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Chapter 12 Sound Science Worksheet for Class 9
Class 9 Science students should refer to the following printable worksheet in Pdf in Class 9. This test paper with questions and solutions for Class 9 Science will be very useful for tests and exams and help you to score better marks
Class 9 Science Chapter 12 Sound Worksheet Pdf
Question. The echo-receiver of a sonar attached to a ship, receives the echo from the bottom of sea 4 seconds after the ultrasonic waves were sent into the sea.
If the speed of sound in water is 1500 m/s, then what is the depth of the sea?
(a) 6000 m
(b) 3000 m
(c) 15000 m
(d) 3500 m
Answer : B
Question. We can easily distinguish between the sounds of the different singers even without having seen them. This is all due the different quality or timber of their sounds. On what factor does the quality of sound depend?
(a) Wavelength
(b) Waveform
(c) Wave frequency
(d) Wave amplitude
Answer : B
Question. Which of the following can produce longitudinal as well as transverse waves under different conditions?
(a) Bats
(b) Slinky
(c) Tuning fork
(d) None of these
Answer : B
Question. On increasing the temperature, the speed of sound in air:
(a) Increases
(b) Decreases
(c) Does not change
(c) First increases then becomes constant
Answer : A
Question. The sound can travel in air when:
(a) Particles of medium travel from one place to another
(b) There is no moisture in the atmosphere.
(c) Disturbance travel from one place to another
(d) Both particles as well as disturbance travel from one place to another
Answer : C
Question. If the speed of a wave is 380 m/s and its frequency is 1900 Hz, then the wavelength of the wave will be:
(a) 20 m
(b) 0.2 m
(c) 200 m
(d) 2 m
Answer : B
Question. Before the main shock waves, the earthquake produces the characteristic sound waves which some animals like rhinoceros can hear. Can you guess the kind of sound waves produced here?
(a) Infrasonic sounds
(b) Ultrasonic sounds
(c) Audible Sounds
(d) None of these
Answer : A
Question. We can distinguish between a man’s voice and a woman’s voice of the same loudness even without seeing them. This is due to a characteristic of sound which measures the shrillness of sounds. Can you choose the correct unit of the quantity on which this characteristic of sound depends?
(a) hertz
(b) meter/second
(c) meter
(d) unitless
Answer : A
Question. You must have seen the doctor checking the sounds produced in the chest or heartbeat of a patient using a medical instrument called Stethoscope. On what principle does this Stethoscope work?
(a) Multiple refractions
(b) Multiple reflections
(c) Wave motion
(d) None of these
Answer : B
Question. Given below is the diagram of human ear. Choose the correct labelling of the parts A, B and C.
(a) A-ear lobe, B-ear canal, C-auditory nerve
(b) A-ear drum, B-auditory nerve, c-ear canal
(c) A-ear canal, B-Eustachian tube, C-auditory nerve
(d) A-ear canal, B-Eustachian tube, C-cochlea
Answer : C
Question. The bats can fly in the darkness of night without colliding with the other objects by emitting special sounds while flying. Which characteristic of sound is used by the bats to navigate?
(a) Ultrasound
(b) Infrasound
(c) Audible sound
(d) None of these
Answer : A
Question. Loudness measures the sound energy reaching the ear per second and depends on the amplitude of the sound wave. What is the unit used to measure the loudness of sound?
(a) hertz
(b) decibel
(c) meter/second
(d) second
Answer : B
Question. Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the string suitably. By doing so, he is adjusting
i. Amplitude of the sound
ii. Frequency of the sitar string with the frequency of other musical instruments
iii. Intensity of sound
iii. Loudness of sound
Choose the correct option:
(a) (i) and (iii)
(b) only (ii)
(c) (ii) and (iv)
(d) only (iv)
Answer : B
Question. A key of mechanical piano is first struck gently and then struck again but much harder this time. What kind of change in sound will you observe in the second case:
(a) Sound will be louder but the pitch will not be different
(b) Sound will be louder and the pitch will also be higher
(c) Sound will be louder but the pitch will be lower
(d) Both loudness and pitch will remain unaffected
Answer : C
Question. The sound travels from particle to particle in the form of vibrations and has different speeds in different mediums.
The maximum speed of vibrations which produce audible sound will be in:
(a) Sea water
(b) Ground glass
(c) Human blood
(d) Dry air
Answer : B
Very Short Answer Type Questions :
Question. What is the frequency of wave with time period 0.025 s?
Answer : Frequency (f) = 1/Time period
= 1/0.025
= 40Hz
Therefore, frequency of the wave = 40 Hz.
Question. What is a wave number?
Answer : The number of wavelengths present per unit length is called wave number. i.e. it is the reciprocal of wavelength = 1/λ.
Question. What type of waves can travel in vacuum? Give example(s).
Answer : Electromagnetic waves can travel in vacuum. Sun light, X-rays are examples of electromagnetic waves.
Question. A baby recognizes her mother by her voice. Name the characteristic of sound involved
Answer : The characteristic of sound involved in uniqueness of the sound is quality of sound or timber.
Question. What is reverberation?
Answer : Reverberation is defined as persistence of sound after the source has stopped emitting sound. This is due to multiple reflections of sound waves.
Question. Give examples of organisms which can hear infrasonic?
Answer : Whales and elephants can produce and hear infrasonic sound.
Question. What is SONAR? For what it is used?
Answer : SONAR is Sound Navigation And Ranging. It is a technique used to measure the depth of the sea, locate the sunken ships or icebergs and submarines.
Question. A vibrating body produces sound. However, no sound is heard when a simple pendulum oscillates in air why?
Answer : Sound is produced only when the frequency of the wave is greater than 20 Hz. As a simple pendulum produces waves less than 20Hz they cannot be heard.
Question. Name two animals which can produce infrasonic waves.
Answer : Hippopotamus and whale
Question. What are infrasonic and ultrasonic sounds?
Answer : Infrasonic sound has frequency less than 20 Hz and ultrasonic sound has frequency higher than 20 kHz.
Question. Are Sound waves mechanical waves?
Answer : Sound waves are mechanical waves as they need material medium for propagation which is the characteristic of the mechanical waves.
Question. What is infrasonic? Give an example.
Answer : Sound having frequency less than 20Hz is known as infrasonic sound or infrasonic. Waves produced during earthquake are infrasonic.
Question. Why do we hear sound of an approaching truck before the truck reaches us?
Answer : This is because velocity of sound is much greater than the velocity of truck
Question. Why Sometimes we hear echo of sound.
Answer : We sometimes hear the echo of a sound produced because the distance between the source of the sound and the obstacle is at least 17.2
Short Answer Type Questions :
Question. An echo is returned in 6 seconds. What is the distance of reflecting surface? from source? [Given that speed of sound is 342 m/s.]
Answer : Given. :- Time in which echo returned, t = 6 s, Speed of sound, v = 342 m/s Distance = Speed × Time = 342 × 6 = 2052m As this distance is twice the distance of reflecting surface from source. So, The distance of reflecting surface from source = 2052 / 2 = 1026 m.
Question. (i) Define the time period of a wave. (ii) Give the relation among speed of sound v, wavelength λ and its frequency f.
Answer : (i) Time period (T)
It is defined as the time required to complete one wave.
(ii) Speed of sound (v) = Wavelength ( λ ) × Frequency ( f )
Question. What is echo?
Answer : The sound waves produced bounce back or gets reflected from the mountain or buildings and come to our ears, this reflected sound is known as Echo. To hear echo, the barrier reflecting the sound should be least at a distance of 17 meters.
Question. A body is vibrating 12000 times in one minute. If the velocity of sound in air is 360 m/s, find:
(a) Frequency of vibration in hertz,
(b) Wavelength of the wave produced.
Answer : (a) Frequency of vibration in hertz
Given,
Number of vibration in one minute = 12000
Number of vibrations in one sec =12000/60
= 200Hz
Therefore, Frequency, f = 200 Hz
(b) Wavelength of the wave produced
Given,
Velocity of speed in air, v = 360 m/s
Frequency, f = 200 Hz
v = f λ
or λ=v/f
=360 /200 = 1.8m
Question. Why Ceiling of good conference halls and concert halls are curved?
Answer : Ceiling of good conference halls and concert halls are curved so that the sound waves after reflecting from these walls reaches every part of the hall and can be easily heard by the listeners.
Question. 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 ms-1?
Answer : Speed of sound (v) = 342 ms-1
Echo returns in time (t) = 3 s
Distance travelled by sound = v × t = 342 × 3 = 1026 m
In the given interval of time, sound must travel a distance which is twice the distance of reflecting surface and source.
Therefore, the distance of reflecting surface from the source =1026/2 = 513 m.
Question. Why are the ceilings of concert halls curved?
Answer : Ceilings of concert halls are curved to uniformly spread sound in all directions after reflecting from the walls.
Question. What is the audible range of the average human ear?
Answer : 20 Hz to 20,000 Hz. Any sound less than 20 Hz or greater than 20,000 Hz frequency is not audible to human ears
Question. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?
Answer :
(a). 20 Hz
(b). 20,000 Hz.
Question. 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−1.
Answer : For sound waves, Speed = Wavelength × frequency
v = λ × v Speed of sound wave in air = 344 m/s
(a) For v = 20 Hz
λ1 = v/v1 = 344/20 = 17.2 m
(b) For v2 = 20,000 Hz
λ2 = v/v2 = 344/20,000 = 0.0172 m
Therefore, for human beings the hearing wavelength is in the range of 0.0172 m to 17.2 m.
Question. 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 the air and in aluminium to reach the second child.
Answer : Consider the length of aluminium rod = d
Speed of sound wave at 25° C, V Al = 6420 ms-1
Time taken to reach other end T Al = d/ (V Al) = d/6420
Speed of sound in air, V air = 346 ms-1
Time taken by sound to each other end,
T air = d/ (V air) = d/346 Therefore, the ratio of time taken by sound in aluminium and air,
Question. What are the laws of reflection of sound ?
Answer : The laws of reflection of sound are:-
1. The angle of reflection is always equal to the angle of incidence .
2. The reflected sound, the incident sound, and the normal sound belong in the same plane.
Question. Define the following terms :- a. Supersonic speed. B. Sonic boom.
Answer : Objects moving with speed greater than speed of sound is said to have supersonic speed.
The air pressure variation associated with shock waves produces a very sharp and loud sound called “sonic boom”.
Question. What is loudness of sound? What factors does it depend on?
Answer: Loudness refers to how loud or soft a sound seems to a listener. Loudness of sound is determined of amplitude. The unit of intensity is the decibel (dB).
Question. Which characteristics of the sound help you to identify your friend by his voice while sitting with others in a dark room?
Answer: The quality (or timbre), pitch and loudness of sound.
Question. Explain how defects in a metal block can be detected using ultrasound.
Answer: Ultrasounds can be used to detect cracks and flaws in metal blocks. Ultrasonic waves are allowed to pass through the metallic 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.
Question. 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 .
Answer: Height of tower = S = 500 m
Acceleration due to gravity = g = 10 m/s2
Initial speed of stone = u = 0
By equation of motion, S = ut + 1/2 at2
500 = 0 x t + 1/2 x 10 x t2
t2 = 500/5 = 100
Hence, t = 10 sec
Now time taken by sound to reach to the top of tower = T = Distance/Speed of sound = 500/340 = 1.47 sec
Question. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Answer: Time = t = 5 sec
Distance of object from submarine = d = 3625 m
Using, d = v x t/2
Speed of sound in water = v = 2 x d/t = 2 x 3625/5 = 1450 m/s
Question. Explain how the human ear works.
Answer: Various sounds produced by particles in our surroundings are collected by pinna that transfers these sounds to the ear drum through the ear canal. The eardrum begins to vibrate back and forth briskly as soon as the sound waves fall on it. The vibrating Eardrum initiates the small bone hammer to vibrate. These vibrations are passed from the hammer to the third bone stirrup via the second bone anvil. The stirrup strikes the membrane of the oval window to pass its vibration to the cochlea. The liquid in the cochlea produces electrical impulses in the nerve cells. These electrical impulses are carried to the brain by the auditory nerve. They are interpreted by the brain as sound and hence we get a sensation of hearing.
Question. What is reverberation? How can it be reduced?
Answer: The continuous multiple reflections of sound in a big enclosed space is reverberation. It can be reduced by covering walls and ceiling of enclosed space with the help of sound absorbing materials such as loose woolens, fibre boards.
Question. Explain how bats use ultrasound to catch prey.
Answer: The ultrasonic waves emitted by the bat are reflected from the prey (e.g., an insect) and are detected by its ears. The nature of reflected waves tells the bat about the location and the nature of its prey.
Question. How is ultrasound used for cleaning?
Answer: Objects that need to be cleansed are put in a cleaning solution and ultrasonic sound waves are passed through the solution. The high frequency of ultrasound waves helps in detaching the dirt from the objects.
Question. A sound wave travels at a speed of 339 m/s. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
Answer: Speed of sound = v = 339 m/s
Wavelength = λ = 1.5 cm = 1.5/100 = 0.015 m
Frequency = v/λ = 339/0.015 = 22600 Hz
The sound is not audible as its frequency lies beyond the audible range (20 Hz to 20000 Hz).
Question. Explain the working and application of a SONAR.
Answer: SONAR;(-Sound Navigation and Ranging).It consists of a transmitter and a detector and is installed in a boat or a ship as shown in figure:
The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the sea bed, get reflected back and are sensed by the detector. SONAR calculates the distance (d) of the object by measuring time taken for transmission and reception of signal (t) by formula:
d = v x t/2 : Where v is speed of sound in sea water.
The SONAR technique is used to determine the depth of the sea and to locate the underwater hills, valleys, submarines, icebergs and sunken ships etc.
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