If we pluck the string of If we pluck the string of VeenaVeena, it , it vibrates and produces soundvibrates and produces soundvibrates and produces sound. vibrates and produces sound. 

In In TablaTabla when we beat the stretched when we beat the stretched membrane it vibrates producingmembrane it vibrates producingmembrane it vibrates producing membrane it vibrates producing sound. Hence sound is producedsound. Hence sound is produced

by vibrating bodies.by vibrating bodies.

Sound may be or may Sound may be or may not be pleasing to not be pleasing to hear. Pleasing sound hear. Pleasing sound i ll di ll d ii ddis called is called musicmusic and and unpleasant sound isunpleasant sound isunpleasant sound is unpleasant sound is calledcalled noisenoisecalled called noisenoise..

Sound waves Sound waves areare longitudinallongitudinalwaves in a medium which producewaves in a medium which producewaves in a medium which produce waves in a medium which produce sensation of hearing.sensation of hearing.

We can only hear the sound of We can only hear the sound of frequencies ranging from 20Hz to frequencies ranging from 20Hz to 20 000Hz called20 000Hz called audible rangeaudible range20,000Hz. called 20,000Hz. called audible rangeaudible range..

• Sounds with frequencies above 20 000Hz are called ultrasonic20,000Hz. are called ultrasonic. Animals like dogs can hear gultrasonic. Sounds with frequencies below 20Hz are called Infrasonicare called Infrasonic. Whales and elephantsWhales and elephants produce infrasonic.p f

How does the sound propagate ?How does the sound propagate ?How does the sound propagate ?How does the sound propagate ?

• When an object vibrates, the particles of the medium in contact with the object arewith the object are

first displaced from their equilibrium positions.

• Then each displaced particle exert a force on its adjacent particle As a result theparticle. As a result, the adjacent particles get displaced j p g pfrom  their equilibrium positions.

• After displacing the adjacent particles, the particles which

are first displaced, come back to their original positions Thistheir original positions. This process continues in the

medium until the sound reaches our ears.

• The particles of the medium has to oscillate for the propagation of sound Forsound. For

this to happen the medium mustthis to happen, the medium must possess both mass(Inertia) and elasticity, which determine

th d f d i dithe speed of sound in a medium.

V=√E/ρ

V= Speed of the sound in the mediummedium.E= elasticity of the medium;E= elasticity of the medium;ρ = density of the medium=ρ   density of the medium  Density of the medium

•• Sound propagates through a Sound propagates through a medium at a finite speedmedium at a finite speed..

•• The sound of a thunderThe sound of a thunder is heardis heard aa•• The sound of a thunder The sound of a thunder is heard is heard a a little later than the flash of light is little later than the flash of light is seen. seen. 

•• SoSo we can make out thatwe can make out that soundsound•• SoSo, we can make out that , we can make out that sound sound travels travels with a speed which is with a speed which is lesser than the speed of light.lesser than the speed of light.

•• The speed of The speed of sound depends sound depends upon the properties of theupon the properties of theupon the properties of the upon the properties of the medium through which it medium through which it ggtravels. travels. 

•• The The speed speed of sound of sound in general in general increases from gaseous stateincreases from gaseous stateincreases from gaseous state increases from gaseous state to solid state. In any medium, to solid state. In any medium, if if the temperature the temperature increases increases the speed of sound increasesthe speed of sound increasesthe speed of sound increases.the speed of sound increases.

Speed of sound varieswithwith……..1‐temperature,2‐pressure,3‐density,y,4‐humidity`

Solids: Speed in ms‐1p

Solids Aluminium 6420Solids Aluminium                  6420Nickel                                        6040Steel                                          5960Iron 5950Iron                                            5950Brass                                         4700

Liquids : Speed in ms‐1

Glycerol                                 1904

Water (Sea) 1531Water (Sea)                           1531

M 1450Mercury                                  1450

Kerosene                                 1324

Gases : Speed in ms‐1

Gases Hydrogen                1284y g

Helium                                      965

Air                                346

Oxygen                                   316yg

Sulphur dioxide             213p

Intensity of sound (I) : It is defined asthe average energy transportedthe average energy transportedby the wave in one second across unit area

d i di l haround a point, perpendicular to thedirection of propagation.It depends upona)amplitude) pb) frequency c)density of the medium andc)density of the medium and d) speed of the wave.

Loudness of Sound : It is thedegree of sensation produced bythe sound in the ear of the listener.t e sou d t e ea o e s e eIt depends on intensity of thesound and sensitivity of the ear ofsound and sensitivity of the ear ofthe listener.If the intensity of the sound ismore its loudness is moremore, its loudness is more.If the intensity of the sound is less,its loudness is less.

Pitch of Sound :It is the degree of shrillness orgraveness (dullness) produced by thegraveness (dullness) produced by thesound in the ear of the listener.i l d i fIt is related to its frequency.

A sound of higher frequency is calledg q yhigh pitch sound or shrill sound.A sound of lower frequency is called lowA sound of lower frequency is called lowpitch sound or grave sound.

Examples : Pitch of the soundExamples : Pitch of the soundproduced by a bird is more than thepitch of roarof a lion Pitch of sound produced byof a lion. Pitch of sound produced bya child or women is more than thepitch of sound produced by a man.

Reflection of Sound :Peep into a very deep well and shoutloudly You hear your own voice afterloudly. You hear your own voice aftera few seconds. The sound waves reachh d b kthe water and come back to you. Herethe sound bounces off the surface ofwater, like a rubber ball bounces off awallwall.

The Reflection of LightWhy are we able to see Why are we able to see

ourselves from mirror?

Such phenomenon is calledreflection of Sound. Here the angleof incidence is equal to the angleof incidence is equal to the angleof reflection as is in the case oflight.Note : “SONAR” works on theNote : SONAR works on theprinciple of reflection of soundwaves

The Principle of Superposition of Waves :Suppose two waves travel at the sameSuppose two waves travel at the same time along the same stretched string. Let Y1 and Y2 be displacements of the tring caused by two waves whenthe tring caused by two waves when each one of them acted alone. The resultant displacement Y of the string is algebraic sumis algebraic sumof Y1 and Y2. 

This is called principle of superposition i e Y = Y + Y.i.e.,... Y = Y1 + Y2

Standing Wave Pattern Animation (SWR).avi

STATIONARY WAVES.Consider a string stretched between twoclamps. When it is plucked, there produces aclamps. When it is plucked, there produces awave in the string which moves to other endand gets reflected Here incoming wave andand gets reflected. Here incoming wave andreflected wave superpose on each other and

d hi hl l li dproduce a highly localized wave patternwhich doesn’t move either the side. ‐‐‐‐‐‐‐‐stationary wave or standing wave.

In this case, different points on the, ff pstring vibrates with different

lit d P i t f lit damplitudes. Points of zero amplitudeare called nodes. Points ofare called nodes. Points ofMAXIMUM amplitude are called

d h l lantinodes. They appear alternativelyon the string.on the string.

Properties of a stationary wave (1)

• Stationary waves have nodes where there is noStationary waves have nodes where there is no displacement at any time.

• In between the nodes are positions called antinodes, p ,where the displacement has maximum amplitude.

Nodes Antinodes

λ

BEATS : Take two tuning forks ofsame frequency and load one of thetuning forks with little amount of waxtuning forks with little amount of waxto decrease its frequency slightly.q y g yVibrate the two tuning forks by gentlyhitti th bb d Plhitting them on a rubber pad. Placethe stems of the vibrating tuning forksg gon a table.

You will hear increase andYou will hear increase anddecrease in intensity ofysound alternatively with

l i t l f tiequal intervals of time.These are called beats.

Doppler Effect

Doppler Effect and light coming from Celestial B di (i O Shift T Shift R d Shift BlBodies (i.e. One Shift, Two Shift, Red Shift, Blue 

Shift)

Doppler Effect : Consider a listenerstanding in a railway platform. Whenthe whistling train is approaching him,the whistling train is approaching him,he experiences an increase in thef f d Wh th t i ifrequency of sound. When the train ismoving away from him, he experiences adecrease in the frequency of sound. Suchmotion related frequency change ismotion related frequency change iscalled “Doppler effect”.

Doppler Effect Animation.avi

Doppler effect is symmetric in lightwhereas it is asymmetric in sound Itwhereas it is asymmetric in sound . Itis due to the fact that the speed ofth b t b l t dthe observer cannot be neglectedcompared to the small speed of thesound .

Here frequency of the source will notchange Frequency of the sound as heard bychange. Frequency of the sound as heard bythe listener only changes which is called anapparent change In general Doppler effectapparent change. In general, Doppler effectis an apparent change in pitch or frequencyof sound when there is a relative motionof sound, when there is a relative motionbetween the source of sound and thelistenerlistener.

Application of Doppler effect :1 Doppler effect in sound can be used to determine1. Doppler effect in sound can be used to determine the velocity of submarines.2. Doppler effect in light is used in tracking of artificial2. Doppler effect in light is used in tracking of artificial satellites.3. Doppler effect is used in the study of binary stars, Saturn rings, galaxies.4. Doppler effect is used to study red shift and 

i l iuniversal expansion.5.When we observe the sun we notice that there is red shift from one end and blue shift from the other endshift from one end and blue shift from the other end. =>the sun is rotating.

The quality of sound (Timbre)The quality of sound (Timbre)

• The quality of sound is determined by the following q y y gfactors:– The particular harmonics present in addition to the fundamental vibrationfundamental vibration,

– The relative amplitude of each harmonic,– The transient sounds produced when the vibration is started.

1st overtoneFundamental

resultant

1 overtone

2nd overtone

3rd overtone

M i t• Moving transverse waves:

Let’s share our answers

1. What are sound waves ?2. What is an audible range ?3 What are Ultrasonic ?3. What are Ultrasonic ?4. What are Infrasonic ?5. Name the sound waves produced in dog’s whistlesin dog s whistles.6. .............. is the medium in which sound travels fastest ?) Ai b) t ) B d) Al i ia) Air b) water c) Brass d) Aluminium

7. Define intensity of a wave at a point.

8 What is loudness of sound ?8. What is loudness of sound ?9. What is pitch of sound ?10. State Principle of Superposition.11 What is a stationary wave ?11. What is a stationary wave ?12. What are nodes and antinodes ?13. What are beats ?14 What is Doppler effect ?14. What is Doppler effect ?

Answers to the Questions1 Sound waves are longitudinal1. Sound waves are longitudinal waves in a medium which produce sensation of hearing.2 We can hear sounds of2. We can hear sounds of frequencies ranging from 20Hz to q g g20,000Hz. Which is called audible range.

3 Sounds with frequencies3. Sounds with frequencies above 20,000 Hz. are called ultrasonic's.4 Sounds with frequencies4. Sounds with frequencies below 20Hz. are infrasonics.5. Ultrasonic's.6 Al i i6. Aluminium.

7 It i d fi d th7. It is defined as the average energy transported g gy pby the wave in one second

iacross unit area surrounding the point,surrounding the point, perpendicular to the 

fdirection of propagation.

8 It is the degree of8. It is the degree of sensation produced by the sound in the ear of the listenerlistenerwhich depends upon p pintensity of the sound and 

iti it f th f thsensitivity of the ear of thelistener.ste e

9. It is the degree of shrillness or graveness produced by the sound in the ear of the listenersound in the ear of the listener related to its frequency.

10. Suppose two waves travel at10. Suppose two waves travel at the same time along the same stretched string. Let Y1 and Y2 be displacements of the string causeddisplacements of the string caused by two waves when each one of them acted alone. The resultant displacement Y of the string isdisplacement Y of the string is algebraic sum of Y1 and Y2. This is called principle of superposition.

11. Highly localized wave pattern hi h d i h id iwhich does not move either side is 

called a stationary wave.called a stationary wave.12. Nodes : They are points of zero amplitude in a stationary wave.Antinodes : They are points ofAntinodes : They are points of maximum amplitude in a stationary wave.

13. Periodic decrease and increase in the intensity of the 

d ll d bsound are called beats.14. In general, Doppler effect is14. In general, Doppler effect is an apparent change in pitch or frequency of sound, when there is a relative motion between theis a relative motion between the source of sound and the listener.