3 sound - the assam valley schoolassamvalleyschool.com/physics today/worksheets/7... · the...

1Class-VII Physics Question Bank

I. Tick (✔✔✔✔✔) the most appropriate answer.1. Rapid back and forth motion of a particle about its mean position is

called(a) displacement (b) acceleration (c) vibration (d) friction

2. In which state of matter does sound travel the fastest?(a) gases (b) solids (c) liquids (d) none of these

3. Sound cannot travel through(a) solids (b) liquids (c) vacuum (d) none of these

4. The intensity of sound is measured in the units of(a) joule (b) decibel (c) watts (d) none of these

5. The speed of sound in air is(a) 3000 m/s (b) 340 m/s (c) 400 m/s (d) 1000 m/s

6. The instrument used in the laboratory to produce sound of a fixedfrequenncy is(a) simple pendulum (b) tuning fork(c) thermometer (d) metre scale

7. Violin is a(a) stringed musical instrument (b) percussion instrument(c) reed instrument (d) wind instrument

8. An instrument based on the principle of echo is(a) sonar (b) metre scale (c) radar (d) barometer

9. An echo of the sound produced can be heard only if it reahes ourear after(a) 1/15th of a seond (b) 1/13th of a seond(c) 1/10th of a seond (d) 1/5th of a seond

10. The characteristic of sound which distinguishes a feeble sound froma loud sound of the same frequeny is(a) loudness (b) pitch (c) music (d) noise

11. To and fro motion of a particle about its mean position is called

3 SOUND3 SOUND


(a) frequency (b) amplitude (c) vibration (d) acceleration12. The approximate speed of sound in air is

(a) 33.2 ms–1 (b) 3320 ms–1 (c) 332 ms–1 (d) 3.32 ms–1

13. A simple instrument used to produce pure sound of fixed frequencyis(a) tuning fork (b) fire tongs(c) simple pendulum (d) metre scale

14. The number of vibrations made by a vibrating body in one secondis called(a) wavelength (b) time period (c) amplitude (d) frequency

15. The frequency of ultrasonic vibrations is(a) less than 20 Hz (b) between 20 and 10000 Hz(c) between 20 and 20000 Hz (d) more than 20000 Hz

16. The loudness of sound is normal between(a) 10 dB – 25 dB (b) 50 dB – 60 dB(c) 90 dB – 105 dB (d) 120 dB – 130 dB

17. The frequency of sound increases when the closed length of an aircolumn(a) increases (b) decreases(c) remain same (d) none of these

18. Pitch of sound does not depend upon the(a) thickness of vibrating wire(b) area of cross-section of vibrating wire(c) amplitude of vibrating wire(d) length of wire

Ans. 1. (c) 2. (b) 3. (c) 4. (b) 5. (b) 6. (b)7. (a) 8. (a) 9. (c) 10. (b) 11. (c) 12. (c)

13. (a) 14. (d) 15. (d) 16. (b) 17. (b) 18. (c)

II (A). Fill in the blanks.1. Every source of sound is a ______________ body.2. Sound travels faster in ______________ than in gases.3. Sound requires a ______________ for propagation.4. The reflected sound is called ______________.


5. The sound produced by an object vibrating with a high frequency iscalled ______________ sound.

6. ______________ produces sensation in the ears.7. Musical instruments in which leather membrane is made to vibrate

are known as ______________ instruments.8. The maximum displacement of the particles of a medium in which a

wave is travelling is called ______________.9. The ______________ of a sound is that characteristic by virtue of

which we can distinguish between two sounds of the same pitchand loudness produced by two different instruments.

10. Pitch increases with the increase in the ______________ of avibrating body.

11. Only vibrating bodies produce ______________.12. The pitch of a sound depends upon the ______________ of a

vibrating string.Ans. 1. vibrating 2. solids 3. medium

4. echo 5. shrill 6. sound7. percussion 8. amplitude 9. quality

10. frequency 11. sound 12. thickness

II (B). Fill in the blank spaces by choosing correct words from the listgiven below.

List : hertz, echo, frequency, twenty, solids, later, depth, vibration,sound, ultrasonic.

1. Only vibrating bodies produce ______________.2. The range of subsonic vibrations is less than _____________ hertz.3. The unit of frequency is ______________.4. The repetition of reflected sound from a distant object is called an

______________.5. The sound travels faster in ______________ as compared to gases.6. The ______________ of the sound produced by a vibrating wire

decreases, if its length is increased.7. The thunder of a cloud is heard ____________, compared to flash

of lightning.


8. The range of ______________ vibrations is above 20,000 Hz.9. Sonar is an instrument used for finding the ____________ of sea.

10. The to and fro motion of a vibrating body about its mean position iscalled ______________.

Ans. 1. sound 2. twenty 3. hertz4. echo 5. solids 6. frequency7. later 8. ultrasonic 9. depth

10. vibration

III (A). Statements given below are incorrect. Write the correct state-ments.

1. The sounds produced by different whistles are the same.Ans. Sounds produced by different whistles are different.

2. We hear echoes only in the mountains.Ans. We hear echoes in the mountains or in the valleys.

3. The ticking of a watch sounds louder when heard through waterthan through a metal.

Ans. The ticking of a watch sound is louder when heard through a metalthan through water.

4. Small vibrations produce loud sounds.Ans. Large vibrations produce loud sounds.

5. All kinds of vibrations in the material bodies produce sensation ofsound.

Ans. The frequency of the vibrations between 20 Hz to 20,000 Hz in thematerial bodies produce sensation of sound.

6. The time taken by a vibrating body to complete one vibration iscalled frequency.

Ans. The time taken by a vibrating body to complete one vibration iscalled time period.

7. The maximum displacement of a vibrating body from one extremeposition to another extreme position is called amplitude.

Ans. The maximum displacement of a vibrating body from its meanposition is called amplitude.

8. Louder the sound, less is the amplitude of vibrating body.


Ans. Louder the sound, more is the amplitude of vibrating body.9. Galton whistle is used to train dolphins.

Ans. Galton whistle is used to train dogs.10. Sound travels slower in water as compared to air.

Ans. Sound travels faster in water as compared to air.11. Material medium is not necessary for the propagation of sound.

Ans. Material medium is necessary for the propagation of sound.12. In order to produce an echo, the minimum distance between source

of sound and reflecting body should be 14 m.Ans. In order to produce an echo, the minimum distance between source

of sound and reflecting body should be 17 m.13. The characteristics of sound which distinguishes between a sharp

sound and dull sound is called loudness.Ans. The characteristics of sound which distinguishes between a sharp

sound and dull sound is called pitch.III (B). Write true or false for each statement given below.

1. The range of sonic vibrations is between 20 Hz to 20,000 Hz.2. The sound can travel through vacuum.3. Amplitude is the maximum displacement of a particle from its mean

position.4. The speed of sound in air is 5100 m/s.5. The number of vibrations produced by a vibrating particle in one

second is called frequency.6. The more the amplitude of a vibrating body, the more is the loudness.7. The minimum distance between source of sound and reflecting body

should be 17 m, for the formation of an echo.8. With the increase in frequency of a vibrating body, the sound

becomes shrill.9. Subsonic vibrations have a range above 20,000 Hz.

Ans. 1. True 2. False 3. True 4. False 5. True 6. True 7. True 8. True 9. FalseIV. Find the odd-one out. Give reasons for your choice.

1. Loudness, reflection, pitch, quality


Ans. Reflection — because it is not the characteristics of sound whileloudness, pitch and quality are characteristics of sound.

2. 40 decibels, 70 decibels, 150 decibels, 90 decibelsAns. 150 decibels — because it is undesirable, very loud and painful

sound and thus considered as noise.3. Soft horns, ear plugs, silencers, loudspeakers

Ans. Loudspeakers — because it causes noise pollution while other devicesare used to prevent the noise pollution.

4. Loudness, reed instrument, pitch, timbreAns. Reed instrument — because it is a type of musical instrument while

other terms are the characteristics of sound.5. Flute, dholak, tabla, mridangam

Ans. Flute — because it is a wind instrument while others are Percussioninstruments.

6. Sitar, guitar, mouth organ, violinAns. Mouth organ — because it is a reed instrument while others are

stringed musical instruments.7. Loudness, pitch, echo, quality

Ans. Echo — because it is the reflection of sound while other terms arecharacteristics of sound.

8. Loudspeakers, aeroplanes, vacuum cleaners, ear plugsAns. Ear plugs — because they are used to avoid noise pollution while

others are the source to cause noise pollution.V. Match the following.

1. reflection of sound (a) sound of high-pitch2. a shriller sound (b) echo3. 120 decibels (c) tuning fork4. a source producing sounds of (d) noise

fixed frequencyAns. 1. (b), 2. (a), 3. (d), 4. (c)

VI. Give reasons for the following.1. Sound cannot reach us from space.

Ans. Sound requires a medium to propagate. It cannot travel in the


vacuum. Since there is a vacuum in the space, therefore, soundcannot reach us from space.

2. A fighter aeroplane is seen before hearing its sound.Ans. A fighter aeroplane is seen before hearing its sound because light

travels at a very high speed of 3 × 108 m/s whereas the sound travelsin air at 330 m/s. Thus as soon as a fighter aeroplane appears it isseen at once whereas its sound take some time to reach us.

3. Bats can locate objects in the dark.Ans. Bats can hear ultrasonic sounds having frequencies upto 1,20,000 Hz.

Infact bats can also produce ultrasonic sounds while screaming.Ultrasonic sounds produced by bats are reflected back by otherobjects in their path and the echoes thus formed are detected bybats. It is due to this echo location of objects around them, the batsbeing nearly blind can locate objects in the dark.

4. A large drum can produce a louder sound than a small drum.Ans. Loudness depends on the area of the vibrating body. Thus, loudness

increases with an increase in the area of the vibrating body. As alarge drum occupies more area than a small drum. Therefore, a largedrum can produce a louder sound than a small drum.

5. Noise pollution can lead to health hazards.Ans. Noise pollution can lead to health hazards because it may cause

partial or permanent hearing loss. It increases nervous tension,irritation and high blood pressure.

6. Echoes can be produced in mountains or valleys.Ans. The repetition of sound when it is reflected from a distant body is

called an echo. Echo can be heard if the minimum distance betweensource of sound and reflecting body is 17 metre. Because there isenough distance in mountains or valleys,. So, echoes can beproduced in such places.

7. We cannot talk to one another on the surface of moon.Ans. Sound requires a medium to propagate. It cannot travel in the

vacuum. Since there is a vacuum on the moon therefore the soundof other person does not reach to our ear, so we cannot talk to


another on the surface of moon.8. A supersonic plane disappears from the line of sight when we hear

its sound.Ans. A supersonic plane disappears from the line of sight when we hear

its sound, because light travels at a very high speed of 3 × 108 m/swhereas sound travels in air at 330 m/s. That is why its sound takessome time to reach us mean while the plane disappears.

VII. Differentiate between the following.1. Wind instrument and percussion instrument

Ans. Wind instruments : These musical instruments are generally madein the form of pipes. When we blow air into them the column of airinside vibrates producing a note. Whistle, flute and shehnai are somecommon wind instruments.Percussion instruments : In these instruments, we make a leathermembrane vibrate by striking it with a hammer or with our hands.Drum, tabla and dholak are the commonly used Percussion musicalinstruments.

2. High-pitch sound and low-pitch soundAns. High-pitch sound : The sound produced by an object vibrating

with a high frequency is called shrill or high-pitched sound.Low-pitch sound : The sound produced by an object vibrating witha low frequency is described as flat or low-pitched sound.

3. Fundamental and harmonicsAns. Fundamental : When a note is played on a sitar, it produces the

basic sound of a certain frequency called the fundamental.Harmonics : Along with the fundamental a note also produces amixture of sound of other frequencies each of which differs inloudness. These are called harmonics.

4. Sound produced by guitar and sound produced by a loudspeakerAns. Sound produced by guitar is a musical sound. It has pleasant effect

on the ears. On the other hand sound produced by a loudspeaker isa noise. It does not have a pleasant effect on the ears.

VIII. Define these terms.


1. VibrationAns. A vibration is a rapid back and forth motion of a particle about its

mean position.2. Echo

Ans. The repetition of sound, when it is reflected from a distant body iscalled an echo.

3. ShrillAns. The sound produced by an object vibrating with a high frequency is

called shrill.4. Sound quality

Ans. The quality of a musical sound is that characteristics by virtue ofwhich we can distinguish between two sounds of the same pitchand loudness produced by two different musical instruments or bydifferent voices.

5. Ultrasonic soundAns. Sounds having frequencies higher than 20,000 Hz are known as

ultrasonic sounds. Some animals like bats, dogs and dolphins canhear ultrasonic sounds.

6. Infrasonic soundAns. Sounds having frequencies lower than 20 Hz are known as infrasonic

sounds. Earthquakes, volcanic eruptions, simple pendulum and someanimals like whales and elephants produce infrasonic sounds.

IX. Answer these questions.1. Describe an experiment to prove that

(a) sound travels faster in solids than in liquids.(b) sound travels faster in solids than in gaseous medium.

Ans. (a) Experiment to prove that sound travels faster in solids than inliquids.Take a squeky toy and put it inside a plastic bag. Seal the bag withthe help of a candle or with a thread. Fill a bucket with water andplace the bag in the water squeeze the toy. You will hear a lowsound. Now place your ear against the side of the bucket and squeezethe toy again. You will hear the sound better.


From the above experiment we conclude that sound travels througha solid medium faster than in liquid medium.(b) Experiment to prove that sound travels faster in solids than ingaseous medium.Palce a wristwatch at one end of a long wooden table stand at theother end of the table. Try to hear the ticking of the watch. Placeyour ear on the table and listen. You will observe that the ticking ofthe watch sound is louder when heard through wood.From the above experiment we conclude that sound travels througha solid medium faster than through a gaseous medium.

2. What do you mean by reflection of sound? Describe an experimentto explain it.

Ans. The phenomenon of sound waves that first strike some solid objectslike walls of a room than get reflected and reach our ears is calledreflection of sound.Experiment.Take a clock and place it at the bottomof a glass container. You cannot hearthe sound of the clock clearly. Hold asmooth wooden sheet inclined at 45°to the glass container. You can nowhear the sound better. This is becausethe sound starts from the clock andreaches the sheet, on striking the sheet,the direction of the sound waveschange and you hear the sound.

3. Why is a woman’s voice different from a man’s voice?Ans. In human beings vocal cords are responsible for the difference of

woman’s voice from man’s voice. When the cords are tight andthin, the sound is of higher pitch or frequency. On the other hand,when the cords are loose and thick the sound is of low pitch orfrequency. Men have longer vocal cords than women. That is whythe frequency of a sound produced by women is of a higher


frequency than the sound produced by men.4. Why is the sound produced from a streched string different when it

is plucked lightly than when plucked strongly?Ans. When a stretched string is plucked lightly it vibrates and produces a

soft sound. If we pluck it strongly it vibrates with a greater amplitudeand a louder sound is produced.

5. What are musical instruments? What are their main types?Ans. Musical instruments : Music is a sound which has pleasant

sensation on our ears. A musical sound is produced by regularvibrations and the instruments used to produce music are calledmusical instruments.Depending upon the way the air vibrates they are of four main types:

(i) Stringed musical instruments(ii) Percussion instruments

(iii) Wind instruments(iv) Reed instruments

6. What is the source of sound in reed instruments? Give examples.Ans. Reed instruments contain metal reeds which produce sound when

air is blown through them. Instruments like harmonium and mouthorgan are examples of reed instruments.

7. How is sound produced in wind instruments? Give examples.Ans. Wind instruments are generally made in the form of pipes. When

we blow air into them the column of air inside vibrates and producea note. Whistle, flute and shehnai are some common wind instruments.

8. How does the pitch of a sound depend on its frequency?Ans. The pitch of sound depends on the frequency of vibration. The higher

the frequency the more shrill is the sound and vice versa.9. How are stringed musical instruments made to produce sounds?

Give two examples.Ans. Stringed instruments have taut strings mounted over specially

designed wooden frames. When the strings are plucked or struck orplayed with bow they vibrate to produce a musical sound of someparticular frequency. The pitch of the sound of a musical instrument


can be changed by altering its length. Guitar, sitar and violin areexamples of stringed musical instruments.

10. How do percussion instruments produce sounds? Give twoexamples.

Ans. In percussion instruments, we make a leather membrane vibrate bystriking it with a hammer or with our hands. Drum, tabla and dholakare the commonly used percussion musical instruments.

11. What steps can be taken to prevent noise pollution?Ans. Steps to prevent noise pollution are as follows :

(i) Automobiles should be fitted with silencers and soft horns.(ii) We should not play radio, television and stereo system too

loudly.(iii) The horns of motor vehicles should not be blown unnecessarily.(iv) Ear plugs shold be used by bus or train drivers, mechanics,

workers in a factory, etc.(v) Airports and noise-making factories should be shifted away

from the residential areas of the city.12. Describe an experiment to prove that sound cannot travel in vacuum.

Ans. To prove that sound cannot travel in vacuum. We can perform asimple experiment to demonstrate this fact.ExperimentConnect the bell to the battery so thatit starts ringing. Arrange the bell jararound the bell and connect it to avacuum pump. Start the vacuum pumpso that it starts evacuating air from thebell jar. Keep on observing the soundof the ringing bell all the time.We observe that we keep on hearingthe sound of the bell after the bell jaris kept over it. As the air inside the belljar is slowly evacuated, the soundbecomes dimmer and dimmer and after


sometime we cannot hear it at all. Wecan however still see the bell vibrating as before.We conclude that the sound of the bell reaches us through the air inthe bell jar. When the vacuum pump removes the air from the belljar, the sound does not reach us even though the bell is still vibrating.It follows that sound cannot travel through vacuum.

13. What is the condition required for a body to produce sound?Describe an experiment to prove it.

Ans. Vibration is a necessary condition for a body to produce sound.ExperimentTake a metallic ruler and pressits one end on a table with theleft hand and pull down the freeend of the ruler with the righthand and let it go, what do weobserve?The metallic ruler is seen vibrating and a humming sound is heard.The humming sound is produced by the to-and-fro motion of themetallic ruler; the vibrations produced by the vibrating body.

14. (a) What is sound and how is it produced?(b) Name some pleasant and unpleasant sound?(c) How many vocal cords are there in our voice boxes? Where is

the voice box situated?(d) What makes our vocal cords vibrate?(e) Do bees have a voice box? How do they produce sound?

Ans. (a) Sound is an energy which can be produced by the vibrations ina material body.

(b) Pleasant sound : Sound produced by musical instruments likesitar, flute, tabla, etc.Unpleasant sound : Sound produced by various machines,the bark of a dog, the sound of a supersonic aircraft etc.

(c) There are two vocal cords in our voice box. Voice box issituated in our throat.


(d) A narrow slit between the vocal cords makes our vocal cordvibrate.

(e) Bees have no voice boxes. They make sound by moving theirwings up and down very fast.

15. Taking the example of tuning fork, explain how a vibrating bodyproduces longitudinal waves in air.

Ans. When a tuning fork is put into vibrations, its prongs movealternatively inwards and outwards between the positions A and B.As it moves from A to B it compresses the layer of air in front of it.This layer because of elasticity, tries to regain its original size. Forthis it has to expand. When it does that, it compresses the next layerand so on. This in turn, compresses the next layer and the processis repeated. The compression therefore advances to the right fromlayer to layer.Now consider the instant when the prong moves from B to A. As itmoves this way it creates a partial vacuum behind it so that thelayer of air in contact with it expands. We call this a rarefaction.Again because of elasticity, it tries to regain its original size, ittherefore creates a partial vacuum behind it. The expansion of onelayer thus allows the next layer to expand and so on. In this way ararefaction moves to the right. When the prong vibrates, thecompressions and rarefactions follow each other and we have thelongitudinal waves travelling through air. Other vibrating bodies,like a ringing bell also produce similar compressions and rarefactionsin air before their sound reaches us.


16. Define the following :(i) Frequency, (ii) Time period, (iii) Amplitude and,

(iv) WavelengthAns. (i) Frequency : The number of complete oscillations made in

one second is known as the frequency of the oscillating body.It is measured in units of hertz (Hz).

(ii) Time period : The time taken to complete one oscillation is


known as the time period.(iii) Amplitude : The amplitude of an oscillating body is the

maximum displacement from its mean or normal position.(iv) Wavelength : The linear distance between two consecutive

particles of a vibrating medium in the same phase is called itswavelength. It is denoted by Greek letter λ (lambda).

17. Name and explain the main parts of human ear.Ans. The Human Ear : A vibrating body produces sound that needs a

medium (solid, liquid or gas) for its propagation. The medium isusually air. When the sound propagating through air reaches ourears we are able to hear it.Our ears are made up of three parts : (i) the outer ear, (ii) the middleear, and (iii) the inner ear. Now as we know, sounds produced makeair vibrate. These vibrations in the air make our ear drum vibratewhen they strike against it. This in turn makes some tiny delicatebones in our middle ear vibrate with the same frequency, as thesource of sound. The vibrations of these tiny bones are passed onto a liquid present in our inner ear. Here there are several nerveswhich send the message to our brain. When these messages reachour brain we are able to hear and recognise the sound.

18. (a) What do you understand by the term ultrasonic vibrations?(b) State three uses of ultrasonic vibrations.(c) Name three animals which can hear ultrasonic vibrations.

Ans. (a) Ultrasonic vibrations : The vibrations whose frequencies aregreater than 20,000 Hz are called ultrasonic vibrations.

(b) Uses of ultrasonic vibrations.(i) Ultrasonic vibrations are used in SONAR to measure the

depth of sea (or ocean) and to locate under water objectslike submarines, sea-rocks and shipwrecks.

(ii) It is used for scanning and imaging the position andgrowth of a foetus and presence of stones in the gallbladder and kidney.

(iii) It is used for homogenising milk in milk plants where fresh


milk is agitated with desired quantity of fat and powderedmilk to obtain toned milk.

(c) Bats can hear ultrasonic sounds having frequencies upto1,20,000 Hz. Some animals like dogs and dolphins can hearsounds having frequencies upto 40,000 Hz.

19. Why is flash of lightning seen first but thunder is heard sometimelater?

Ans. Flash of lightning seen first but thunder is heard sometime laterthis is because, light travels at a very high speed of 3 × 108 m/swhereas the sound travels in air at 330 m/s. Thus, whenever lightningtakes place it is seen at once whereas sound will take some time toreach us.

20. (a) What is an echo?(b) State two conditions necessary for hearing an echo.

Ans. (a) Echo : The repetition of the sound which is reflected from ahigh building or any such object is called an echo.

(b) Conditions necessary for hearing an echo.(i) There should be a high rise building or hill or wall or

well.(ii) Minimum distance between source of sound and reflecting

body should be 17 metre.21. Why is sound of a sitar different from sound of a drum?

Ans. Sound of a sitar is different from sound of a drum because of thequality of musical sound by virtue of which we can distinguishbetween two sounds of the same pitch and loudness produced bytwo different musical instruments.

22. What do you understand by the following terms:(a) Pitch, (b) Loudness (c) Timbre

Ans. (a) Pitch : Pitch is the effect or sensation produced in the braindue to the sound of some particular frequency. Pitch dependsupon the frequency of a vibrating body. It means that higherthe pitch more is the frequency and lower the pitch less is thefrequency.


(b) Loudness : Loudness is the characteristic property of a soundwhich distinguishes a feeble sound from a loud sound of thesame frequency. The loudness of sound depends upon theamplitude of the vibrating body. It means more the amplitude ,more is the loudness.

(c) Timbre : Timbre or sound quality is that characteristic by virtueof which we can distingish between two sounds of the samepitch and loudness produced by two different musicalinstruments or by different voices. For example when twomusician produce the same note on two different instrumentslike piano and harmonium we can easily distinguish between thetwo notes because of their sound quality.

23. How can we measure the depth of a sea by using “Sonar”?Ans. Sonar is an instrument based on the reflection of sound. The Sonar

sends out ultrasonic (high frequency) sound towards the bottom ofthe sea. The sound reflected from the sea-bed is received by it. Bymeasuring the time taken by the sound to return to the ship andknowing the speed of sound in sea-water, we can find the depth ofthe sea or ocean at that place.

24. What are (i) infrasonic vibrations, (ii) audible vibrations? Write theirrange.

Ans. (i) Infrasonic vibrations : The vibrations whose frequencies areless than 20 Hz are called infrasonic vibrations. Human earcannot hear these vibrations.

(ii) Audible vibrations : The vibrations whose frequencies arefrom 20 Hz to 20,000 Hz constitute the audible vibrations.Human ear can hear only these vibrations.

25. Which part of a musical instrument vibrates to produce sound in (i)drum, (ii) flute, (iii) violin, (iv) tuning fork?

Ans. (i) Drum : In drum we make a leather membrane vibrate bystriking it with a hammer or with our hands to produce sound.

(ii) Flute : In flute we blow air into it, the column of air inside theflute vibrates and produce sound.


(iii) Violin : In violin we draw a bow across the strings to put theminto vibrations to produce sound.

(iv) Tuning fork : We hit the tuning fork hard against a rubberpad and prongs of the tuning fork begins to vibrate to producethe sound.

26. (a) Name the unit for the measurement of loudness of sound?(b) What do you understand by the term noise pollution?(c) Suggest two ways of minimising noise pollution in

(i) home, (ii) surrounding, (iii) automobile(d) State four harmful effects of noise pollution on the workers

working in factories.Ans. (a) Loudness of sound is measured in decibels (dB). A sound of

about 50 to 80 decibels is considered as audible sound.(b) Unwanted, unpleasant and very loud sounds are called noise.

The running of automobiles, loudspeakers and music systemscause noise pollution.

(c) Ways of minimising noise pollution.(i) Home

(A) Television, radio and power music system should beplayed at low volume.(B) Electric generator should be provided with improvedand modified silencers.

(ii) Surrounding(A) Exploding crackers on various functions should beavoided.(B) Loudspeakers used in marriages and religious placesshould be played at low volume.

(iii) Automobile(A) Horns should not be blown unnecessarily.(B) Automobiles should be provided with improved andmodified silencers.

(d) Harmful effects of noise pollution in factories(i) Noise in the surroundings interfaces with speech and talk


with another person.(ii) A long exposure to noise pollution may result in the loss

of hearing to deafness.(iii) Noise pollution reduces concentration and results in the

loss of work efficiency.(iv) Noise produces headaches, irritability and nervous

tension.

X. Practice for numerical problems.1. The thunder of cloud is heard 9.8 s later than flash of lightning. If

the speed of sound in air is 330 m/s. Find the height of cloud.Ans. Time = 9.8 s

Speed of sound = 330 m/sHeight of cloud = Distance travelled by sound

= Speed of sound × Time= 330 m/s × 9.8 s= 3234 m

2. Two observers are stationed in two boats, 4.5 km apart. A soundsignal sent by one under water reaches the other after 3 s. What isthe speed of sound in water?

Ans. Distance between two observers = 4.5 kmTime = 3 s

Speed of sound in water =Distance between two observers

Time

=4.5 km

3 s = 1.5 km/s = 1500 m/s

3. An echo is heard from a distant cliff after an interval of 0.8 s. If thespeed of sound in air is 340 m/s, how far away is the cliff?

Ans. Time taken by the sound to reach the cliff = 0.8 s

2 = 0.4 s

Speed of sound in air = 340 m/sDistance of the cliff = Speed × Time


= 340 m/s × 0.4 s= 136 m

4. A strong sound signal is sent from a ship towards the bottom of thesea. It is received back after 1 s. What is the depth of the sea?Given that the speed of sound in water 1450 m/s?

Ans. Time taken by the signal to reach the bottom of the sea = 12

sSpeed of sound in water = 1450 m/s

Depth of the sea = Distance travelled by the signal= Speed × Time

= 1450 m/s × 12

s

= 725 m5. A boy stands at a distance of 175 metres from a high wall. He claps

his hands and hears the echo after one second. Calculate the speedof sound in air.

Ans. Distance of the boy from the wall = 175 m

Time taken by the clap sound to reach the wall = 12

s

Speed of sound in air =Distance

Time

=175 m

12

s = 350 m/s

6. During a thunderstorm, the thunder is heard 2 seconds after theflash of lightning is seen. If the speed of sound is 300 m/s, calculatethe distance at which the lightning takes place.

Ans. Time = 2 sSpeed of sound = 300 m/s

Distance = Speed × Time= 300 m/s × 2 s= 600 m

3 sound - the assam valley schoolassamvalleyschool.com/physics today/worksheets/7... · the...

Documents