Lakhmir Singh & Manjit Kaur: Sound, Solutions- 2

Page No:206

Solution 1
The reflection of sound leads to formation of echoes
Solution 2
Echo is repetition of sound caused by the reflection of sound waves.
Solution 3
The persistence or sound in a big hall or auditorium is called reverberation.
Solution 4
a) Megaphone and bulb horn
b) Stethoscope
c) Soundboard
Solution 5
Megaphone
Solution 6
a) Loudness
b) Pitch
c) Timbre or Quality
Solution 7
The loudness of sound is measured in decibel. Its symbol is dB.
Solution 8
Pitch helps us to distinguish between a man's voice and a woman's voice, even without seeing them.
Solution 9
Pitch of a sound is directly proportional to frequency. Higher the frequency, higher is the pitch of the sound.
Solution 10
(i) Loudness
(ii) Pitch
(iii) Timbre
Solution 11
Quality or timbre
Solution 12
Ears enable us to hear sounds.
Solution 13
Ear drum starts vibrating when outside sound falls on it.
Solution 14
There are three small bones in the middle ear- anvil, hammer and stirrup.
Solution 15
a) Hammer
b) Stirrup
Solution 16
The function of three tiny bones in the ear is to increase the strength of vibrations coming from the ear drum before passing them onto the inner ear. 

Page No:207 

Solution 17
Eustachian tube
Solution 18
Auditory nerve
Solution 19
Ear canal
Solution 20
We should not put a pin or pencil or any other sharp pointed objects in our ears because they can damage the ear-drum and damaging of ear drum can make us deaf.
Solution 21
Ultrasound scans are used to monitor the growth of developing baby in the uterus of the mother.
Solution 22
An ultrasound scan for fetus is better than X-rays because X-rays can damage the delicate body cells of the fetus.
Solution 23
SONAR is used to find the depth of sea by using ultrasonic sound waves.
Solution 24
SO und Navigation And Ranging
Solution 25
Soundboard works on the principle of reflection of sound.
Solution 26
A megaphone is used to address a small gathering of people.
Solution 27
A stethoscope, based on the principle of reflection of sound, is used by doctors to listen to our heartbeats.
Solution 28
Soundboard is a concave board which is kept behind the speaker on the stage of a big hall.
Solution 29
Curtains and carpets can make our big room less echoey.
Solution 30
No we cannot hear infrasonic waves and ultrasonic waves. That's because the frequencies of both these waves fall beyond the human audible range of frequencies.
Solution 31
Infrasonic sound
Solution 32
Ultrasonic sounds
Solution 33
Infrasonic sound waves
Solution 34
As the frequency increases the pitch of the sound also increases.
Solution 35
The loudness decreases with the decrease in the amplitude of sound.
Solution 36
Ultrasonic sound waves
Solution 37
a) reflected
b) frequency
c) amplitude
d) waveform
e) reflection
Solution 38
An echo is heard sooner on a hot day because the speed of sound in air increases with temperature. So the speed of sound in air is more on a hot day, and an echo is heard sooner.
Solution 39
An echo is heard sooner in water because the speed of sound in water is higher than the speed of sound in air.
Solution 40
The persistence of sound in a big hall due to repeated reflections from the walls, ceiling and floor of the hall is called reverberation. If the reverberation time in a big hall is too long, then the sound becomes blurred, distorted and confusing due to overlapping.
Solution 41
Reverberations in a big hall or auditorium can be reduced by the following methods :
i) Panels made of sound absorbing materials are put on the walls and ceilings of hall and auditorium.
ii) Carpets are put on the floor to absorb sound and reduce reverberations
iii) Heavy curtains are put on doors and windows to absorb sound and reduce reverberations
iv) The seats in the hall are made from materials having sound absorbing properties
Solution 42
We hear more clearly in a room with curtains than in a room without curtains because curtains are bad reflectors of sound. They absorb most of the sound falling on them, and hence do not produce echoes. On the other hand, in rooms without curtains, there is a greater reflection of sound due to which some echoes are produced.
These echoes cause a hindrance to hearing.
Solution 43
A megaphone is a large, cone-shaped (or funnel-shaped) device for amplifying and directing the voice of a person who speaks into it. A megaphone works on the principle of multiple reflections of sound.
Solution 44
A bulb horn is a cone shaped wind instrument which used for signaling in bicycles, cars, buses, trucks and boats, etc. A bulb horn works on the principle of multiple
reflections of sound.
Solution 45
Stethoscope is a medical instrument used by the doctors for listening to the sounds produced within the human body, mainly in the heart and lungs. It works on the
principle of multiple reflections of sound.

Solution 46 

The soundboard is a concave board (curved board) which is placed behind the speaker in large halls or auditoriums so that his speech can be easily heard even by the persons sitting at a considerable distance. The sound board works as follows: the speaker is made to stand at the focus of the concave soundboard. The concave surface of the soundboard reflects the sound waves of the speaker towards the audience (and hence prevents the spreading of sound in various directions). Due to this, sound is distributed uniformly throughout the hall and even the persons sitting at the back of the hall can hear the speech easily.
Solution 47
a) The loudness of sound is a measure of the sound energy reaching the ear per second. It depends on the amplitude of the sound waves. 

b)

Solution 48
(a)Pitch is that characteristic of the sound by which we can distinguish between different sounds of same loudness. It depends on the frequency of the sound waves
(b) 

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Page No:208 

Solution 49
Quality (or timbre) is that characteristic of the sound by which we can distinguish between sounds of same pitch and loudness produced by different musical instruments (and different singers). The quality (or timbre) depends on the shape of the sound wave (or waveform) produced by it.
Solution 50
When we strike a table lightly, then due to less energy supplied, the table top vibrates with a small amplitude and hence a soft sound is produced. However if we hit the table hard, due to greater energy supplied, the table top vibrates with a large amplitude and hence a loud sound is produced.
Solution 51
Ultrasound is used in industry for detecting flaws in metal blocks without damaging them. In hospitals, ultrasound is used to investigate the internal organs of the human body such as liver, kidneys, uterus, etc.
Solution 52
Bats are able to fly at night without colliding with other objects because they emit high frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from the objects or obstacles in their path. From the time taken by the echo to be heard, bats can judge the distance of the object in their path and avoid it by changing the direction.
Solution 53
Bats emit high frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from their prey. From the time taken by the echo to be heard, bats can judge the distance of the prey in their path and catch it.
Solution 54
Ultrasound waves are made to pass through one face of the metal block and ultrasound detectors are placed on the opposite face of the block to detect the transmitted ultrasound waves.
(i) If the ultrasound waves pass uninterrupted through all parts of the metal block, then the block is flawless.
(ii) However, if the ultrasound waves are not able to pass through a part of the metal block and get reflected back, then there is a flaw in the metal block.
Solution 55 

The ceilings of concert halls are made curved so that sound, after reflection from the ceiling, reaches all part of the hall.
Solution 56 

Solution 57
(a) Infrasounds include sounds of frequencies below 20 Hz.
(b) Audible sounds include sounds of frequencies between 20 Hz and 20,000 Hz
(c) Ultrasounds include sounds of frequencies above 20,000 Hz.
Solution 58
(b) Infrasonic waves: 10 Hz, 18 Hz
Ultrasonic waves: 30,000 Hz ,50,000 Hz 

(a)

(b) Infrasonic waves : 10 Hz, 18 Hz

Ultrasonic waves : 30,000 Hz, 50,000 Hz

Solution 59
(a) Range of hearing in humans is 20 Hz to 20,000 Hz
(b) The sound frequencies that cannot be heard by a human ear are 10 Hz, 15 Hz and 40,000 Hz.
Solution 60
Time taken to listen the echo = 5 s
So time taken for 

time taken to listen he echo = 5 s

so time taken for sound to reach the reflecting surface, t = 5/2 s

Speed of sound in air v= 342 m/s

Distance of reflecting surface = v x t = 

Solution 61
We can hear original sound and reflected sound separately only if there is a time interval of at least 0.1 sec between them.
So, time taken to listen to echo = 0.1 s
Time taken for sound to reach the reflecting surface, 

Speed of sound in water v = 1500 m/s

Distance of reflecting surface =

Solution 62
(a) The bouncing back of sound when it strikes a hard surface is called reflection of sound.Hard, solid surfaces are the best for reflecting sound waves.
(b) Metal sheet, hard wood are good reflectors of sound.
(c) The laws of reflection of sound are:
1. The incident sound wave, the reflected sound wave, and the normal at the point of incidence, all lie in the same plane.
2. The angle of reflection of sound is always equal to the angle of incidence of sound.
Solution 63
(a) The repetition of sound caused by the reflection of sound waves is called echo. An echo is produced when sound is reflected from a hard surface such as a tall brick wall or a cliff.
(b) The minimum distance in air required from a sound reflecting surface to hear an echo (at 20^oC) is 17.2 metrers
(c) Distance s = 825 m
speed of sound v = 330 m/s
Time taken for sound to reach the reflecting surface, t = 

So, time taken to hear the echo = 2 x 2.5 = 5s

Solution 64
(a) Ultrasounds are the sounds having very high frequency which cannot be heard by human beings. 

(b)Applications of ultrasound are:
1. Ultrasound is used in industry for detecting flaws in metal blocks without damaging them
2. In hospitals, ultrasounds are used to investigate the internal organs of the human body such as liver, kidneys, uterus, etc
3. Ultrasounds are also used to monitor the growth of fetus inside the mother's uterus. 

Solution 65
(a)Infrasonic waves include sounds of frequencies below 20 Hz. These sounds are produced by objects vibrating very slowly. Whales and elephants can produce these sounds.
(b)Ultrasonic waves include sounds of frequencies above 20,000 Hz. These sounds are produced by objects vibrating very rapidly. Bats and dolphins can produce these sounds.
(c)Speed of sound v = 344 m/s
Lower frequency f_l= 20 Hz
Higher frequency f_h = 20,000Hz 

Solution 66
(a)
(a) Echolocation is the method used by some animals to locate the objects by hearing the echoes of their ultrasonic squeaks.
(b) Echocardiography is the use of ultrasound waves to investigate the action of the heart.
(c) Ultrasonography is the technique of obtaining pictures of internal organs of the body by using echoes of ultrasound pulses.
(b) Bat navigates and finds its food by echolocation.
(c) Porpoise produces ultrasonic waves.
Solution 67
(a) SONAR stands for SOund Navigation And Ranging. A sonar is an apparatus (or device) which is used to find the depth of a sea or to locate the underwater things like shoals of fish, shipwrecks and enemy submarines.
(b) Time taken to listen to the return signal = 3 s 

So time taken to reach the object = 3/2 = 1.5 s

Speed of sound in water = 1440 m/s

Distance of object = 1440 x 1.5 = 2160 m

Solution 68 

Construction of human ear:
The ear consists of three compartments: outer ear, middle ear and inner ear. The outer ear consists of broad part called pinna and about 2 to 3 centimeters long passage called ear canal. At the end of ear canal is a thin, elastic, circular membrane called tympanum or ear-drum. The middle ear contains three small delicate bones called hammer, anvil and stirrup. These bones are linked to one another. The one end of hammer is touching the ear drum and its other end is connected to the second bone called anvil. The other end of anvil is connected to the third bone called stirrup. And the free end of stirrup is held against the membrane over the oval window of the inner ear. The lower part of middle ear has Eustachian tube going to the throat. The inner ear has a coiled structure called cochlea. The cochlea is filled with liquid containing sound sensitive nerve cells. The other side of cochlea is connected to the auditory nerve which goes to the brain.
Working of human ear:
The sound waves are collected by the pinna. These sound waves pass through ear canal and fall on the ear-drum. Sound waves consist of compressions and rarefactions. When the compression strikes the ear drum, the pressure on the outside of ear drum increases and pushes the ear drum inwards. And when rarefaction strikes the ear drum, the pressure on the outside of ear drum decreases and it moves outwards. Thus, when sound waves fall on the ear drum, it vibrates back and forth rapidly. These vibrations are passed onto the three bones in the middle ear and finally to the liquid in the cochlea. Due to this, liquid in the cochlea starts to vibrate, setting up electrical impulses in the nerve cells present in it. These impulses are carried to the brain by auditory nerve. The brain interprets the impulses and we get the sensation of hearing. 

Page No:209 

Solution 85
(a) Echo
(b) Sound gets reflected back
(c) Incident sound travels distance = 800 m
Sound after reflection travels = 800 m
Total distance travelled by sound = 800 + 800 = 1600 m
(d) Distance of cliff, d = 800 m
Time taken to listen to the echo = 5 s 

So time taken by sound to reach the cliff, t = 5/2 = 2.5 s

Speed of sound = d/t= 800/2.5 = 320 m/s

Page No:210

Solution 86
(a) A and D
(b) B and D
(c) Same vibrating body produced all the sound wvaes
(d) Tuning forks
Solution 87
(a) The air in-between Anhad and the loudspeaker vibrates with the frequency of 200 Hz
(b) Anhad receives sound in the right ear by the sound waves coming directly from the loudspeaker and in his left ear he receives sound from sound waves reflected from the classroom wall.
Solution 88 

Solution 89
a)
(i) X is ultrasonic sound
(ii) Y is infrasonic sound
(iii) Z is audible sound
(b) Ultrasound machine in hospitals
(c) Simple pendulum
(d) Loudspeaker
(e) 20 Hz to 20000 Hz
Solution 90
(a) The person would hear a lot of noise of heavy traffic in a city.
(b) The person would hear very little noise of traffic in a village.
(c) The person would hear echoes of persons talking in a bare room.
(d) The person will find furnished room less echoic.