12
Q 1. An airplane produces a
sound wave with frequency of 5 KHz and wavelength 30 m. In how much time would
the sound wave cover the distance of 4 Km?
Solution
Frequency, ν = 5 KHz =
5000 Hz Wavelength, λ = 30 cm = 0.3 m Speed, v = ν λ = 0 .3 m x 5000 Hz =
1500 m/s Time = distance/ speed = 4000/1500 = 2.6 seconds
Q 2. How is sound produced
in the following things? The buzzing of mosquito The sound of drums The
sound of guitar The sound of trumpet
Solution
Vibration of mosquito
wings produces buzzing sound. The sound of drum is caused by the vibration of
its membrane when struck by drumsticks. The vibration of strings produces
various sounds in guitar. The vibration of air inside tube of the trumpet
produces sound.
Q 3. The frequency of a
sound source is 600 Hz. The speed of sound in a medium is
330 m/s. What will be the
linear distance between an adjacent crest and trough.
Solution
v = ν λ 330 = 600
x λ Hence, λ = 330/600 = 11/20 m The distance between adjacent crest and
trough = λ/2 = 11/40 m
Q 4. What is meant by
echocardiography?
Solution
It is used to study the
heart-valve action. An image of the heart is obtained by getting ultrasonic
waves reflected from various parts of the heart.
Q 5. What is reverberation?
Solution
The repeated reflection
that results in the persistence of sound in a large hall is called
reverberation.
Q 6. State laws of
reflection of sound. Explain three applications based on reflection of sound.
Solution
(1) Angle of incidence =
angle of reflection (2) Incident sound, reflected sound and normal lie in same
plane Applications: (a) Stethoscope is a medical instrument used for listening
to sounds produced within the body, chiefly in the heart or lungs. In
stethoscopes, the sound of the patient's heartbeat reaches the doctor's ears by
multiple reflection of sound. (b) Megaphones are designed to send sound in a
particular direction without spreading it in all directions. In it, a tube
followed by a conical opening reflects sound successively to guide most of the
sound waves from the source in the forward direction towards the audience. (c)
Generally the ceilings of concert halls, conference halls and cinema halls are
curved so that sound after reflection reaches all corners of the hall.
Q 7. Do the medium
particles have permanent displacement in wave motion?
Solution
No, medium particles
simply oscillate to and fro about their equilibrium positions.
Q 8. What is an echo?
Solution
The sound returning back
towards the source after getting reflected from a distant obstacle is called an
echo.
Q 9. Sound produced by a
thunderstorm is heard 10 s after the lightning is seen.
Calculate the approximate
distance of the thunder cloud. (Given speed of sound = 340 m s–1)
Solution
Given that: Speed of
sound = 340 m s–1 and t = 10 s Distance = Speed of
sound × Time =
340 m s–1 × 10 s →Distance = 3400 m
Q 10. When a person uses
deodorant spray, the other person standing at a distance would hear the sound
of spraying first and the fragrance of spray would reach him later. Why so?
Solution
The sound of spraying
deodorant travels through the vibrations of air layers so it reaches first.
But, the fragrance of deodorant reaches the other person through actual
movement of air particles, therefore takes more time. This example shows that
in sound waves there is no actual movement of particles from sound producing
source to the ear otherwise the person standing at a distance would have heard
and smelt the deodorant at same time.
Q 11. (i) Why is the
ceiling and wall behind the stage of good conference halls or concert halls
made curved? (ii) Which property of sound leads to the formation of echoes?
Briefly explain? (iii) What is reverberation? What will happen if the
reverberation time in a big hall is too long? How can we reduce it?
Solution
(i) Ceiling and walls are
made curved so that sound after reflection reaches all corners of the hall.
(ii) Reflection leads to echoes. An echo is the phenomenon of repetition of
sound of a source by reflection from an obstacle. To hear a distinct echo of a
sound, the time taken by this sound to reach the listener after reflection
should be (1/10) of a second. If speed of sound in air is 344 m/s, then the
minimum distance of obstacle from the source of sound must be 17.2 m. (iii)
Reverberation- It is repeated reflection of sound that results in persistence
of audible sound after the source has stopped emitting it. If the reverberation
time in a big hall is too long, sound gets blurred, distorted and confusing to
hear. This can be reduced by using sound absorbent material on roof, walls and
seats.
Q 12. How does a megaphone
work?
Solution
Megaphones are designed
to send sound in a particular direction without spreading it in all directions.
In these, a tube followed by a conical opening reflects sound multiple times to
guide the sound waves in the direction towards audience.
Q 13. Name few instruments
which are based on multiple reflection of sound?
Solution
Megaphone, Horns,
stethoscope, trumpet, shehanais.
Q 14. Would the speed of
sound on a warm day be different from the speed of sound on a cold day? If yes
what would be the difference?
Solution
Speed of sound depends on
the temperature, pressure and characteristics of the medium through which it is
travelling. Therefore, the speed of sound would be greater on a warm day as
compared to speed of sound on a cold day because with increase in temperature
speed of sound increases.
Q 15. Can we hear the
ringing of a mobile phone placed in a vacuum chamber?
Solution
No, we could not hear the
ringing of a mobile phone placed in a vacuum chamber as sound waves needs a
medium to travel and in vacuum there are no air particles.
Q 16. Can two astronauts
talk on moon a they does on the surface of the earth?
Solution
No, the astronauts cannot
talk on the surface of the moon because there is no atmosphere/medium present
on the moon and sound cannot travel through vacuum. On earth air is present and
hence they can talk with each other.
Q 17. What is cause of
sensation of sound due to rolling of thunder?
Solution
Repeated reflection and
reverberation is the cause of sound due to rolling of thunder.
Q 18. Wavelength of a siren
in air is 1.2m. And if same siren is blown under water it has wavelength 6 m in
water. Find the velocity of sound in water, if the speed of sound in air is 330
m/s.
Solution
Let v1, v2 be
the velocities of sound in air and water respectively. Wavelength of sound
waves in air λ1 = 1.2 m Wavelength of sound waves in water λ2 =
6 m Since, source of sound is same, frequency will be same vl = νλ1 330
m/s = ν x 1.2 ν =
330/1.2
…(1) Velocity of sound in water v2 = ν λ2 ν
= v2/6
…(2) From equation 1 and 2 we get 330/ 1.2 = v2 /6 v2 =
330 x 6/1.2 = 1650 m/s is the velocity of sound wave in water.
Q 19. Sound of engine of a
submarine would travel faster in water or in air?
Solution
Sound of engine of a
submarine could be detected earlier (with help of instruments called
hydrophones) in water as compared to in air. As sound waves travel faster in
liquids as compared to in gases.
Q 20. How far does sound
travel in air when a tuning fork of frequency 12 Hz makes 30 vibrations?
Velocity of sound is 336 m/s.
Solution
Q 21. List three
characteristics of sound waves. State the factors on which each of these
characteristics depends.
Solution
1. Loudness- The
sensation produced in the ear which enables us to distinguish between a loud
and a faint sound is called loudness. Loudness is determined basically by the
amplitude of the wave. 2. Pitch- It is that characteristic of sound which helps
in differentiating between a shrill and a grave sound. It depends on the
frequency of sound wave. 3. Quality or timbre- The characteristic of a sound
which distinguishes it from another of the same pitch and loudness is called
quality. It depends on the wave-form of the sound wave.
Q 22. List in tabular form
two differences between longitudinal waves and transverse waves. Name a wave
which does not require a material medium for its propagation.
Solution
Longitudinal wave
Transverse waves (i) The particles of the medium vibrate in a direction
parallel to the direction of propagation of the wave. (i) The particles of the
medium vibrate in a direction perpendicular to the direction of propagation of
wave. (ii) Compressions and rarefactions are formed. (ii) Crests and troughs
are formed. (iii) Eg.- sound waves (iii) Eg.- Light waves Light waves, radio
waves, etc. do not require any material medium for their propagation.
Q 23. Why ceilings of
concert halls and conference halls are made curved?
Solution
The ceilings of concert
halls and conference halls are generally curved. It is done to ensure that the
sound after reflection from the ceilings reaches all corners of the hall.
Q 24. How is ultrasound
used in various industries?
Solution
Ultrasound can be used
for various commercial uses as follows: 1)Ultrasound is used in industry for
detecting flaws (cracks, etc) in metal blocks. If there is a crack in metal
block then ultrasounds get reflected and do not reach the ultrasound detector,
thereby indicating a flaw. 2)Ultrasound is used in medical industry to
investigate internal organs of the human body such as liver, gall bladder,
uterus, kidneys, pancreas, heart, etc. 3)Ultrasound is also used to remove
kidney stones by first breaking the kidney stones into fine granules which then
could pass through urine. 4)Ultrasound is also now a days used extensively to
see the well being of foetus (unborn child) inside the mother’s uterus.
5)Ultrasound is used in sonar apparatus to measure the depth of sea and to
locate objects under sea like shipwrecks, submarines, sea rocks, hidden
ice-bergs etc.
Q 25. Why do animals like
dogs etc get uneasy before earthquakes?
Solution
Earthquakes produce low
frequency infrasound before the main shock waves begin, therefore the animals
like dog which could hear sounds below 20 hz, i.e infrasound get a warning and
become disturbed.
Q 26. How does a hearing
aid work?
Solution
A hearing aid is a
battery operated electronic device. It receives sound through a microphone
which converts the sound waves to electrical signals. These electrical signals
are amplified by an amplifier and sent to speakers of hearing aid. The speaker
converts the amplified electrical signal to sound and sends to the ear for clear
hearing.
Q 27. In which medium the
speed of sound will be greater -air or water? Give reason for your answer.
Solution
Sound travels faster in
water than air because particles of water are closer to each other, that is
water is denser than air, and hence vibrations could travel faster.
Q 28. (a) What is the role
of hammer bone, anvil bone, stirrup bone and cochlea of human ear in hearing a
sound? (b) Write a difference between pitch of sound and loudness.
Solution
(a) The vibrations due to
the ear drum are amplified several times by three bones hammer, anvil and
stirrup in the middle ear. The pressure variations in the inner ear are turned
into electrical signals by the cochlea. (b) S.No Pitch Loudness 1 It is the way
in which the brain interprets the frequency of a given sound. It is the
physiological response of the ear to the intensity of sound. 2 It depends on
frequency of the wave. It depends on amplitude (or intensity) of the wave.
Q 29. (i) Why is sound
called a longitudinal wave? (ii)Sound is produced due to vibrating motion, then
why a vibrating pendulum does not produce sound?
Solution
(i) In a sound wave,
particles of medium vibrate along the direction of motion of the wave. Thus,
the sound wave is a longitudinal.(ii) The frequency of the vibrating pendulum
does not lie within the audible range (20Hz to 20,000Hz), and hence it does not
produce audible sound.
Q 30. Does the sound of an
exploded cracker in air travel faster than the sound produced by a humming bee?
Why?
Solution
In air, both the sound,
that is sound of an exploded cracker and the sound produced by a humming bee
travel with same speed in air. It is because, the speed of sound in a medium is
independent of the frequency of the sound.
Comments
Post a Comment