This post provides you a complete solution of Chapter 11 Class 9th for Rajasthan Board of Secondary Education.
This post contains all the main and important topics
which have complete and detailed description.
Sound Questions- Answers
These solutions are based on NCERT Science book
which includes :
Production of Sound
Propagation of Sound
Reflection of Sound
Range of Hearing
Application of Ultrasound
Structure of Human Ear
Intext Questions
Answers of Questions on Page 129
Q1. How does the sound
produced by a vibrating object in a medium reach your ear?
Ans.
Air is the most common medium through which sound propagates. When an object vibrates, it makes the particles of the medium vibrating. This creates a region of high pressure and high density called compression and when the object move backward, it creates a region of low pressure and low density called rarefaction.
So the particles of the medium
vibrates and displace the adjacent particles and come back to the original
position, this process continues in the
medium till the waves reaches our ear.it forces the tympanic membrane to
vibrate and thus causing the sensation of hearing.
Q2. Explain how sound is
produced by your school bell.
Ans.
When the school bell is hit by a hammer, it begins to vibrate, and it
makes the particles of the medium vibrating. This creates a region of high
pressure and high density called compression and when the object move backward,
it creates a region of low pressure and low density called rarefaction. So the
particles of the medium vibrates and displace the adjacent particles and come
back to the original position, this
process continues in the medium till the waves reaches our ear.it forces
the tympanic membrane to vibrate and thus causing the sensation of hearing.
Q3. Why are sound waves
called mechanical waves?
Ans.
Sound waves need a medium to travel in
it. Mechanical energy is needed to make an object to vibrate. This mechanical
energy of vibrating object travel through a medium and reaches our ear. That’s
why sound waves are called mechanical waves.
Q4. Suppose you and your
friend are on the moon, will you be able to hear any sound produced by your friend?
Ans.
Sound needs medium (air, water etc.) to
propagate. There is no atmosphere on the moon, so sound cannot propagate.
That’s why I will not able to hear any sound produced by my friend.
Answers of Questions on Page 132
Q1. Which wave property
determines (a) loudness (b) pitch?
Ans.
(a). Loudness is determined by the
amplitude of the sound waves. More the amplitude then more is the loudness.
(b)The pitch is determined by the frequency
of the sound waves.
Q2. Guess which sound has a
higher pitch guitar or car horn?
Ans.
Guitar has a sound of higher pitch
because frequency of sound produced by guitar is higher than that of car horn.
Answers of Questions on Page 132
Q1. What are wavelength,
frequency, time period and amplitude of a sound wave?
Ans.
(i). Wave
length – The distance between two successive regions
of high pressure and density (compression) or low pressure and low density
(rarefaction) is known as wavelength. It is denoted by 
(ii). Frequency
– The number of vibrations (compression or
rarefaction) is called frequency. It is denoted by
and its unit is 
(iii). Time
period – Time taken by two consecutive compressions
or rarefaction to cross a fixed point is called the time period of the wave.
(iv). Amplitude –
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 represented by letter ‘A’
Q2. How are the wavelength
and frequency of a sound wave related to its speed?
Ans.
Suppose s mechanical wave passing through a
medium
Speed of wave (v) = 
Speed of wave = Frequency ×Wavelength
Q3. Calculate the wavelength
of sound wave whose frequency is 220Hz and speed is 440m/s in a
given medium.
Ans.
Q4. A person is listening to
a tone of 500Hz sitting at a distance of 450m form the source of the
sound. What is the time interval between successive compressions from the
source?
Ans.
Answers of Questions on Page 133
Q1. Distinguish between
loudness and intensity of sound.
Ans.
Loudness is physiological response of the
ear.it depend on intensity of sound and response of ear.
The intensity of sound wave is the amount of
sound energy passing each second through a unit area.
Answers of Questions on Page 133
Q1. In which of the three
media air, water or iron, does sound travel the fastest at a particular
temperature?
Ans. Sound travel fastest through iron.
Answers of Questions on Page 134
Q1. An echo was heard after
3s. What is the distance of the reflecting surface from the source given that
the speed of sound is 342m/s?
Ans.
Answers of Questions on Page 135
Q1. Why are the celling of
concert halls curved?
Ans.
The ceilings of concert halls are curved
because the sound reaches all the parts of the hall after reflecting from the
ceiling and is audible to each person in the hall.
Answers of Questions on Page 136
Q1. What is the audible
range of the average human ear?
Ans. 20 Hz to 20,000Hz
Q2. What is the range of
frequencies associated with?
(a). infrasound?
(b) Ultrasound?
Ans.
(a). Infrasound- less than 20Hz
(b).
Ultrasound- higher than 20,000Hz
Answers of End Exercise Questions on Page 138-139
Q.1 What
is sound and how is it produced?
Ans.
Sound is a form of energy
which produces a sensation of hearing in our ears. It is produced when a body
vibrates.
Q.2 Describe
with the help of a diagram, how compressions and rarefactions are produced in
air near a source of sound?
Ans.
A wave is a disturbance
that moves through a medium when the particles of the medium set adjacent
particles into motion. Air is the common medium through which sound travels.
When a vibrating object moves forward, it pushes and compress the air in front
of it and create a region of high pressure, this region is called compression.
When the vibrating
object moves backward, it creates a region of low pressure called rarefaction.
The object moves back and forth rapidly. So a series of compression and
rarefaction is created which makes the sound wave propagates in air.
Q.3 Why
is sound wave called a longitudinal wave?
Ans.
Sound waves travel
through a medium in the form of alternate compressions and rarefactions. The
particles of the medium thus move to and fro along the direction of propagation
of the sound wave. That is why, the sound waves are called longitudinal waves.
Q.4 Which
characteristic of the sound helps you to identify your friend by his voice
while sitting with others in a dark room?
Ans.
The quality or timbre of sound is the characteristic which helps
us to identify our friends’ voice.
Q.5 Flash
and thunder are produced simultaneously. But thunder is heard a few seconds
after the flash is seen, why?
Ans.
Speed of sound is 330m/s
in air at 0
whereas speed of light is 3× 108 m/s. The speed of sound is smaller than the speed
of light, so thunder is heard a few seconds after the flash is seen.
Q.6 A
person has a hearing range from 20 Hz to 20 kHz. What are the typical
wavelengths of sound wave in air corresponding to these two frequencies? Take
the speed of sound in air as 344 ms-1.
Ans.
Q.7 Two
children are at opposite ends of an aluminum 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
aluminum to reach the second child.
Ans.
Q.8 The
frequency of a source of sound is 100 Hz. How many times does it vibrate in a
minute?
Ans.
Q.9 Does
sound follow the same laws of reflection as light does? Explain.
Ans.
Yes, sound follows the
same laws of reflection as light does.
(i).The directions in
which sound is incident and reflected make equal angles with the normal to the
reflecting surface and three lie in the same plane.
(ii). The direction in
which sound is incident, the direction of reflected and normal lie in the same
plane.
Q.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
remain the same. Do you hear echo sound on a hotter day?
Ans.
On the hotter day, the
speed of sound increases. So the reflected sound returns to source earlier than
0.1 s. Hence, a distinct echo cannot be heard.
Q.11 Give
two practical applications of reflection of sound waves.
Ans.
(i) In a stethoscope, the
sound of a patient’s heartbeat is guided along the tube of the stethoscope to
the doctor’s ears by multiple reflections of sound.
(ii) Ceiling of concert halls are
curved so that sound after reflection reaches all corners of the hall.
Q.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 ms-2
and speed of sound = 340 ms-1.
Ans.
Q.13 A
sound wave travels at a speed of 339 ms-1, if its wavelength is 1.5 cm, what is
the frequency of the wave? Will it be audible?
Ans.
Q.14 What
is reverberation? How can it be reduced?
Ans.
The persistence of sound in a big hall is the result of repeated reflections of sound and is called reverberation.
Excessive
reverberation is undesirable so following measures are taken to reduce
reverberation
(i). The large plane surface of the wall and roofs are covered with sound absorbing material like compresses fireboard, rough plaster.
(ii). Sound absorbing panels are kept near
the stage.
Q.15 What
is loudness of sound? What factors does it depend on?
Ans.
Loudness is determined by the amplitude of the sound waves. More the amplitude then more is the loudness. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. Loudness is a subjective quantity. Loud sound can travel a larger distance because it is associated with higher energy.
Q.16Explain
how defects in a metal block can be detected using ultrasound.
Ans.
Ultrasounds can be used to detect cracks and flaws in
metal blocks. Metallic components are generally used in construction of big
structures like buildings, bridges, machines and also scientific equipment. The
cracks or holes inside the metal blocks, which are invisible from outside
reduces the strength of the structure.
When ultrasonic waves are sent in the metal
block, the waves get partly reflected from a crack and so te intensity of emerging waves from the
metal block is less in the portion of crack or defect. Detectors are used to
detect the transmitted waves.
Q.17 Explain
how the human ear works.
Ans.
We can hear with the help of an extremely sensitive
device called the ear. It allows us to convert pressure variations in air with
audible frequencies into electric signals that travel to the brain via the
auditory nerve.
The outer ear is called ‘pinna’. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane. When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. At the same time, the eardrum moves outward when a rarefaction reaches it.
The vibrations are amplified several times by three bones (the hammer,
anvil and stirrup) in the middle ear. The middle ear transmits the amplified
pressure variations received from the sound wave to the inner ear. In the inner
ear, the pressure variations are turned into electrical signals by the cochlea.
These electrical signals are sent to the brain via the auditory nerve, and the
brain interprets them as sound.
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