does it vibrate in
a minute?
Ans.
๐ = 100 ๐ป๐ง
๐ก = 1 ๐๐๐
= 60 ๐
๐๐๐๐๐ข๐๐๐๐ฆ =
๐๐ข๐๐๐๐ ๐๐ ๐ฃ๐๐๐๐๐ก๐๐๐
๐๐๐ก๐๐ ๐ก๐๐๐
100 ๐ป๐ง =
๐๐ข๐๐๐๐ ๐๐ ๐ฃ๐๐๐๐๐ก๐๐๐
60
๐๐ข๐๐๐๐ ๐๐ ๐ฃ๐๐๐๐๐ก๐๐๐ = 100 ร 60
๐๐ข๐๐๐๐ ๐๐ ๐ฃ๐๐๐๐๐ก๐๐๐ = 6000 ๐ก๐๐๐๐
9. Does sound follow the same laws of reflection as light does? Explain.
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Ans. Yes, it follows the same laws of reflection as light. The directions in which
the sound is incident and is reflected make equal angles with the normal to the
reflecting surface at the point of incidence, and the three are in the same plane.
10. When a sound is reflected from a distant object, an echo is produced. Let the
distance between the reflecting surface and the source of sound production
remains the same. Do you hear echo sound on a hotter day?
Ans. Maybe not, because on hotter days the temperature of air is high so the
speed will also be high and time taken to reach the air is less and if it takes less
than 0.1 s then the echo will not be heard.
11. Give two practical applications of reflection of sound waves.
Ans. In stethoscopes and musical instruments, reflection of sound waves are
used.
12. A stone is dropped from the top of a tower 500 m high into a pond of water at the
base of the tower. When is the splash heard at the top? Given, g = 10 m s
โ2
and
speed of sound = 340 m s
โ1
.
Ans.
๐น๐๐ ๐ ๐ก๐๐๐ : โ ๐น๐๐ ๐ ๐ก๐๐๐ : โ
๐ข = 0 ๐ / ๐ ๐ฃ = 340 ๐ / ๐
๐ = 500 ๐ ๐ = 500 ๐
๐ = 10 ๐ ๐
2
๐ = ๐ข๐ก +
1
2
๐ ๐ก
2
๐ก๐๐๐ =
๐๐๐ ๐ก๐๐๐๐
๐ ๐๐๐
500 = 0 ร ๐ก +
1
2
ร 10 ร ๐ก
2
๐ก๐๐๐ =
500
340
500 = 0 + 5 ๐ก
2
๐ก๐๐๐ = 1 . 47 ๐
100 = ๐ก
๐ก = 10 ๐
So, the splash be heard at the top = 10 + 1.47 = 11.47 s
13. A sound wave travels at a speed of 339 m s
โ1
. If its wavelength is 1.5 cm, what is
the frequency of the wave? Will it be audible?
Ans.
๐ ๐๐๐๐ = 339 ๐ / ๐
๐ = 1 . 5 ๐๐
= 0 . 015 ๐
๐ = ?
Will it be audible?
๐๐๐๐๐ = ๐ ร ๐
๐ =
๐ ๐๐๐๐
๐
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๐ =
339
0 . 015
๐ = 22600 ๐ป๐ง
It will not be audible.
14. What is reverberation? How can it be reduced?
Ans. Reverberation is the phenomenon of the persistence of sound after it is
produced because of repeated reflection. To reduce reverberation, the roof and
walls of the auditorium are generally covered with sound-absorbent materials like
compressed fibreboard, rough plaster or draperies. The seat materials are also
selected on the basis of their sound absorbing.
15. What is the loudness of sound? What factors does it depend on?
Ans. Loudness is a measure of the response of the ear to the sound. It depends
on the amplitude of the sound wave.
16. How is ultrasound used for cleaning?
Ans. Ultrasound is generally used to clean parts located in hard-to-reach places,
for example, spiral tube, odd shaped parts, electronic components, etc. Objects
to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into
the solution. Due to the high frequency, the particles of dust, grease and dirt get
detached and drop out. The objects thus get thoroughly cleaned.
17. Explain how defects in a metal block can be detected using ultrasound.
Ans. The cracks or holes inside a metal block, which are invisible from outside,
reduce the strength of the structure. Ultrasonic waves are allowed to pass
through the metal block and detectors are used to detect the transmitted waves.
If there is even a small defect, the ultrasound gets reflected back indicating the
presence of the flaw or defect
