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Doppler Effect

Doppler Effect. Moving Source. Sound moves equally in all directions from a source. Circular (or spherical) pattern If the source is moving the origin of successive circles moves. Wavelength Shift. The normal period is T and the observer moves at speed u .

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Doppler Effect

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  1. Doppler Effect

  2. Moving Source • Sound moves equally in all directions from a source. • Circular (or spherical) pattern • If the source is moving the origin of successive circles moves.

  3. Wavelength Shift • The normal period is T and the observer moves at speed u. • The wavelength ahead of the source is shorter. • l’= l - uT • The wavelength behind the source is longer. • l’= l + uT

  4. The period is related to the wavelength. This can be converted into a frequency shift. Frequency Shift Insert + for – to get the shift behind the source.

  5. An ambulance is approaching at 130 km/h with a siren at 1.2 kHz. What pitch do you hear? Change speed to m/s (130 km/h)/3.6 = 36 m/s Find the Doppler shift. u/v = (36 m/s)/(343 m/s) u/v = 0.105 f’ = f/(1-1.05) = 1340 Hz Ambulance

  6. Moving Observer • If the observer is moving toward the sound the effect is similar, but the wave crests have a different relative spacing. A speed gun uses a double Doppler shift. There is a shift for both the pulse out and reflected signal.

  7. A moving source can exceed the speed of sound. The sound waves constructively interfere on a front at an angle to the motion. This is called a shock wave. Shock Wave

  8. Sonic Boom • The speed of sound is called Mach 1. • Faster than that speed causes a shock wave – a sonic boom. next

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