Sound class 9 physics MCQ & SAQ

Sound class 9 physics NCERT SOLUTION

1. How does the sound produced by a vibrating object in a medium reach your ear ?

OR

Explain how sound is produced by your school bell.

Ans. Air is the commonest material through which sound propagates. When a vibrating object, like the prongs of a tuning fork, moves forward, they push the molecules of the air in front of them. This in turn compresses the air, thus creating a region of high pressure and high density called compression. This compression in the air travels forward. When the prongs of the tuning fork move backward, they create a region of low pressure in the air, commonly called rarefaction. This region has low pressure, low density and more volume. As the tuning fork continues to vibrate, the regions of compression in the air alternate with the regions of rarefaction. These regions alternate at the same place. The energy of the vibrating tuning fork travels outward. This energy which reaches the ears, makes the eardrums to vibrate and thus we hear sound.

 

2. 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.

Ans. Speed of sound =

 

3. Why are sound waves called mechanical waves ?

Ans. Some mechanical energy is required to make an object vibrate. Sound energy cannot be produced on its own. The mechanical energy of a vibrating object travels through a medium and finally reaches the ear. Therefore, the sound waves are called mechanical waves.

 

4. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Ans. No, I will not be able to hear sound, because the Moon has no atmosphere. Therefore, no sound waves can travel to your ears and therefore, no sound is heard.

 

5. Which wave property determines

(a) loudness, (b) pitch ?

Ans. (a) The amplitude of the wave determines the loudness; more the amplitude of a wave, more is-the loudness produced.

(b) The pitch is determined by the frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound.

 

6. Guess which sound has a higher pitch; guitar or car horn ?

Ans. Car horn has a higher pitch than a guitar, because the sound produced by the former is shriller than the later.

 

7. What are wavelength, frequency, time period and amplitude of a sound wave ?

Ans. Wavelength : It is the linear distance between two consecutive compressions or two consecutive rarefactions.

Frequency : The number of compressions or rarefactions taken together passing through a point in-one second is called frequency.

Time Period : It is the time taken by two consecutive compressions or rarefactions to cross a point.

Amplitude : It is the magnitude of maximum displacement of a vibrating particle about its mean position.

 

8. How are the wavelength and frequency of a sound wave related to its speed ?

Ans. Speed of sound = Frequency × Wavelength

 

9. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 ms1 in a given medium.

Ans. Frequency = 220 Hz

Speed of sound 440 ms1

We know speed of sound = Frequency × wavelength

Þ Wavelength = = 2m

 

10. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source ?

Ans. Time interval =

 

11. Distinguish between loudness and intensity of sound.

Ans. The loudness depends on energy per unit area of the wave and on the response of the ear but intensity depends only on the energy per unit area of the wave and is independent of the response of the ear.

 

12. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Ans. Sound travels fastest in iron as compared to water and air.

 

13. An echo is returned in 3s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 ms1 ?

Ans. Distance of reflecting body from the source of sound

 

14. Why are the ceiling of concert halls curved ?

Ans. The ceilings of concert halls are curved because sound after reflection from it reaches all the corners of the hall and is audible to each person in the hall.

 

15. What is the audible range of the average human ear ?

Ans. An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

 

16. What is the range of frequencies associated with

(a) Infrasound ? (b) Ultrasound ?

Ans. (a) Infrasound : Sound waves between the frequencies 1 to 20-Hz

(b) Ultrasound : Sound waves of the frequencies above 20,000 Hz.

17. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 ms1, how far away is the cliff ?

Ans. Distance of cliff

 

18. What is sound and how is it produced ?

Ans. Sound is mechanical energy which produces the sensation of hearing. When an object is set into vibrations, sound is produced.

 

20. Give an experiment to show that sound needs a material medium for its Propagation.

Ans. Take an electric circuit which consists of a cell, a switch and an electric bell arranged inside a bell jar, which stands on the platform of an evacuating pump.

The switch of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped out of the bell jar. When the air is completely removed from the bell jar, no sound is heard as it is obvious from fig. because the medium of air which has to carry energy from the bell to the bell jar, is removed. It shows that sound needs a material medium for its propagation.

 

21. Why is a sound wave called a longitudinal wave ?

Ans. Sound wave is called a longitudinal wave because the particles of the medium vibrate in the direction of the propagation of the wave.

 

22. Which characteristic of the sound helps you to identify your friend by his voice while sitting with Miens in a dark room?

Ans. The characteristic of sound is quality or timbre.

 

23. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why ?

Ans. The speed of light is 3 × 108 ms1 and the speed of sound is 344 ms1 in air. Thus, a flash of lightning is seen at first, but sound takes a few seconds to reach the ears.

 

24. 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 ms1.

Ans. Wavelength of sound of frequency 20 Hz

wavelength of sound of frequency 20,000 Hz

 

25. 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. Given velocity of sound in air and aluminum are 346 ms1 and 6420 ms1 respectively.

Ans.

Time taken by sound in air Speed of sound in aluminum

Þ

Þ TAIu : TAir = 346 : 6420 = 1 : 18.55

26. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute ?

Ans. No. of vibrations produced in 1 s = 100 Þ No. of vibration produced in 60 (sec)

(1 min) = 100 × 60 = 6000

 

27. Does sound follow the same laws of reflection as light does ? Explain.

Ans. Yes, sound and light follow the same laws of reflection given below :

(a) Angle of incidence at the point of incidence = Angle of reflection.

(b) At the point of incidence the incident sound wave, the normal and the reflected sound wave lie in the

same plane.

 

28. 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. If the temperature rises the speed of sound will increase. This in turn will increase the minimum distance required for hearing an echo. No echo is heard because the distance between the source of sound and the reflecting body does not increase.

 

29. Give two practical applications of reflection of sound waves.

Ans. (i) Megaphones are designed to send sound waves in a particular direction based on the reflection of sound. (ii) In a stethoscope the sound of a patient's heartbeat reaches the doctor's ears by multiple reflections in the tubes.

 

30. 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 ms2 and speed of sound = 340 ms1.

Ans. For the downward journey of stone.

Initial velocity (u) = 0

Distance i.e., (height) of tower (S) = 500 m

Time of fall (t1) = ?

Acceleration due to gravity (g) = 10 ms2

We know ; S = ut gt2

500 = 0 × t1   × 10 × t12

Þ 500 = 5t12

Þ t12 = 100

t1 = 10 s

For the sound traveling upward

Time taken (t2)

For the sound traveling upward

Time taken (t2) =

Time required to hear splash = t1  t2 = 10 1.47 = 11.47 s

 

31. A sound wave travels at a speed of 339 ms1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible ?

Ans. Speed of sound wave = 339 ms1

Wavelength of sound wave = 1.5 cm = 0.015 m

Frequency of sound wave =

The sound will not be audible, because human beings can hear only upto 20,000 Hz.

 

32. What is reverberation? How can it be reduced ?

Ans. Reverberation is the repeated multiple reflections of sound in any big enclosed space. It can be reduced by covering the ceiling and walls of the enclosed space with some absorbing materials like fiber board, loose woolens etc.

 

33. What is the loudness of sound ? What factors does it depend on ?

Ans. Loudness of sound is the effect produced in the brain by the sound of different frequencies. The loudness of the sound depends on the distance of the observer from the source of sound ; lesser the distance louder the sound. It increases with the increase in amplitude and the area of the vibrating body.

 

34. Explain how bats use ultrasound to catch their prey.

Ans. The bats produce high pitched ultrasonic waves which are not heard by human beings. The ultrasonic waves on striking the insect send back an echo, which is heard by the bat. As the echo is heard by the bat it hooves on the insect and catches it.

 

35. How is ultrasound used for cleaning ?

Ans. The object to be cleaned is put in a tank fitted with an ultrasonic vibrator. The tank is filled with water containing detergent. As the ultrasonic vibrator is switched on the detergent rubs against the object at a very high speed and hence cleans it.

 

36. Explain the working and application of a sonar.

Ans. SONAR is a device for determining water depth and locating underwater objects like reefs, submarines and schools of fish.

On striking the bottom of the ocean, the ultrasonic wave is reflected upward toward the ship. This wave is received by a suitable receiver. The time of travel from the source of sound to the receiver is noted. We can calculate the depth of ocean floor if the velocity of sound in water is known

Depth of ocean floor =

For Example : If it takes 2.4 s to record echo by the sonar

Velocity of sound in sea water = 1450 ms1

Depth of ocean floor =

Sound class 9 physics MCQ

Question: If the wave is propagating having the number of waves shown in the figure per second ,then the frequency of the wave is

• a) 5Hz

• b) 6Hz

• c) 3Hz

• d) 10Hz

Answer: 5Hz

 

Question: The characteristic of sound which enables us to distinguish one sound from another having the same pitch and loudness

• a) Timber

• b) Amplitude

• c) Shrillness

• d) Intensity

Answer: Timber

 

Question: Sound waves with frequency less than 20 Hz and more than 20000 Hz is called

• a) Infrasonic and ultrasonic

• b) Sonic boom and supersonic

• c) Infrasonic and sonic boom

• d) Supersonic and ultrasonic

Answer: Infrasonic and ultrasonic

 

Question: The product of time-period and frequency is

• a) Unity

• b) Zero

• c) Infinity

• d) None of these

Answer: Unity

 

Question: Sound waves are

• a) Longitudinal

• b) Partly longitudinal, partly transverse

• c) Transverse

• d) Sometimes longitudinal, sometimes transverse

Answer: Longitudinal

 

Question: The time period of a simple pendulum in a spacecraft orbiting the earth is

• a) Infinity

• b) Zero

• c) One second

• d) Two seconds

Answer: Infinity

 

Question: Speed of sound in vacuum is

• a) 340 m/s

• b) 600m/s

• c) 380m/s

• d) 300m/s

Answer: 340 m/s

 

Question: The sound which is produced due to a mixture of several frequencies is called

• a) A note

• b) A tone

• c) Frequency

• d) Amplitude

Answer: A note

 

Question: Pitch of the wave is measured in terms of

• a) Frequency of the wave

• b) Amplitude of the wave

• c) Speed of the wave

• d) Time period of the wave

Answer: Frequency of the wave

 

Question: The distance between any two compressions or rarefactions in a longitudinal wave is

• a) One wavelength

• b) Half wavelength

• c) Twice the wavelength

• d) One fourth wavelength

Answer: One wavelength

Question: Sound waves are

• a) Longitudinal in n nature

• b) Transverse in nature

• c) Electromagnetic in nature

• d) Magnetic in nature

Answer: Longitudinal in n nature

 

Question: Compression is a region of

• a) High pressure

• b) Low pressure

• c) Atmospheric pressure

• d) No pressure

Answer: High pressure

 

Question: For its propagation sound requires

• a) All of the above

• b) Air

• c) Water

• d) Iron

Answer: All of the above

 

Question: SI unit of frequency is

• a) Hertz

• b) Meter

• c) M/s

• d) M/s2

Answer: Hertz

 

Question: In gasses a sound wave is

• a) Both Transverse and Longitudinal

• b) Transverse only

• c) Longitudinal only

• d) Neither Transverse Nor Longitudinal

Answer: Both Transverse and Longitudinal

 

Question. A wave in slinky traveled two and fro in 5 sec the length of the slinky is 5m. What is the velocity of a wave?

(a) 10m.s

(b) 5m/s

(c) 2m/s

(d) 25m/s

 

Answer : C

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. 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. Speed of sound depends upon

(a) Temperature of the medium

(b) Pressure of the medium

(c) Temperature of source producing sound

(d) Temperature and pressure of medium

 

Answer :  A

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. You must have seen the doctor checking the sounds produced in the chest or heartbeat of a patient using a medical instrument called a Stethoscope. On what principle does this Stethoscope work?

(a) Multiple refractions

(b) Multiple reflections

(c) Wave motion

(d) None of these

Answer : B

Question. What do dolphins, bats and porpoise use

(a) Ultrasound

(b) Infrasound

(c) Both a and b

(d) None of them

 

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 a Stethoscope. On what principle does this Stethoscope work?

(a) Multiple refractions

(b) Multiple reflections

(c) Wave motion

(d) None of these

Answer : B

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. 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. On increasing the temperature, the speed of sound in air:

(a) Increases

(b) Decreases

(c) Does not change

(d) First increases then becomes constant

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 . 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 . 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. 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. 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. 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. A mechanical piano key 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. 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

1. Sound is produced by _________objects.

(a) fast moving

(b) vibrating

(c) stationary

(d) rotating

► (b) vibrating

2. A mechanical piano key 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

► (c) Sound will be louder but the pitch will be lower

3. In case of transverse waves the particles of a medium vibrate:

(a) In the direction of wave propagation

(b) Opposite to the direction of wave propagation

(c) At the right angles to the direction of wave propagation

(d) None of the above

► (c) At the right angles to the direction of wave propagation

4. The upper frequency limit of the audible range of human hearing is about

(a) 20 kHz

(b) 2000 Hz

(c) 2 kHz

(d) 2,000,000 Hz

► (a) 20 kHz

5. The distance between a consecutive crest and trough is x. The wavelength of the wave is

(a) 4x  

(b) x

(c) x/2

(d) 2x

► (d) 2x

6. Sound wave is a

(a) transverse wave

(b) longitudinal wave

(c) both a and b

(d) None of these

► (b) longitudinal wave

7. You can not talk to each other on the moon because

(a) atmosphere is not present, that is, there is vacuum

(b) you would not feel like talking

(c) compressions and rarefactions can only travel on earth’s surface

(d) gravity is 1/6th that on the earth

► (a) atmosphere is not present, that is, there is vacuum

8. Sonic booms are caused by the combination of

(a) supersonic speed and pressure variation.

(b) infrasonic speed and pressure variation.

(c) ultrasonic sound and pressure variation.

(d) pressure variation only.

► (a) supersonic speed and pressure variation.

9. Frequency of infrasonic waves is ____________ than the frequency of ultrasound waves.

(a) less

(b) more

(c) almost same

(d) none of these.

► (a) less

10. Sound travels

(a) slower in warm air than in cold air

(b) faster in solids and liquids than in air

(c) slower in water than air

(d) none of these

► (b) faster in solids and liquids than in air

11. In longitudinal waves, how does the particle of the medium vibrate compared to the direction of propagation of the wave?

(a) Perpendicular

(b) Parallel

(c) At 45°

(d) At 60°

► (b) Parallel

12. The sound waves in a medium are characterized by the:

(a) Linear motion of particles in the medium

(b) Rotatory motion of particles in the medium

(c) Oscillatory motion of particles in the medium

(d) None of the above

► (c) Oscillatory motion of particles in the medium

13. Sound can travel through

(a) gasses only

(b) vacuum only

(c) gasses and liquids only

(d) solids, liquids and gasses.

► (d) solids, liquids and gasses.

14. The motion of the particles of a medium when a sound wave is passing through it is:

(a) translatory

(b) random

(c) rotatory

(d) oscillatory

► (d) oscillatory

15. The transfer of energy in a material medium due to the periodic motion of its particles is called:

(a) Wave front

(b) Wave motion

(c) Pulse

(d) None of the above

► (b) Wave motion

16. A wave moves a distance of 10m in 0.05 second. Find the wavelength of the wave if the frequency is 200Hz?

(a) 0.8M

(b) 1m

(c) 0.5m

(d) 0./m

► (b) 1m

17. The maximum displacement of the particle of the mean from its mean position is called the

(a) Amplitude

(b) wavelength

(c) wave number

(d) none of these

► (a) Amplitude

18. Which of the following is a mechanical wave?

(a) Sound

(b) Light

(c) Radio waves

(d) X-rays

► (a) Sound

19. Wave motion transfers

(a) momentum

(b) velocity

(c) energy

(d) mass

► (c) energy

20. The longitudinal waves can propagate only in :

(a) Solids

(b) Liquids

(c) Gasses

(d) All the above

► (d) All the above

21. The sound waves which travel in the air are called:

(a) Transverse waves

(b) Longitudinal waves

(c) Polarized waves

(d) None of the above

► (b) Longitudinal waves

22. Infrasonic waves have frequency

(a) below 20 Hz

(b) between 20 Hz and 20,000 Hz 

(c) above 20,000 Hz

(d) no upper or lower limit of frequencies

► (a) below 20 Hz

23. In the compression region of the medium in case of longitudinal wave:

(a) The volume monetarily decreases

(b) The density momentarily

(c) The pressure monetarily increases

(d) All the above

► (d) All the above

24. Which wave property determines loudness?

(a) pitch

(b) frequency

(c) amplitude

(d) All of these

► (c) amplitude

25. Which is not the condition for hearing sound?

(a) There must be a vibrating body capable of transferring energy

(b) There must be a material medium to pick up and propagate energy

(c) The medium must have a large density

(d) There must be receiver to receive the energy and interpret it

► (c) The medium must have a large density

26. In case of transverse wave :

(a) The hump on the + y axis is called crest

(b) The hump on the – y axis is called crest

(c) The highest point on the hump on + y axis is called crest

(d) The highest point on the hump on the – y axis is called crest

► (c) The highest point on the hump on + y axis is called crest

27. Sound cannot travel through

(a) air

(b) water

(c) iron

(d) space

► (d) space

28. In the bell jar experiment, as air is removed

(a) intensity falls

(b) speed of sound falls

(c) both (a) and (b)

(d) elasticity falls

► (a) intensity falls

29. A bat hears the echo of its squeak after 0.1 second. How far is the obstacle from the bat? Speed of sound is 344 m/s.

(a) 170m

(b) 17.2m

(c) 180m

(d) 18m

► (b) 17.2m

30. The method of detecting the presence, position and direction of motion of distant objects by reflecting a beam of sound waves is known as _____.

(a) RADAR

(b) SONAR

(c) MIR

(d) CRO

► (b) SONAR

31. Which of the following is not the character of mechanical waves ?

(a) Propagation depends on the elasticity of medium

(b) Can be both transverse or longitudinal

(c) Requires a material medium

(d) Speed is comparatively higher

► (d) Speed is comparatively higher

32. A thunder clap is heard 5.5 seconds after lightning flash. The distance of the flash is (Velocity of sound in air = 330 m/s)

(a) 1750 m

(b) 1815 m

(c) 3000 m

(d) 3500 m

► (b) 1815 m

33. In the region of compression or rarefaction, in a longitudinal wave the physical quantity which does not change is:

(a) Pressure

(b) Mass

(c) Density

(d) Volume

► (b) Mass

34. A part of longitudinal wave in which particles of medium are farther away than the normal particles is called:

(a) Rarefaction

(b) Trough

(c) Compression

(d) Crest

► (a) Rarefaction

Sound class 9 physics SAQ

 

Question.1 What is SONAR? [SAII-2014]

Answer. SONAR (Sound Navigation And Ranging) is a technique for determining water depth and locating underwater objects, such as reefs, submarines, and schools of fish.

Question.2 Define one hertz. [SAII-2013]

Answer. One hertz is one vibration per second.

 Define wavelength. [SAII-2013]

Answer. It is the distance between two nearest points in a wave which are in the same phase of vibration.

What is the audible range of the average human ear? [SAII-2011]

Answer. An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why ? [SAll-2012]

Answer. The speed of light is 3 x 108 ms-1 whereas that of sound is 344 ms-1 in air. Thus, a flash of lightning is seen at once, but sound takes a few seconds to reach our ears.

What is a transverse wave? [SAII-2010]

Answer. It is a wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave. ,

 What is a trough? [SAII-2011]

Answer. A trough is a depression in a wave /.e., maximum displacement in the negative direction (below the mean position).

 What do you understand by the term echo? [SAII-2012]

Answer. The sound heard after reflection from a rigid obstacle is called an echo.

 Can you produce both types of waves (i.e., longitudinal and transverse) on a slinky? [SAII-2010]

Answer. Yes.

. Is the amplitude of a wave the same, as the amplitude of the vibrating body producing the wave? [SAII-2011]

Answer. Yes.

How are moths of certain families able to escape captures from bats ? What is the range of frequencies associated with

(a) infrasound ? (b) ultrasound ? [SAII-2014]

Answer. They have very sensitive hearing equipment, can hear the squeaks (ultrasound) of bats and know when a bat is flying nearby.

(a) Less than 20 Hz. (b) More than 20,000 Hz.

A person fires a gun standing at a distance of 55 m from a wall. If the speed of sound is 330 ms-1, find the time for an echo to be heard. [SAII-2014]

Answer. Given S = 55 m, v = 330 ms-1 t = ?

2d = v x t

or t = 2d/v = 2 x 55/330 = 0.3 s

The pulse rate of a man is 80 beats in one minute. Calculate its frequency. [SAII-2014]

Answer.

No. of beats per minute = 80

No. of beats per second = 80/60 = 1.3

So, frequency = 1.3 Hz

 Which wave property determines: (a) loudness, (b) pitch ? [SAII-2011]

Answer.

(a) The amplitude of the wave determines the loudness. More the amplitude of a wave, more is the loudness produced.

(b) The pitch is determined by the frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound.

 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? [SAII-2012]

Answer. If the temperature rises, the speed of sound will increase. This in turn will increase the minimum distance required for hearing an echo. No echo is heard because the distance between the source of sound and reflecting body does not increase

Explain how bats use ultrasound to catch their prey. [SAII-2011]

Answer. The bats produce high pitched ultrasonic waves which are not heard by human beings. The ultrasonic waves on striking the insect send back an echo, which is heard by the bat. As the echo is heard by the bat it hovers on the insect and catches it.

 How is ultrasound used for cleaning?

Answer. The object to be cleaned is put in a tank fitted with an ultrasonic vibrator. The tank is filled with water containing detergent. As the ultrasonic vibrator is switched on the detergent rubs against the object at a very high speed and hence cleans it.

Explain how defects in a metal block can be detected using ultrasound.

Answer. The ultrasound waves are allowed to pass through metal blocks to which detectors are fitted. If there is a small defect in the metal block like an air bubble or a crack, then the ultrasound waves are reflected from such spots. Metal block if defective is indicated by the reflected ultrasonic waves.

 Explain why echoes can not be heard in a small room. [SAll-2012]

Answer. For hearing echo, there should be at least a distance of 17 m between the source of sound and the body from which’ sound is reflected. In small rooms this is not the case, hence echoes are heard.

 Sound is produced due to a vibratory motion, then why does a vibrating pendulum not produce sound? [SAII-2010]

Answer. The frequency of the vibrating pendulum does not lie within the audible range (20 Hz to 20,000 Hz) and hence it does not produce audible sound.

Prove that: v = fλ, where the symbols have their usual meanings.

Answer. The particle takes time to equal the time period to complete one vibration. Let v be the velocity of the wave, then

Question.17 Two sound waves A and B are shown in the figure. Identify the sound wave hiving :

(i) high frequency

(ii) low frequency.

Answer.

(i) Wave B has high frequency as it repeats itself after smaller intervals of time.

(ii) Wave A has low frequency as it repeats itself after longer intervals of time.

Distinguish between transverse and longitudinal waves. (Three points) [SAII-2014]

Answer.

Transverse waves

(a) Particles the medium vibrate at right angles.

(b) Alt. crests and troughs formed.

(c) e.g., water waves

Longitudinal waves

(a) Particles vibrate parallel to the direction of waves.

(b) Alt. compressions, rarefaction formed.

(c) e.g., sound waves.

State three characteristics of a musical sound. On what factors they depend. [SAII-2014]

Answer.

Characteristics of musical sound are:

•  Loudness-Amplitude affects loudness-more amplitude loud/Less amplitude soft sound.

•  Pitch-frequency affects pitch-more frequency more pitch, Less freq. less pitch.

•  Quality or Timbre.

A stone is dropped into a well 44 m deep and the sound of splash is heard after 3.12 sec. Find the speed of sound in air. [SAII-2014]

Answer.

The sound of which of the following frequencies are audible to human ears : 2 Hz, 5 Hz, 20 Hz, 200 Hz, 2000 Hz.

(b) How [paths of, certain families able to escape capture ? [SAII-2014]

Answer.

(a) 20 Hz, 200 Hz and 2000 Hz.

(b) They do not reflect the ultrasounds sent towards them by bats.

 How does the sound produced by a musical instrument reach your ears? Astronauts need radio transmitters to talk to each other on the moon. Why ? [SAII-2014]

Answer. The sound produced by the musical instrument makes the molecules of air vibrate. These vibrations are carried forward by the other molecules till they reach our ear. These then vibrate our eardrum to produce sound. Since sound requires a medium to propagate, therefore, sound cannot travel between astronauts on the moon, hence they use radio transmitters

.7 What are wavelength, frequency, time period and amplitude of a sound wave? [SAII-2013]

Answer. Wavelength: It is the linear distance between two consecutive compressions or two consecutive rarefactions.

Frequency: The number of compressions or rarefactions taken together passing through a point in one second is called frequency.

Time period : It is the time taken by two consecutive compressions or rarefactions to cross a point.

Amplitude: It is the magnitude of maximum displacement of a vibrating particle about its mean position

Does sound follow the same laws of reflection as light does? Explain. [SAII-2010]

Answer. Yes, sound and light follow the same laws of reflection given below :

(a) Angle of incidence at the point of incidence = Angle of reflection.

(b) At the point of incidence, the incident sound wave, the normal and the reflected sound wave lie in the same plane.

 Explain the working and application of SONAR ?

Answer. SONAR is a device for determining water depth and locating underwater objects like reefs, submarines and schools of fish. To find the depth of an ocean, a strong ultrasonic wave is sent from the ship towards the bottom of the ocean.

On striking the bottom of the ocean, the ultrasonic wave is reflected upward toward the ship. This wave is received by a suitable receiver. The time of travel from the source of sound to the receiver is noted. We can calculate the depth of ocean floor if the velocity of sound in water is known :

Depth of ocean floor = v x t/2

 What are longitudinal waves? Give two examples. [SAII-2011]

Answer. A wave in which the particles of the medium vibrate back and forth along the same direction, in which the wave is moving, is called a longitudinal wave. Examples :

(a) The sound waves in the air.

(b) The waves produced in air when a sitar wire is plucked.

(a) Draw the sound waves for a low pitched and the high pitched sound.

(b) Write one use of ultrasonography.

(c) Which wave property determines pitch? [SAII-2013]

Answer.

(a) The diagram is as shown :

(b) Ultrasonography is used for examination of the fetus during pregnancy to detect congenital defects and growth abnormalities.

(c) Frequency.

State any two characteristics of a wave motion.

Answer.

The characteristics of wave motion are :

(i)It is a periodic disturbance.

(ii)Energy transfer takes place at a constant speed.

A longitudinal wave of wavelength 1 cm travels in air with a speed of 330 

ms

−1

. Calculate the frequency of the wave. Can (his wave be heard by a normal human being?

Answer. Given λ = 1 cm = 0.01m

v = 330 ms-1

Using v = v λ, we have v = v/λ = 330/0.01 = 33000 Hz

No, this cannot be heard by a normal human being.

 How do the frequency and amplitudes affect a musical sound?

Answer. The ‘frequency’ of a musical sound affects its ‘pitch’. The more the frequency of a (musical) sound, the ‘sharper’, and ‘shriller’ the sound becomes.

The ‘amplitude’ of a musical sound affects its loudness, or intensity. The more the amplitude of the sound, the louder, (or more intense) the sound is.

Give one example each of natural vibration, forced vibration and resonance.

Answer.

(i) Natural vibration : The vibrations of a simple pendulum about its mean position.

(ii) Forced vibration : A sonometer wire, under tension, vibrating under the influence of a vibrating tuning fork.

(iii) Resonance: A correctly adjusted length of a sonometer wire under proper tension, vibrating under the influence of a vibrating tuning fork.

How does a stretched string on being set into vibration, produce the audible sound ?

Answer. On being set into vibrations, the stretched string, forces the surrounding air to vibrate. This vibrating air, in turn, affects our eardrum and produces an audible sound.

 Will the sound be audible if the string is set into vibration on the surface of the moon? Give reason for your answer. [SAII-2012]

Answer. No, we will not hear any audible sound on the surface of the moon. This is because sound requires a medium to propagate, since there is no atmosphere on the surface of the moon, therefore, the sound will not be heard.

What change, if any, would you expect in the characteristics of musical sound when we increase:

(i) its frequency, and (ii) its amplitude?

Answer. (i) Pitch of sound will increases (ii) Loudness of sound will increases

Q1. What is a medium? Give two examples.

 Ans. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Example, air, water, metal.

 

Q2. Define wave-motion.

 Ans. A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. The particles of the medium do not move forward but the disturbance is carried forward.

 

Q3. What is ‘sonic boom’?

 Ans. When an object just attains a supersonic speed, it causes shock waves in air. As a result there is a large change in air pressure. This results in a sonic boom.

 

Q4. Why does sound become faint with distance?

 Ans. Sound is a form of energy. As it moves away from the source its amplitude as well as its loudness decreases. The energy also gets transformed in vibration of the particles of the medium.

 

Q5. Why do we say that sound waves are longitudinal?

 Ans. Longitudinal waves need medium for propagation. The sound energy travels in the same line as the particles oscillate.

It forms compression and rarefaction for the longitudinal wave motion.

Sound wave shows all the characteristics of a longitudinal wave so it is called a longitudinal wave.

 

Q6. Differentiate between longitudinal wave and transverse wave.

 Ans.

Longitudinal Wave	Transverse Wave
1. It needs a medium for propagation.	1. It needs a medium for propagation.
2.Particles of the medium move in a direction parallel to the direction of propagation of the disturbance. Example, sound waves.	2.Particles of the medium move in the perpendicular direction of propagation of the disturbance. Example, light wave, seismic waves.

 

Q7. What is crest and trough?

 Ans. When a wave is propagated as represented below. A peak is called the crest and a valley is called the trough of a wave.The maximum oscillation disturbance of particles of air forms crest and trough.

 

Q8. What is echo? Why don’t we echo in a small room?

 Ans. The distinct sound heard after reflection of sound from the source is called echo. For echo, the distance of the reflecting surface from the source should be more than 17.2 m.

 

Q9. What is velocity of sound? Why does sound travel faster in the summer season than in winter?

 Ans. Velocity of sound is the speed of sound in a given medium at a given temperature. As the temperature increases the speed of sound also increases, hence in summer the sound travels faster than in winter.

 

Q10. Draw a graphical representation of the wave shape for (a) low pitched sound and

 (b) a high pitched sound.

 Ans. 

Q11. Give two applications of echo/reflection of sound.

 Ans. (i) Ships use reflection of sound technique ‘‘SONAR’’ which helps in locating the depth, distance, direction and speed of underwater objects.

(ii) Ceilings of concert halls are curved so that sound after reflection reaches all corners of the hall.

 

 

 

Q12. Define amplitude, time period and frequency of sound wave.

 Ans. Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called amplitude of the wave. Its unit ismetre.

Time Period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave.

Frequency: The number of oscillations occurring per unit time is called the frequency of sound waves.

 

Q13. A sound wave causes the density of air at a place to oscillate 1200 times in 2 minutes. Find the time period and frequency of the wave.

 Ans. 

Q14. Give 3 uses of ultrasound.

 Ans. Uses of ultrasound:

1. Ultrasound is used to detect cracks and flaws in metal blocks.

2. It is used in ‘echo-cardiography’, the ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart.

3. It is used in ‘ultrasonography’, to detect the image of organs or to detect the abnormalities in the organs. It is also used to examine the fetus during pregnancy to detect congenital defects.

 

Q15. What is the function of the middle ear?

 Ans. Middle ear consists of three small bones called hammer, anvil and stirrup. These three bones receive the sound vibrations and increase the strength of these vibrations to amplify the vibrations received by the ear-drum. These amplified vibrations are further passed to the inner ear.

 

Q16. A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s. What is the distance of the seabed from the ship?

 Ans. Time between transmission and detection t = 3.42 s.

Speed of ultrasound in seawater = 1531 m/s.

Distance traveled by the ultrasound = 2 × depth of sea = 2d.

2d = speed of sound × time

= 1531 × 3.42 = 5236 m

∴ 2d = 5236 m

∴ d = (5236/2) = 2618 m.

The distance of the seabed from the ship is 2618 m.

 

Q17. Distinguish between tone, note and noise.

 Ans. Tone: A sound of a single frequency is called a tone.

Note: The sound which is produced due to a mixture of several frequencies is called a note.

Noise: The sound which is produced due to a mixture of several frequencies but is unpleasant to the ear is called noise.

 

Q18. Establish the relationship between speed, wavelength and frequency of sound.

 Ans. Speed of sound → The distance traveled by a wave or a point on a wave (compression or rarefaction) per unit time.

Distance = wavelength of the sound wave, it is the distance traveled by the sound wave in one time period (T)of the wave.

Speed = Wavelength × Frequency.

 

Q19. Which wave property determines (a) loudness? (b) pitch?

 Name the characteristic of the sound which helps you to distinguish your friend’s voice while talking in a dark room.

 Ans. (a) Loudness is determined by amplitude.

(b) Pitch is determined by frequency. The quality or timber of sound helps us to distinguish our friend’s voice while talking in a dark room.

 

Sound class 9 physics long question

With the help of a labeled diagram shows that sound needs a material medium for its propagation.

Answer. A well labeled diagram is as shown: Take an electric circuit which consists of a cell, a switch and an electric bell arranged inside a bell jar, which stands on the platform of an evacuating pump.

The switch Of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped out of the bell jar. When the air is completely removed from the bell jar, no sound is heard as it is obvious from fig. because the medium of air which has to carry energy from the bell to the bell jar is removed. It shows that sound needs a material medium for its propagation.

A particular transmitter of Aakashvani broadcasts at 420.5 m wavelength. Given the speed of radio waves 3 x 108 ms-1. Calculate the frequency at which the radio station broadcasts its programme.

(b) What is the direction of oscillations of the medium particles through which a :

(i) transverse waves are propagating ?

(ii) longitudinal waves are propagating ? [SAII-2014]

Answer.

Question.3 Draw a well labeled diagram of the human ear. Explain its working.

Answer. A well labeled diagram of the human ear is as shown :

The sound waves are collected by the pinna and then are directed through the ear canal. The sound waves on striking the eardrum make it vibrate exactly the same way as the given sound emitting object. The bones in the middle ear start vibrating when the eardrum vibrates. It helps in magnifying the vibrations. When the magnified vibrations reach the cochlea in the inner ear, the fluid in it starts vibrating. These vibrations are picked up by sensory receptors and are converted into electrical signals. These electrical signals then travel to the brain which interprets sound.

 What is reverberation ? Write two ways of reducing reverberation.

(b) Distinguish between tone and note.

(c) With the help of a simple diagram, explain how defects in a metal block can be detected using ultrasound. [SAII-2013]

Answer.

(a) The persistence of sound in an auditorium as a result of repeated reflections of sound is called reverberation.

(b) A sound of a single frequency is called a tone. The sound which is produced due to a mixture of several frequencies is called a note.

(c) Ultrasounds can be used to detect cracks and flaws in metal blocks. The cracks or holes

inside the metal blocks, which are invisible from outside reduce the strength of the structure. Ultrasonic waves are allowed to pass through the metal 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.

 The given graph (Fig.)shows the displacement versus time relation for a disturbance traveling with velocity of 1500 ms-1. Calculate the wavelength of the disturbance.

Answer.

Question.2 Which of the below two graphs (a) and (b) (Fig.) representing the human voice is likely to be the male voice? Give reason for your answer.

Answer. The difference in male and female voice lies in its frequency. Female voice is of higher frequency. Hence, graph ‘a’ represents a male voice.

Q20. A sound produces 13 crests and 15 troughs in 3 seconds. When the second crest is produced the first is 2 cm away from the source? Calculate.

 (a) the wavelength

 (b) the frequency

 (c) the wave speed.

 Ans.

(a) Wavelength = distance between two consecutive crests or troughs is 2 cm.

Q21. A construction site near school is bringing a lot of behavioral changes in the students. Explain

 Ans. The constant work on the construction site makes loud sounds and this leads to irritation, high blood pressure, loss in concentration, hearing loss and sick feeling. All these changes affect a lot of students' performance and behavior.

 

Q22. Lot of houses are located just next to the railway tracks. The train honks every 20 minutes when it passes from the locality. What could be the measure taken by the families to prevent them from loud sound effects of the train?

 Ans. The loud sound can be avoided from entering into the house to some extent by using sound proof windows and doors. One can use the insulators to prevent the sound waves from entering the rooms.

 

Q23. What is the Doppler effect?

 Ans. As the train approaches, the sound of its horn is heard at a high pitch and as the train moves away, the sound of its horn is heard at a low pitch. This is the Doppler effect.

 

Q24. Sound reflects like light. How is this property of sound useful?

 Ans. The reflection of sound is used in all musical instruments which have a long tube like flute to produce a peculiar sound. It is also used in construction. If the hard material is used it would absorb the sound and in cinema halls the soft materials are used like fiberglass and acoustic tiles which will reflect the sound.

 

Q25. A sound wave is described as: Mechanical wave, longitudinal in nature and is a pressure wave. Discuss.

 Ans. Sound is a wave that is created by vibrating objects and propagated through a medium from one location to another. Since a sound wave is a disturbance that is transported through a medium via the mechanism of particle-to-particle interaction, a sound wave is characterized as a mechanical wave.

Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport. This is seen in sound waves.

Pressure wave: Since a sound wave consists of a repeating pattern of high-pressure and low-pressure regions moving through a medium, it is sometimes referred to as a pressure wave. The continuous crests and trough in the sound wave causes the pressure in the sound wave.