Doppler EffectMoving SourceWavelength ShiftFrequency ShiftAmbulanceMoving ObserverShock WaveSonic BoomDoppler EffectDoppler EffectMoving SourceMoving SourceSound moves equally in all directions from a source.Sound moves equally in all directions from a source.•Circular (or spherical) patternCircular (or spherical) patternIf the source is moving the origin of successive If the source is moving the origin of successive circles moves.circles moves.Wavelength ShiftWavelength ShiftThe normal period is The normal period is TT and the and the observer moves at speed observer moves at speed uu..The wavelength ahead of the The wavelength ahead of the source is shorter.source is shorter.• ’’= = - - uTuTThe wavelength behind the source The wavelength behind the source is longer.is longer.• ’’= = + + uTuTFrequency ShiftFrequency ShiftThe period is related to the The period is related to the wavelength.wavelength.This can be converted into a This can be converted into a frequency shift.frequency shift.)/1()/(/vuvuvTvuffvufvfvfv/1)/1)(/(//Insert + for – to get the shift behind the source.AmbulanceAmbulanceAn ambulance is An ambulance is approaching at 130 km/h approaching at 130 km/h with a siren at 1.2 kHz.with a siren at 1.2 kHz.What pitch do you hear?What pitch do you hear?Change speed to m/sChange speed to m/s•(130 km/h)/3.6 = 36 m/s(130 km/h)/3.6 = 36 m/sFind the Doppler shift.Find the Doppler shift.•u/vu/v = (36 m/s)/(343 m/s) = (36 m/s)/(343 m/s)•u/vu/v = 0.105 = 0.105•f’f’ = = ff/(1-1.05) = 1340 Hz/(1-1.05) = 1340 Hzvuff/1Moving ObserverMoving ObserverIf the observer is moving toward the sound the effect If the observer is moving toward the sound the effect is similar, but the wave crests have a different relative is similar, but the wave crests have a different relative spacing.spacing.)/1( vuff A speed gun uses a double Doppler shift. There is a shift for both the pulse out and reflected signal.Shock WaveShock WaveA moving source can A moving source can exceed the speed of exceed the speed of sound.sound.The sound waves The sound waves constructively interfere on constructively interfere on a front at an angle to the a front at an angle to the motion.motion.This is called a This is called a shock shock wavewave..Sonic BoomSonic BoomThe speed of sound is called The speed of sound is called Mach 1.Mach 1.Faster than that speed Faster than that speed causes a shock wave – a causes a shock wave – a sonic boom.sonic
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