Basic physics nature of sound doppler effect

Basic physics nature of sound doppler effect

1. Sound
UNIT -V
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2. Objectives
•Theobjectiveofthisunitistopresentthestudentthenatureof
soundwave,studentwillhaveanunderstandingof
•Thenatureandpropagationofsoundwave
•Transverseandlongitudinalwaves
•Interfaceofsoundwaves
•Beats
•Decibel
Objectives
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•Decibel
•Speedofsoundinmaterialmedium
•Intensityofsound
•Superimpositionofthewave
•Dopplereffectprinciple

3. The nature and
propagation of sound
wave
•Thenatureandpropagationofsoundwave
•Soundisproducedbyvibratingobjects.Thematterorsubstancethroughwhich
soundistransmittediscalledamedium.Itcanbesolid,liquidorgas.Sound
movesthroughamediumfromthepointofgenerationtothelistener.
•Whenanobjectvibrates,itsetstheparticlesofthemediumarounditvibrating.
Theparticlesdonottravelallthewayfromthevibratingobjecttotheear.Aparticle
ofthemediumincontactwiththevibratingobjectisfirstdisplacedfromits
equilibriumposition.Itthenexertsaforceontheadjacentparticle.
•Asaresultofwhichtheadjacentparticlegetsdisplacedfromitspositionofrest.
Afterdisplacingtheadjacentparticlethefirstparticlecomesbacktoitsoriginal
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Afterdisplacingtheadjacentparticlethefirstparticlecomesbacktoitsoriginal
position.Thisprocesscontinuesinthemediumtillthesoundreachesyourear
•Thedisturbancecreatedbyasourceofsoundinthemediumtravelsthroughthe
mediumandnottheparticlesofthemedium
•Awaveisadisturbancethatmovesthroughamediumwhentheparticlesofthe
mediumsetneighbouringparticlesintomotion.Theyinturnproducesimilar
motioninothers.Theparticlesofthemediumdonotmoveforwardthemselves,
butthedisturbanceiscarriedforward.Thisiswhathappensduringpropagationof
soundinamedium,hencesoundcanbevisualisedasawave.Soundwavesare
characterisedbythemotionofparticlesinthemediumandarecalledmechanical
waves

4. The nature and
propagation of sound
wave
•Thenatureandpropagationofsoundwave
•Airisthemostcommonmediumthroughwhichsoundtravels.Whenavibrating
objectmovesforward,itpushesandcompressestheairinfrontofitcreatinga
regionofhighpressure.Thisregioniscalledacompression(C),asshowninFig.
•Thiscompressionstartstomoveawayfromthevibratingobject.Whenthe
vibratingobjectmovesbackwards,itcreatesaregionoflowpressurecalled
rarefaction(R),asshowninFig.
•Astheobjectmovesbackandforthrapidly,aseriesofcompressionsand
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•Astheobjectmovesbackandforthrapidly,aseriesofcompressionsand
rarefactionsiscreatedintheair.Thesemakethesoundwavethatpropagates
throughthemedium.Compressionistheregionofhighpressureandrarefactionis
theregionoflowpressure
•Pressureisrelatedtothenumberofparticlesofamediuminagivenvolume.
Moredensityoftheparticlesinthemediumgivesmorepressureandviceversa.
Thus,propagationofsoundcanbevisualisedaspropagationofdensityvariations
orpressurevariationsinthemedium

5. •Longitudinal
waves •Longitudinalwaves
•Theregionswherethecoilsbecomecloserarecalledcompressions(C)andthe
regionswherethecoilsarefurtherapartarecalledrarefactions(R).Aswealready
know,soundpropagatesinthemediumasaseriesofcompressionsand
rarefactions.Now,wecancomparethepropagationofdisturbanceinaslinkywith
thesoundpropagationinthemedium.
•Thesewavesarecalledlongitudinalwaves.Inthesewavestheindividualparticles
ofthemediummoveinadirectionparalleltothedirectionofpropagationofthe
disturbance.Theparticlesdonotmovefromoneplacetoanotherbuttheysimply
oscillatebackandforthabouttheirpositionofrest.
•Thisisexactlyhowasoundwavepropagates,hencesoundwavesarelongitudinal
waves.Thereisalsoanothertypeofwave,calledatransversewave.Ina
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waves.Thereisalsoanothertypeofwave,calledatransversewave.Ina
transversewaveparticlesdonotoscillatealongthelineofwavepropagationbut
oscillateupanddownabouttheirmeanpositionasthewavetravels.
•Thusatransversewaveistheoneinwhichtheindividualparticlesofthemedium
moveabouttheirmeanpositionsinadirectionperpendiculartothedirectionof
wavepropagation.Lightisatransversewavebutforlight,theoscillationsarenotof
themediumparticlesortheirpressureordensity–itisnotamechanicalwave.You
willcometoknowmoreabouttransversewavesinhigherclasses

6. Interference of
sound waves
•Interference of sound waves
When two longitudinal (sound) waves meet at a point where the compression of
one wave coincides with the compression of the other wave and the rarefaction of
one wave coincides with the rarefaction of the other wave. Then the resultant
amplitude of a wave is maximum, or when compression of one wave falls on the
rarefaction of the other lock and vice versa. Then the amplitude of the resultant
wave is minimum. Then these effects are interference of longitudinal (sound)
waves. Interference of longitudinal
A new sound wave is produced whenever two or more sound waves from different
origins interact with one other at the same instant in time to create a new resultant
wave. This phenomenon, known as sound interference, causes the final wave to
total all previously existing waves.
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total all previously existing waves.

7. Interference of
sound waves
•Interference of sound waves
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8. Beats
•Beats
In acoustics, a beat isan interference pattern between two sounds of slightly
different frequencies, perceived as a periodic variation in volume whose rate is
the difference of the two frequencies.
A wave not only travels in space, but it also propagates through time, so if the two
waves can produce interference by overlapping in space, they should also produce
an interference pattern when they overlap in time; this phenomenon is called beats.
Beats occur when two waves of nearby frequencies overlap and create a new
resultant wave
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9. Decibel
•Decibel
Adecibel(dB)isaunitofmeasurementforsound.A-weighteddecibels,
abbreviateddBA,areanexpressionoftherelativeloudnessofsoundsinairas
perceivedbyourears.
A1dBchangeinasoundequatesto abouta26%differenceinsound
energy(rememberthata3dBdifferenceisadoublingofenergylevels).Interms
ofsubjectiveloudness,a1dBchangeyieldsjustovera7%change.A3dB
changeyieldsa100%increaseinsoundenergyandjustovera23%increasein
loudness
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10. Decibel
•Decibel
Safe and Unsafe Decibel Levels
•Noise above 70 dB over a prolonged period of time may start to damage hearing.
•Loud noise above 120 dB can cause immediate harm to ears
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11. Speed of sound
•Speed of sound
Speedofsoundisthespeedatwhichsoundwavesmovethroughthemediumslike
gas,liquid,solidandvacuum.Thespeedofsoundindryairis343m/s.
The speed of sound varies from substance to substance: typically, sound travels
most slowly in gases, faster in liquids, and fastest in solids. For example, while
sound travels at343 m/sin air, it travels at 1,481 m/s in water (almost 4.3 times as
fast) and at 5,120 m/s in iron (almost 15 times as fast).
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12. Sound Intensity
•Sound Intensity
Soundintensity,alsoknownasacousticintensity,isdefinedasthepowercarried
bysoundwavesperunitareainadirectionperpendiculartothatarea .TheSI
unitofintensity,whichincludessoundintensity,isthewattpersquaremeter(W/m
2
)
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13. •Superposition
of the waves
•Superposition of the waves
Theprincipleofsuperpositionmaybeappliedtowaves whenevertwo(ormore)
wavestravellingthroughthesamemediumatthesametime .Thewavespass
througheachotherwithoutbeingdisturbed.Thenetdisplacementofthemediumat
anypointinspaceortime,issimplythesumoftheindividualwavedisplacements
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14. •Doppler Effect
•Doppler Effect
Dopplereffectisdefinedastheincrease(ordecrease)inthefrequencyof
sound,light,orotherwavesasthesourceandobservermovetowards(or
awayfrom)eachother.
•TheDopplereffectisobservedwheneverthesourceofwavesismovingwith
respecttoanobserver.
•TheDopplereffectcanbedescribedastheeffectproducedbyamovingsource
ofwavesinwhichthereisanapparentupwardshiftinfrequencyforobservers
towardswhomthesourceisapproachingandanapparentdownwardshiftin
frequencyforobserversfromwhomthesourceisreceding.
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frequencyforobserversfromwhomthesourceisreceding.
•Itisimportanttonotethattheeffectdoesnotresultbecauseofanactualchangein
thefrequencyofthesource

15. •Doppler Effect
•Doppler Effect
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