Lakhmir Singh Physics "Sound" Solutions


Lakhmir Singh Physics Class 9 Solutions Page No:185


Question 1 Can sound travel through (a) iron, and (b) water?

Solution 1 Yes, sound can travel through iron and as well as water.


Question 2 Can sound travel through vacuum?

Solution 2 No, sound cannot travel through vacuum.


Question 3 Name the type of waves which are used by astronauts to communicate with one another on moon (or in outer space).

Solution 3 Radio waves are used to communicate with one another on moon.


Question 4 Name one solid, one liquid and one gas through which sound can travel.

Solution 4

Solid – table

Liquid – water

Gas – air


Question 5 Which of the following cannot transmit sound?

Water, Vacuum, Aluminium, Oxygen gas.

Solution 5 Vacuum


Question 6 Name the physical quantity whose SI unit is 'hertz'.

Solution 6 Frequency


Question 7 What is the SI unit of frequency?

Solution 7 SI unit of frequency is hertz.


Question 8 What type of wave is represented:

(a) By density – distance graph?

(b) By displacement – distance graph?

Solution 8

(a) Longitudinal wave

(b) Transverse wave


Question 9 Is the speed of sound more in water or in steel?

Solution 9 Speed of sound is more in steel (solid medium) as compared to water (liquid medium).


Question 10 In which medium sound travels faster: air or iron?

Solution 10 Sound travels faster in iron (being a solid medium).


Question 11 In which medium sound travels fastest: air, water or steel?

Solution 11 Sound travels fastest in steel (solid medium).


Question 12 Out of solids, liquids and gases:

(a) In which medium sound travels slowest?

(b) In which medium sound travels fastest?

Solution 12

(a) Sound travels slowest in gases.

(b) Sound travels fastest in solids.


Question 13 Which of the following is the speed of sound in copper and which in aluminium?

(a) 5100 m/s (b) 1500 m/s (c) 3750m/s

Solution 13

(a) Speed of sound in copper = 3750m/s

(b) Speed of sound in aluminium = 5100m/s


Question 14 If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track?

Solution 14 It is more convenient to put the ear to the track to hear a train approaching from far away because sound travels faster in solids than in air.


Question 15 What is the speed of sound:

(a) in air?

(b) in water?

(c) in iron?

Solution 15

Speed of sound (at 20oC) in:

(a) Air = 344 m/s

(b) Water =1498 m/s

(c) Iron =5130 m/s


Question 16 What name is given to those aircrafts which fly at speeds greater than the speed of sound?

Solution 16 Supersonic aircrafts


Question 17 A jet aircraft flies at a speed of 410m/s. What is this speed known as?

Solution 17 Supersonic Speed


Question 18 What is meant by supersonic speed?

Solution 18 Supersonic speed refers to the speed of an object which is greater than the speed of sound.


Question 19 State an observation from everyday life which shows that sound travels much more slower than light.

Solution 19 It is common observation that in the rainy season, the flash of lightning is seen first and the sound of thunder is heard a little later. That's because, speed of light is very high as compared to speed of sound in air.


Question 20 Name two types of waves which can be generated in a long flexible spring (or slinky).

Solution 20 Transverse and Longitudinal waves.


Question 21 A stone is dropped on the surface of water in the pond. Name the type of waves produced.

Solution 21 Transverse (water) waves.


Question 22 Name the type of waves produced in air when a tuning fork is struck.

Solution 22 Longitudinal (sound) waves.


Question 23 What is the general name of the waves consisting of :

(a) Compressions and rarefactions?

(b) Crests and troughs?

Solution 23

(a) Longitudinal waves

(b) Transverse waves.


Question 24 State the general name of the waves in which the particles of the medium vibrate:

(a) In the same direction as wave.

(b) At right angles to the direction of wave.

Solution 24

(a) Longitudinal waves

(b) Transverse waves


Question 25 What type of waves are illustrated by the movement of a rope whose end is fixed to a pole and the other end is moved up and down?

Solution 25 Transverse waves


Question 26 What should an object do to produce sound?

Solution 26 An object should vibrate in order to produce sound.


Question 27 What is the name of the strings which vibrate in our voice box when we talk?

Solution 27 Vocal cords vibrate in our voice box when we talk.


Question 28 Name the device which is used to produce sound in laboratory experiments.

Solution 28 Tuning fork is used to produce sound in laboratory experiments.


uestion 29 What is the nature of sound waves in air?

Solution 29 The sound waves in air are longitudinal waves.


Question 30 What conclusion can be drawn from the observation that when the prongs of a sound making tuning fork touch the surface of water in a beaker, the water gets splashed?

Solution 30 The conclusion from the observation is that the prongs of tuning fork are vibrating, and the vibrating prongs carry energy which gets transmitted to surrounding medium.


Lakhmir Singh Physics Class 9 Solutions Page No:186


Question 31 State whether the following statement is true or false:

Sound produced by a vibrating body travels faster to our ears by the actual movement of air.

Solution 31 False


Question 32 Which of the following travels slowest in air and which one fastest?

Supersonic aircraft, light, Sound.

Solution 32

Slowest: Sound

Fastest: light


Question 33 Which term is used to denote a speed greater than the speed of sound?

Solution 33 Supersonic is used to denote a speed greater than the speed of sound.


Question 34 In which medium sound travels faster: air or hydrogen?

Solution 34 Sound travels faster in hydrogen ( speed of sound in hydrogen is 1284m/s)


Question 35 A tuning fork has a number 256 marked on it. What does this number signify?

Solution 35 The number 256 on tuning fork signifies the frequency of tuning fork.


Question 36 What is the time period of a tuning fork whose frequency is 200 Hz?

Solution 36


Question 37 Calculate the frequency of a wave whose time period is 0.02 s.

Solution 37


Question 38 What will be the change in the wavelength of a sound wave in air if its frequency is doubled?

Solution 38 Velocity of sound = Frequency x wavelength

Speed of sound in air is constant.

Hence, frequency x wavelength = constant

If frequency is doubled, wavelength is reduced to half.


Question 39 If 20 waves are produced per second, what is the frequency in hertz?

Solution 39 The frequency in hertz is equal to the number of waves produced per second. In this case, 20 waves are produced per second, so the frequency of sound waves is 20 hertz.


Question 40 Fill in the blanks:

(a) A sound is caused by _________.

(b) A sound wave consists of places of higher pressure called ______ and places of _____ pressure called ____

(c) Wave speed in metres per second equals frequency in _______ multiplied by _____ in _______.

(d) Sound cannot travel through _______.

(e) The speed of sound in a solid is _____than the speed of sound in air.

(f) When the frequency of the sound is increased, the wavelength_________.

Solution 40

(a) Vibrations

(b) Compressions; lower; rarefactions

(c) Hertz; wavelength; metres

(d) Vacuum

(e) Greater

(f) Decreases


Question 41 What is vacuum? Explain why, sound cannot travel through vacuum?

Solution 41 Vacuum means empty space, region with no matter particles. Sound cannot travel through vacuum because vacuum has no molecules which can vibrate and carry sound waves.


Question 42 Explain the term 'amplitude' of a wave. Draw the diagram of a wave and mark its amplitude on it.

Solution 42 The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium, is called amplitude (A) of the wave.


Question 43

(a) Distinguish between longitudinal and transverse waves.

(b) Are sound waves longitudinal or transverse?


Solution 43

(a) A wave in which the particles of the medium vibrate back and forth in the 'same direction', in which the wave is moving, is called a longitudinal wave. These waves can be produced in all the three media: solids, liquids and gases.

A wave in which the particles of the medium vibrate up and down, 'at right angles' to the direction in which the wave is moving, is called a transverse wave. It can be produced in solids and liquids but not in gases.

(b) Sound is a longitudinal wave.


Question 44 A cricket ball is seen to hit the bat first and the sound of hitting is heard a little later. Why?

Solution 44 Due to the very high speed of light we see the ball hitting the bat first. And it is due to comparatively lower speed of sound that the sound of hitting is heard a little later.


Question 45 Explain why, the flash of lightning reaches us first and the sound of thunder is heard a little later.

Solution 45 Light travels much faster than sound. Due to this, the flash of lightning is seen first and the sound of thunder is heard a little later.


Question 46 Explain why, the flash of a gun shot reaches us before the sound of a gunshot.

Solution 46 Light travels much faster than sound. Due to this, the flash of gun shot is seen first and the sound of gun shot is heard a little later.


Question 47 Which of the following terms apply to sound waves in air and which to water waves?

Transverse, Rarefaction, Trough, Crest, Compression, Longitudinal

Solution 47 Sound waves in air: Longitudinal, Compression, Rarefaction Water waves: Transverse, Crest, Trough


Question 48 (a) Name four ways in which sound can be produced.

(b) Calculate the speed of a sound wave whose frequency is 2kHz and wavelength 65 cm.

Solution 48

(a) Sound can be produced by the following methods:

(i) By vibrating strings (as in a sitar),

(ii) By vibrating air (as in a flute),

(iii) By vibrating membranes (as in a drum)

(iv) By vibrating plates (as in cymbals)


Question 49 If a ringing bicycle bell is held tightly by hand, it stops producing sound. Why?

Solution 49 This is due to the fact that when the ringing bell is held tightly with our hand, it stops vibrating and the sound coming from it also stops.


Question 50 Which object is vibrating when the following sounds are produced?

(i) The sound of a sitar

(ii) The sound of a tabla

(iii) The sound of a tuning fork

(iv) The buzzing of a bee or mosquito

(v) The sound of a flute


Solution 50 Sound is produced by the following objects:

(i) Vibrating stretched strings of sitar

(ii) Vibrating stretched membranes of tabla

(iii) Vibrating prongs of a tuning fork

(iv) Vibrating wings of mosquito

(v) Vibrating air columns in flute.


Question 51 Give reason for the following:

In most of the cases, we cannot see the vibrations of sound producing object with our eyes.

Solution 51 In most of the cases, a sound producing object vibrates so fast that we cannot see its vibrations with our eyes. The time inetrval between two successive vibration is lower than the persistence of vision. Hence we see the object in static state and not in vibration mode.


Question 52 Describe a simple experiment to show that the prongs of a sound producing tuning fork are vibrating.

Solution 52 Fill water in a beaker up to its brim. Touch the surface of water with the prongs of a sound making tuning fork (which has been struck on a hard rubber pad). The prongs of tuning fork producing sound splash water. This shows that the prongs of a sound producing tuning fork are vibrating (moving forwards and backwards rapidly).

The prongs of a sound producing tuning fork splash water, so they are vibrating.


Question 53 When we open a gas tap for a few seconds, the sound of escaping gas is heard first but the smell of gas comes later. Why?

Solution 53 The sound of a gas travels through the vibrations of air layers so it reaches first, but the smell of gas reaches the person through the actual movement of the air layers, which takes more time.


Question 54 A sound signal of 128 vibrations per second has a wavelength of 2.7m. Calculate the speed with which the wave travels.

Solution 54 Frequency is number of vibrations produced per second i.e. 128 Hz.

Wavelength, λ=2.7m

Speed of sound wave= frequency x wavelength

V=f xλ

Velocity, v= 128 x 2.7 =345.6 m/s


Question 55 A wave is moving in air with a velocity of 340m/s. Calculate the wavelength if its frequency is 512 vibrations/sec.

Solution 55

Velocity of wave= 340m/s

Frequency= 512 Hz

Wavelength=?

Speed of sound wave= frequency x wavelength

V=f xλ


Question 56 Define the 'frequency' and 'time period' of a wave. What is the relation between the two?

Solution 56 The number of complete waves (or cycles) produced in one second is called frequency of the wave.

The time required to produce one complete wave (or cycle) is called time-period of the wave.

The time taken to complete one vibration is called time-period.

Relation between time-period and frequency of a wave is:


Question 57 Explain why, a ringing bell suspended in a vacuum chamber cannot be heard outside.

Solution 57 A ringing bell suspended in a vacuum chamber cannot be heard outside because sound cannot travel through vacuum as it has no molecules which can vibrate and carry sound waves.


Question 58 The frequency of sound emitted by the loudspeaker is 1020 Hz. Calculate the wavelength of the sound wave in air in cm where its velocity is 340m/s.

Solution 58

Frequency, f=1020Hz

Velocity, v=340m/s

Wavelength =?

Speed of sound wave= frequency x wavelength


Question 59 What is the difference between a compression and a rarefaction in a sound wave? Illustrate your answer with a sketch.

Solution 59 A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium. It is a region of high pressure.

A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium. It is a region of low pressure


Lakhmir Singh Physics Class 9 Solutions Page No:187


Question 60

(a) What is sound? What type of waves are sound waves in air?

(b) Describe an experiment to show that sound cannot pass through vacuum.

Solution 60

(a) Sound is that form of energy which makes us hear. Sound waves are longitudinal waves in air.

(b) Sound cannot travel through vacuum. This can be shown by the following experiment:

(i) A ringing electric bell is placed inside an air tight glass jar containing air. We can hear the sound of ringing bell clearly. Thus, when air is present as medium in the bell jar, sound can travel through it and reach our ears.

(ii) The bell jar containing ringing bell is placed over the plate of a vacuum pump. Air is gradually removed from the bell jar by switching on the vacuum pump. As more and more air is removed from the bell jar, the sound of ringing bell becomes fainter and fainter. And when all the air is removed from the bell jar, no sound can be heard at all. Thus, when vacuum is created in the bell jar, then the sound of ringing bell placed inside it cannot be heard.

This shows that sound cannot travel through vacuum.


Question 61

(a) How is sound produced? Explain with the help of an example.

(b) How does sound from a sound producing body travel through air to reach our ears? Illustrate your answer with the help of a labeled diagram.

Solution 61

(a) Sound is produced when an object vibrates. For example, the sound of our voice is produced by the vibrations of two vocal cords in our throat caused by air coming from the lungs.

(b) When an object vibrates (and makes sound), then the air layers around it also start vibrating in exactly the same way and carry sound waves from the sound producing object to our ears.

Suppose a tuning fork is vibrating and producing sound waves in air. Since the prongs of the tuning fork are vibrating, the individual layers of air are also vibrating. Sound travels in the form of longitudinal waves in which the back and forth vibrations of the air layers are in the same direction as the movement of sound wave.


Question 62

(a) An electric bell is suspended by thin wires in a glass vessel and set ringing. Describe and explain what happens if the air is gradually pumped out of the glass vessel.

(b) Why cannot a sound be heard on the moon? How do astronauts talk to one another on the surface of moon?

Solution 62

(a) If the air is gradually pumped out of the glass vessel, no sound of the electric bell can be heard because vacuum is created in the vessel and there are no air molecules to carry sound vibrations.

(b) Sound cannot be heard on the surface of moon because there is no air on the moon to carry the sound waves.

Astronauts talk to one another on the surface of moon through wireless sets using radio waves. This is because radio waves can travel even through vacuum though sound waves cannot travel through vacuum.


Question 63

(a) Define the terms 'frequency', 'wavelength' and 'velocity' of a sound wave. What is the relation between them?

(b) A body vibrating with a time-period of 1/256 sec produces a sound wave which travels in air with a velocity of 350m/s. Calculate the wavelength.

Solution 63

(a) The number of vibrations per second is called frequency.

The minimum distance in which a sound wave repeats itself is called its wavelength.

The distance travelled by a wave in one second is called velocity of wave.

Relation between velocity, frequency and wavelength of a wave:

Velocity of wave= frequency x wavelength


Question 64

(a) What are longitudinal waves and transverse waves? Explain with the help of labeled diagrams.

(b) Give two examples each of longitudinal waves and transverse waves.

Solution 64

(a) A wave in which the particles of the medium vibrate back and forth in the 'same direction', in which the wave is moving, is called a longitudinal wave. These waves can be produced in all the three media: solids, liquids and gases.

A wave in which the particles of the medium vibrate up and down 'at right angles' to the direction in which the wave is moving, is called a transverse wave. It can be produced in solids and liquids but not in gases.

(b) Longitudinal waves:

(i) The waves which travel along a spring when it is pushed and pulled at one end.

(ii) Sound waves in air

Transverse waves:

(i) The waves produced by moving one end of a long spring up and down rapidly, while other end is fixed.

(ii) The water waves or ripples formed on the surface of water in a pond.


Question 65

(a) Explain the terms 'compressions' and rarefactions' of a wave. What type of waves consists of compressions and rarefactions?

(b) A worker lives at a distance of 1.32 km from the factory. If the speed of sound in air be 330 m/s, how much time will the sound of factory siren take to reach the worker?

Solution 65

(a) A compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are, and there is a momentary reduction in volume of the medium.

A rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than normal, and there is a momentary increase in the volume of the medium.

Longitudinal waves consist of compressions and rarefactions.


Question 66

(a) Explain the terms 'crests' and troughs' of a wave? What types of wave consist of crests and troughs?

(b) The flash of a gun is seen by a man 3 seconds before the sound is heard. Calculate the distance of the gun from the man (Speed of sound in air is 332m/s).

Solution 66

(a) The 'elevation' or 'hump' in a transverse wave is called crest. It is that part of the transverse wave which is above the line of zero disturbance of the medium.

The 'depression' or 'hollow' in a transverse wave is called trough. It is that part of the transverse wave which is below the line of zero disturbance of medium.

A transverse wave consists of crests and troughs.

(b) Speed of sound= 332m/s

Time =3 sec


Question 67

(a) When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air. Why?

(b) How could you convince a small child that when you speak, it is not necessary for air to travel from your mouth to the ear of a listener?

Solution 67

(a) When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air. This is due to the fact that sound travels much more fast through the railway line made of steel than through air.

(b) There is no actual movement of air from the sound-producing body to our ear. The air layers only vibrate back and forth, and transfer the sound energy from one layer to the next layer till it reaches our ear. This will be clear from an example: If we turn on a gas tap for a few seconds, a person standing a few metres away will hear the sound of escaping gas first and the smell of gas reaches him afterwards. The sound of gas travels through the vibrations of air layers so it reaches first, but the smell of gas reaches the person through the actual movement of the air layers, which takes more time. So, it is clear that the sound is not being transmitted by the actual movement of air from the gas tap to person, otherwise he would hear and smell the gas at the same time.


Lakhmir Singh Physics Class 9 Solutions Page No:188


Question 81 Draw the sketches of two waves A and B such that wave A has twice the wavelength and half the amplitude of wave B.

Solution 81


Question 82 A device called oscillator is used to send waves along a stretched string. The string is 20 cm long, and four complete waves fit along its length when the oscillator vibrates 30 times per second. For the waves on the string:

(a) What is their wavelength?

(b) What is their frequency?

(c) What is their speed?

Solution 82

(a) Given that there are four complete waves.


Question 83 Through which of the following materials can sound travel?

wood, air, water, steam, ice , hydrogen, steel, diamond

Solution 83

Sound can travel through all the given materials.


Question 84 A sound producing body is at considerable distance from a man. There can be four different media W, X, Y and Z between the sound producing body and the man. The medium X brings the sound to man most quickly whereas medium Z takes the maximum time. The time taken by medium W in bringing sound to man is more than that of X but less than that of Z. The medium Y, however, fails to bring the sound from the sound producing body to the man. Which medium could be the one:

(a) Having no fixed shape and no fixed volume?

(b) Having a fixed volume but no fixed shape?

(c) Having the same composition as that on the moon?

(d) Having a fixed shape and a fixed volume?

Solution 84

(a) Z medium has no fixed shape and no fixed volume.

(b) W medium has a fixed volume but no fixed shape.

(c) Y medium has the same composition as that on the moon.

(d) X medium has a fixed shape and a fixed volume.


Question 85 The longitudinal waves travel in a coiled spring at a rate of 4m/s. The distance between two consecutive compressions is 20 cm. Find:

(i) Wavelength of the wave

(ii) Frequency of the wave.

Solution 85

(i) The distance between two consecutive compressions or rarefactions is equal to its wavelength.

Hence, wavelength is =20 cm= 0.20 m

(ii) Speed of wave =4 m/s

Wavelength=0.20 m

Speed of wave =frequency x wavelength

4 m/s = frequency x 0.20 m


Lakhmir Singh Physics Class 9 Solutions Page No:206


Question 1 Which property of sound leads to the formation of echoes?

Solution 1 The reflection of sound leads to formation of echoes


Question 2 What name is given to the repetition of sound caused by the reflection of sound waves?

Solution 2 Echo is repetition of sound caused by the reflection of sound waves.


Question 3 What name is given to the persistence of sound in a big hall or auditorium?

Solution 3 The persistence or sound in a big hall or auditorium is called reverberation.


Question 4 Name three devices which work on the reflection of sound.

Solution 4 a) Megaphone and bulb horn

b) Stethoscope

c) Soundboard


Question 5 What is the other name for a loud-hailer?

Solution 5 Megaphone


Question 6 Name three characteristics of sound.

Solution 6

a) Loudness

b) Pitch

c) Timbre or Quality


Question 7 Name the unit used to measure the loudness of sound. Also write its symbol.

Solution 7 The loudness of sound is measured in decibel. Its symbol is dB.


Question 8 Name the characteristic that helps us to distinguish between a man's voice and a woman's voice, even without seeing them.

Solution 8 Pitch helps us to distinguish between a man's voice and a woman's voice, even without seeing them.


Question 9 How does the pitch of a sound depend on frequency?

Solution 9 Pitch of a sound is directly proportional to frequency. Higher the frequency, higher is the pitch of the sound.


Question 10 Name the characteristics of sound which depend on

(i) amplitude

(ii) frequency

(iii) waveform.

Solution 10

(i) Loudness

(ii) Pitch

(iii) Timbre


Question 11 Name the characteristic of sound which can distinguish between the notes played on a flute and a sitar(both having the same pitch and loudness).

Solution 11 Quality or timbre


Question 12 Name the organs of hearing in our body

Solution 12 Ears enable us to hear sounds.


Question 13 Name the part of the ear which vibrates when outside sound falls on it.

Solution 13 Ear drum starts vibrating when outside sound falls on it.


Question 14 Name three tiny bones present in the middle part of ear.

Solution 14 There are three small bones in the middle ear- anvil, hammer and stirrup.


Question 15 There are three small bones in the middle ear- anvil, hammer and stirrup

a) Which of these is in touch with ear drum?

b) Which of these is in touch with oval window?

Solution 15

a) Hammer

b) Stirrup


Question 16 What is the function of three tiny bones in the ear?

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.


Lakhmir Singh Physics Class 9 Solutions Page No:207


Question 17 Name the tube which connects the middle ear to throat.

Solution 17 Eustachian tube


Question 18 Name the nerve which carries electrical impulses from the cochlea of ear to the brain.

Solution 18 Auditory nerve


Question 19 What is the name of passage in outer ear which carries sound waves to the ear drum?

Solution 19 Ear canal


Question 20 Why should we not put a pin or pencil in our ears?

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.


Question 21 What types of scans are used these days to monitor the growth of developing baby in the uterus of the mother?

Solution 21 Ultrasound scans are used to monitor the growth of developing baby in the uterus of the mother.


Question 22 How is an ultrasound scan for fetus better than X-ray?

Solution 22 An ultrasound scan for fetus is better than X-rays because X-rays can damage the delicate body cells of the fetus.


Question 23 What is the name of the device which is used to find the depth of sea by using ultrasonic sound waves?

Solution 23 SONAR is used to find the depth of sea by using ultrasonic sound waves.


Question 24 What is the full name of SONAR?

Solution 24 SOund Navigation And Ranging


Question 25 Name the principle on which a soundboard works.

Solution 25 Soundboard works on the principle of reflection of sound.



Question 26 Name the device which is used to address a small gathering of people.

Solution 26 A megaphone is used to address a small gathering of people.


Question 27 Name the device used by doctors to listen to our heartbeats.

Solution 27 A stethoscope, based on the principle of reflection of sound, is used by doctors to listen to our heartbeats.


Question 28 What is the shape of soundboard kept behind the speaker on the stage of a big hall?

Solution 28 Soundboard is a concave board which is kept behind the speaker on the stage of a big hall.


Question 29 Name two sound absorbing materials which can make our big room less echoey.

Solution 29 Curtains and carpets can make our big room less echoey.


Question 30 Can we hear a) infrasonic waves b) ultrasonic waves ?

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.


Question 31 What name is given to the sound waves of frequency too low for humans to hear?

Solution 31 Infrasonic sound


Question 32 What name is given to the sound waves of frequency too high for humans to hear?

Solution 32 Ultrasonic sounds


Question 33 What type of sound waves is produced by a vibrating simple pendulum?

Solution 33 Infrasonic sound waves


Question 34 What happens to the pitch of a sound if its frequency increases?

Solution 34 As the frequency increases the pitch of the sound also increases.


Question 35 What happens to the loudness of a sound if its amplitude decreases?

Solution 35 The loudness decreases with the decrease in the amplitude of sound.


Question 36 What name is given to the sound waves of frequencies above 20 kHz?

Solution 36 Ultrasonic sound waves

Question 37 Fill in the blanks with suitable words

a) An echo is simply a ________sound

b) Pitch of sound depends on______

c) Loudness of sound depends on______

d) Quality of sound depends on______

e) Echoes are caused by the ________of sound

Solution 37

a) reflected

b) frequency

c) amplitude

d) waveform

e) reflection


Question 38 On which day, a hot day or a cold day, an echo is heard sooner? Give reason for your answer.

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.


Question 39 In which medium, air or water, an echo is heard much sooner? Why?

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.


Question 40 What is reverberation? What will happen if the reverberation time in a big hall is too long?

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.


Question 41 How can reverberations in a big hall or auditorium be reduced?

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


Question 42 Why do we hear more clearly in a room with curtains than in a room without curtains?

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.


Question 43 What is a megaphone?

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.


Question 44 What is a bulb horn? Name the principle on which it works.

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.


Question 45 What is a stethoscope? Name the principle on which it works.

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.


Question 46 What is a soundboard?Explain the working of a soundboard with the help of a diagram.

Solution 46

The soundboard is a concave board (curved board) which is placed behind the speaker in large halls or auditoriums so that his/her speech can be easily heard even by the persons sitting at a considerable distance. Working of the soundboard: 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). Because of this, sound is distributed uniformly throughout the hall and even the persons sitting at the back of the hall can hear the speech easily.


Question 47

(a) What is meant by the loudness of the sound? On what factor does the loudness of a sound depend?

(b) Draw labeled diagram to represent (a) soft sound (b) loud sound, of the same frequency.

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)


Question 48

a) What is meant by the pitch of the sound? On what factors does the pitch of a sound depend?

b) Draw labeled diagram to represent sound of (a) low pitch, and (b) high pitch, of the same loudness.

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)


Lakhmir Singh Physics Class 9 Solutions Page No:208


Question 49 What is meant by the quality (or timbre) of the sound? On what factors does the quality (or timbre) of a sound depend?

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.


Question 50 Explain why, if we strike a table lightly, we hear a soft sound but if we hit the table hard, we hear a loud sound?

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.


Question 51 Give one use of ultrasound in industry and one in hospital.

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.


Question 52 How is it that bats are able to fly at night without colliding with other objects?

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.


Question 53 Explain how bats use ultrasound to catch their prey.

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.


Question 54 Explain how flaws in a metal block can be detected by using ultrasound.

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.


Question 55 Why are the ceilings of concert halls made curved? Draw a labeled diagram to illustrate your answer.

Solution 55

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


Question 56 Draw a labeled diagram to show multiple reflections of sound in a part of stethoscope tube.

Solution 56


Question 57 What is the range of frequencies associated with

a) Infrasound

b) Audible sound

c) Ultrasound?

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.


Question 58 (a) What is the difference between infrasonic and ultrasonic waves?

(b) Choose the infrasonic waves and ultrasonic waves from the following frequencies

(i) 10,000 Hz

(ii) 30,000 Hz

(iii) 18 Hz

(iv) 50,000 Hz

(v) 10 Hz

Solution 58

(b) Infrasonic waves: 10 Hz, 18 Hz

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


Question 59

(a) What is the frequency range of hearing in humans?

(b) Which of the the following sound frequencies cannot be heard by a human ear?

(i) 10 Hz

(ii) 100 Hz

(iii) 10,000 Hz

(iv) 15 Hz

(v) 40,000 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.


Question 60 The echo of sound is heard after 5 seconds. If the speed of sound in air is 342 m/s, calculate the distance of the reflecting surface.

Solution 60 Time taken to listen the 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 = 342 x (5/2) = 855 m


Question 61 The speed of sound in water is 1500 m/s. how far away from an under-sea rock should a diver be so that he can hear his own echo?

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, t = 0.1/2 s

Speed of sound in water v = 1500 m/s

Distance of reflecting surface = v x t = 1500 x (0.1/2) Sound = 75 m


Question 62

(a) What is meant by the reflection of sound? What types of surfaces are best for reflecting sound?

(b) Name any two objects which are good reflectors of sound.

(c) State the laws of reflection.

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.


Question 63

(a) What is an echo? How is echo formed?

(b) What is the minimum distance in air required from a sound reflecting surface to hear an echo (at 20oC)?

(c) A man standing 825 metres away from a cliff (steep rock) fires a gun. After how long will he hear its echo? Speed of sound in air is 330 m/s.

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 20oC) 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,

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


Question 64 (a) What is ultrasound? What is the difference between ordinary sound and ultrasound?

(b) Write any three applications of ultrasound.

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.


Question 65 (a) What are infrasonic waves? Name two animals which produce infrasonic waves.

(b) What are ultrasonic waves? Name two animals which produce ultrasonic waves.

(c) The audible range of frequencies of an average human ear is from 20 Hz to 20 kHz. Calculate the corresponding wavelengths. (Speed of sound in air is 344 m/s.)

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 fl= 20 Hz

Higher frequency fh = 20,000Hz

So, the wavelength range is 0.0172 m to 17.2 m.


Question 66 (a) Define the following terms: (a) Echolocation (b) Echocardiography, and (c) Ultrasonography.

(b) Name an animal which navigates and finds its food by echolocation.

(c) Which of the two animals produce ultrasonic waves: porpoise or whale?

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.


Question 67 (a) What is SONAR? Explain its use.

(b) A sonar station picks up a return signal after 3 seconds. How far away is the object? Speed of sound in water = 1440 m/s

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


Question 68 Draw a neat and labeled diagram of the human ear. With the help of this diagram, explain the construction and working of human ear.

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.


Lakhmir Singh Physics Class 9 Solutions Page No:209


Question 85

The drawing shows a ship 880 m from cliff. A gun is fired on the ship. After 5 seconds, the people at the front of the hear the sound of the gun again.

a) What is the name of this effect?

b) What happens to the sound at the cliff?

c) How far does the sound travel in 5 seconds?

d) Calculate the speed of sound in air.

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


Lakhmir Singh Physics Class 9 Solutions Page No:210


Question 86 Consider the following sound waves marked A, B, C and D

(a) Which two waves represents sound of same loudness but different pitch?

(b) Which two waves represents sound of same frequency but different loudness?

(c) State whether all these sound waves have been produced by same vibrating body or different vibrating bodies.

(d) Which vibrating body/bodies could have generated the sound waves shown here?

Solution 86

(a) A and D

(b) B and D

(c) Same vibrating body produced all the sound wvaes

(d) Tuning forks


Question 87 In an experiment, Anhad studies sound waves. He sets up a loudspeaker to produce as shown below:

Anhad adjusts the signal to the loudspeaker to give a sound of frequency of 200 Hz

(a) What happens to air in between Anhad and speaker?

(b) Explain how Anhad receives sound in both ears.

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.


Question 88 Figure X shows a trace of sound wave produced by a particular tuning fork:

(a) On the graph paper given in Figure Y, draw a trace of the sound wave which has a higher frequency than that shown in Figure X.

(b) On the graph paper shown in Figure Z, draw a trace of the sound wave which has a larger amplitude than that shown in Figure X

Solution 88


Question 89 Three different vibrating objects produce three types of sounds X, Y, Z. Sounds X, Y cannot be heard by a man having a normal range of hearing but sound Z can be easily heard. The sound X is used in hospitals to break kidney stones of a patient into fine grains which then get flushed out with urine. The sound Y is similar to that which produced during an earthquake before the main shock wave is generated.

(a) What type of sounds are (i) X (ii) Y, and (iii) Z?

(b) Name one device which can produce sound like X.

(c) Name one device in a science laboratory which can produce sound like Y.

(d) Name one device in our homes which can produce sound like Z.

(e) What is the frequency range of sounds like Z?

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


Question 90 A man is kidnapped, blindfolded and imprisoned in a big room. How could the man tell if he was in:

(a) a city

(b) a village

(c) a bare room

(d) a furnished room?

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.

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