Q.

10 Why is sound wave called a

Class – 9th Chapter-Sound longitudinal wave?
Ans. Sound wave is called longitudinal wave
because the particles of the medium vibrate
Q.1 What is SONAR?
in the direction of the propagation of wave.
Ans. SONAR (Sound Navigation And
Ranging) is a technique for determining
water depth and locating underwater Q.11 Which characteristic of the sound
objects, such as reefs, submarines, and helps you to identify your friend by his voice
schools of fish. while sitting with others in a darkroom?
Ans. The characteristic of sound is quality or
timbre.
Q.2 Define one hertz.
Ans. One hertz is one vibration per second.
Q.12 Flash and thunder are produced
simultaneously. But thunder is heard a few
Q.3 Define wavelength.
seconds after the flash is seen, why ?
Ans. It is the distance between two nearest
Ans. The speed of light is 3 x 108 ms-
points in a wave which are in the same 1
whereas that of sound is 344 ms-1 in air.
phase of vibration.
Thus, flash of lightning is seen at once, but
sound takes few seconds to reach our ears.
Q.4 Suppose you and your friend are on the
moon. Will you be able to hear any sound
Q.13 The frequency of a source of sound is
produced by your friend?
100 Hz. How many times does it vibrate in a
Ans. No, I will not be able to hear sound,
minute?
because moon has no atmosphere.
Ans.No. of vibrations produced in 1 s = 100
Therefore, no sound waves can travel to
No. of vibrations produced in 60 (sec) = (1
your ears and, therefore, no sound is heard.
min) = 100 x 60 = 6000.

Q.5 Guess which sound has a higher pitch;

Q.14 Name the two types of mechanical
guitar or car horn ?
waves.
Ans. Car horn has a higher pitch than a
Ans. The two types of mechanical waves
guitar, because sound produced by the
are:
former is shriller than the latter.
(i) Transverse wave and (ii) Longitudinal
wave.
Q.6 How are the wavelength and frequency
of a sound wave related to its speed?
Q.15 What is a wave?
Ans. Speed of sound = Frequency x
Ans. A wave is a disturbance that travels in
Wavelength.
a medium due to repeated periodic motion
of particles about their mean position,- such
Q.7 In which of the three media, air, water that the disturbance is handed over from
or iron, does sound travel the fastest at a one particle to the other without the actual
particular temperature ? motion of the medium.
Ans. Sound travels fastest in iron as
compared to water and air.
Q.16 What is a transverse wave?
Ans. It is a wave in which the particles of the
Q.8 What is the audible range of the medium vibrate perpendicular to the
average human ear? direction of propagation of the wave. ,
Ans. An average human ear can hear sound
waves between frequencies 20 Hz to 20,000
Q.17 What is a longitudinal wave?
Hz.
Ans. It is a wave in which the particles of the
medium vibrate in the direction of
Q.9 What is sound and how is it produced? propagation of the wave.
Ans. Sound is mechanical energy which
produces a sensation of hearing. When an
Q.18 What is a trough?
object is set into vibrations, sound is
Ans. A trough is a depression in a wave /.e.,
produced.
maximum displacement in the negative Q.31 Where is the density of air higher; at
direction (below the mean position). compressions or at rarefactions?
Ans. At the compression.
Q.19 What do you understand by*the term
infrasonic vibrations? Q.32 Name the quantity that represents the
Ans. The sounds of frequency lower than 20 length of one complete wave.
Hz are called the infrasonics or subsonics. Ans. Wavelength.

Q.20 Which of the following Q.33 What is the distance between two
sound wavers we can hear: 10 Hz, 500 Hz, consecutive crests in a wave called?
1500 Hz, 12000 Hz, 25000 Hz ? Ans. Wavelength.
Ans. 500 Hz, 1500 Hz, 12000 Hz.
Q.34. Is the amplitude of a wave the same,
Q.21 What do you understand by the term as the amplitude of the vibrating body
ultrasonic vibrations? producing the wave? Ans. Yes.
Ans. Sounds of frequency higher than
20,000 Hz are called the ultrasonics.
Q.35 How moths of certain families are able
to escape captures from bats ? What is the
Q.22 What do you understand by the term range of frequencies assciated with
echo? (a) infra sound ? (b) ultrasound ?
Ans. The sound heard after reflection from a Ans. They have very sensitive hearing
rigid obstacle is called an echo. equipment, can hear the squeaks
(ultrasound) of bat and know when a bat is
flying nearby.
Q.23 Name the term associated with the
(a) Less than 20 Hz. (b) More than 20,000
travelling disturbance in a medium.
Hz.
Ans. Wave.

Q.36 A person fires a gun standing at a

Q.24 Do waves transport energy? Ans. Yes.
distance of 55 m from a wall. If the speed of
sound is 330 ms-1, find the time for an echo
Q.25 Do waves transport matter? Ans. No. to be heard.
Ans. Given S = 55 m, v = 330 ms-1 t = ?
2d = v x t
Q.26 Do the particles of the medium move or t = 2d/v = 2 x 55/330 = 0.3 s
from one place to another in a medium?
Ans. No.
Q.37 The pulse rate of a man is 80 beats in
one minute. Calculate its frequency. [SAII-
Q.27 Does the velocity of wave motion 2014]
depend on the nature of the medium ? Ans.No. of beats per minute = 80
Ans. Yes. No. of beats per second = 80/60 = 1.3
So, frequency = 1.3 Hz
Q.28 Does the velocity of wave motion
depend on the nature or motion of the Q.38 Which wave property determines: (a)
source? Ans. No. loudness, (b) pitch ?
Ans.(a) The amplitude of the wave
Q.29 What is the other name of a long determines the loudness. More the
flexible spring? Ans. Slinky. amplitude of a wave, more is the loudness
produced.
(b) The pitch is determined by tl e frequency
Q.30 Can you produce both types of waves of the wave. Higher the frequency of a
(i.e., longitudinal and transverse) on a wave, more is its pitch and shriller is the
slinky? sound.
Ans. Yes.

Q.39 Distinguish between loudness and

intensity of sound.
Ans. The loudness depends on energy per human beings. The ultrasonic waves on
unit area of the wave and on the response striking the insect send back an echo, which
of the ear, but intensity depends only on the is heard by the bat. As the echo is heard by
energy per unit area of the wave and is the bat it hovers on the insect and catches
independent of the response of the ear. it.

Q.40 Why are the ceilings of concert halls Q.46 How is ultrasound used for cleaning?
curved? Ans. The object to be cleaned is put in a
Ans. The ceilings of concert halls are curved tank fitted with ultrasonic vibrator. The tank
because sound after reflection from it is filled with water containing detergent. As
reaches all the corners of the hall and is the ultrasonic vibrator is switched on the
audible to each person in the hall. detergent rub against the object at a very
high speed and hence cleans it.
Q.41 What is the range of frequencies
associated with : (a) Infra sound? (b) Q.47 Explain how defects in a metal block
Ultrasound? can be detected using ultrasound.
Ans.(a) Infra sound : Sound waves between Ans. The ultrasound waves are allowed to
the frequencies 1 to 20 Hz. pass through metal block to which detectors
(b) Ultrasound : Sound waves of the are fitted. If there is a small defect in the
frequencies above 20,000 Hz. metal block like an air bubble or a crack,
then the ultrasound waves are reflected
from such spots. Metal block if defective is
Q.42 When a sound is reflected from a
indicated by the reflected ultrasonic waves.
distant object, an echo is produced. Let the
distance between the reflecting surface and
the source of sound production remains the Q.48 Explain why can echoes not be heard
same. Do you hear echo sound on a hotter in a small room.
day? Ans. For hearing echo, there should be at
Ans. If the temperature rises, the speed of least a distance of 17 m between the source
sound will increase. This in turn will increase of sound and the body from which’ sound is
the minimum distance required for hearing reflected. In small rooms this is not the
an echo. No echo is heard because the case, hence echoes are heard.
distance between the source of sound and
reflecting body does not increase.
Q.49 Sound is produced due to a vibratory
motion, then why a vibrating pendulum
Q.43 Give two practical applications of does not produce sound?
reflection of sound waves. Ans. The frequency of the vibrating
Ans.(i) Megaphones are designed tp send pendulum does not lie within the audible
sound waves in particular direction are range (20 Hz to 20,000 Hz) and hence it
based on the reflection of sound. does not produce audible sound.
(ii) In stethoscope, the sound of patient’s
heartbeat reaches the doctor’s ears by
Q.50 Two sound waves A and B are shown
multiple reflections in the tubes.
in the figure. Identify the sound wave hiving
:
Q.44 What is reverberation? How can it be (i) high frequency
reduced? (ii) low frequency.
Ans. Reverberation is the repeated multiple Ans.(i) Wave B has high frequency as it
reflections of sound in any big enclosed repeats itself after smaller intervals of time.
space. It can be reduced by covering the (ii) Wave A has low frequency as it repeats
ceiling and walls of the enclosed space with itself after longer intervals of time.
some absorbing materials like fibre board,
loose woolens etc.
Q.51 Two sound waves A and B are shown
in figure. Identify the sound wave having :
Q.45 Explain how bats use ultrasound to (i) small amplitude
catch their prey. (ii) large amplitude
Ans. The bats produce high pitched Ans.(i) Sound wave B has small amplitude.
ultrasonic waves which are not heard by (ii) Sound wave A has large amplitude.
Q.52. A sound wave travelling in a medium medium to propagate, therefore, sound
is represented as shown in figure, cannot travel between astronauts on the
(i) Which letter represents the amplitude of moon, hence they use radio transmitters.
the sound wave?
(ii) Which letter represents the wavelength
Q.57 How does the sound produced by a
of the sound wave?
vibrating object in a medium reach your
Ans.(i) Letter P represents the amplitude of
ear?
the sound wave.
Ans. Air is the commonest material through
(ii) Letter Q represents the wavelength of
which sound propagates. When vibrating
the sound wave.
objects, like prongs of a tuning fork move
forward, they push the molecules of the air
in front of them. This in turn compresses the
Q.53 Distinguish between transverse and air, thus, creating a region of high pressure
longitudinal waves. (Three points) and high density called compression. This
Ans.Transverse waves: compression in the air travels forward.
(a) Particles the medium vibrate at right When the prongs of the tuning fork move
angles. backward, they create a region of low
(b) Alt. crests and troughs formed. pressure in the air, commonly called
(c) e.g., water wavesv rarefaction. This region has low pressure,
Longitudinal waves: low density and more volume. As the tuning
(a) Particles vibrate parallel to the direction fork continues to vibrate, the regions of
of waves. compression in the air alternate with the
(b) Alt. compressions, rarefaction formed. regions of rarefaction. These regions
(c) e.g., sound waves. alternate at the same place. The energy of
vibrating tuning fork travels outward. This
energy which reaches the ears, makes the
Q.54 State three characteristics of a musical
eardrums to vibrate and thus we hear
sound. On what factors do they depend.
sound.
Ans.Characteristics of musical sound are:

 Loudness-Amplitude affect Q.58 What are wavelength, frequency, time

loudness-more amplitude period and amplitude of a sound wave?
loud/Less amplitude soft sound. Ans. Wavelength: It is the linear distance
 Pitch-frequency affects pitch- between two consecutive compressions or
more frequency more pitch, two consecutive rarefactions.
Less freq. less pitch. Frequency: The number of compressions or
 Quality or Timber. rarefactions taken together passing through
a point in one second is called frequency.
Q.55 (a) The sound of which of the following Time period : It is the time taken by two
frequencies are audible to human ears : 2 consecutive compressions or rarefactions to
Hz, 5 Hz, . 20 Hz, 200 Hz, 2000 Hz. cross a point.
(b) How [paths of, certain families are able Amplitude: It is the magnitude of maximum
to escape capture ? displacement of a vibrating particle about
Ans. its mean position.
(a) 20 Hz, 200 Hz and 2000 Hz.
(b) They do not reflect the ultrasounds sent Q.59 Does sound follow the same laws of
towards them by bats. reflection as light does? Explain.
Ans. Yes, sound and light follow the same
Q.56 How does the sound produced by a laws of reflection given below :
musical instrument, reach your ears? (a) Angle of incidence at the point of
Astronauts need radio transmitter to talk to incidence = Angle of reflection.
each other on moon. Why ? (b) At the point of incidence, the incident
Ans. The sound produced by the musical sound wave, the normal and the reflected
instrument makes the molecules of air sound wave lie in the same plane.
vibrate. These vibrations are carried forward
by the other molecules till they reach our Q.60 State any two characteristics of a wave
ear. These then vibrate our eardrum to motion.
produce sound. Since sound requires a Ans.The characteristics of wave motion are :
(i)It is a periodic disturbance. our eardrum and produces an audible
(ii)Energy transfer takes place at a constant sound.
speed.
Q.67 Will the sound be audible if the string
Q. 61 A longitudinal wave of wavelength 1 is set into vibration on the surface of the
cm travels in air with a speed of 330 m/s. moon? Give reason for your Question.
Calculate the frequency of the wave. Can Ans. No, we will not hear any audible sound
(his wave be heard by a normal human on the surface of the moon. This is because
being? sound requires a medium to propagate,
Ans. Given λ = 1 cm = 0.01m since there is no atmosphere on the surface
v = 330 ms-1 of moon, therefore, the sound will not be
Using v = v λ, we have v = v/λ = 330/0.01 = heard.
33000 Hz
No, this cannot be heard by a normal human Q.68 What change, if any, would you expect
being. in the characteristics of musical sound when
we increase: (i) its frequency, and (ii) its
Q.62 If the amplitude of a wave is doubled, amplitude?
what will be the effect on its loudness? Ans. (i) Pitch of sound will increases (ii)
Ans. Loudness depends upon the square of Loudness of sound will increases
the amplitude of the wave, therefore, when
the amplitude of wave is doubled, the Q.69: If the speed of ultrasound through
loudness becomes four times. seawater is 1531 m/s. What is the distance
of the seabed from the ship?
Q.63 How do the frequency and amplitudes Ans: Time between transmission and
affect a musical sound? detection t = 342 s.
Ans. The ‘frequency’ of a musical sound Speed of ultrasound in seawater = 1531
affects its ‘pitch’. The more the frequency of m/s.
a (musical) sound, the ‘sharper’, and Distance travelled by the ultra sound =
‘shriller’ the sound becomes. 2 × depth of sea = 2d
The ‘amplitude’ of a musical sound affects
its loudness, or intensity. The more the 2d = speed of sound × time
amplitude of thd sound, the louder, (or more = 1531 × 3.42
intense) the sound is. = 5236 m
∴ d = 5236/2
Q. 64 Give one example each of natural = 2618 m.
vibration, forced vibration and resonance. The distance of the seabed from the ship is
Ans.(i) Natural vibration : The vibrations of a 2618 m.
simple pendulum about its mean position.
(ii) Forced vibration : A sonometer wire, Q. 70. What is the unit of sound intensity?
under tension, vibrating under the influence Ans. Decibel (dB)
of a vibrating tuning fork.
(iii) Resonance: A correctly adjusted length
of a sonometer wire under proper tension, Q.71 What is the effect of temperature on
vibrating under the influence of a vibrating the speed of sound? Answer: The speed of
tuning fork. sound increases with temperature.

Q.65 Mention one practical use of echoes. Q.72. What happens when sound waves
Ans. Echoes are used in radars to estimate strike a surface? Answer: Reflection.
the distance of flying objects.
Q.73. What does a sound wave with a high
Q.66 How does a stretched string on being frequency sound like? Answer: A high-
set into vibration,produce the audible pitched sound.
sound?
Ans. On being set into vibrations, the
stretched string, forces the surrounding air
to vibrate. This vibrating air, in turn, affects
Q.74. How do animals like bats use sound?
Answer: Bats use echolocation, emitting
ultrasonic waves to detect objects.

Q.75. What happens when sound waves

pass from air into water? Answer: The
waves bend due to change in speed.

Q.76. Can sound travel in a gas like helium?

Answer: Yes, but it travels faster in helium
due to lower density.

Q.77. Why does sound travel faster in solids

than liquids and gases? Answer: Solids
have particles that are closer together.

Q.78. What is a sound wave’s wavelength?

Answer: The distance between two
successive compressions or rarefactions.

Q.79. How do sound waves reflect off a

surface? Answer: The angle of incidence
equals the angle of reflection.

Q.80. What is the purpose of soundproofing?

Answer: To reduce unwanted sound
transmission.