### Observer receding from the source wave sound

Observer receding rom the source Again, suppose that the bank alarm is emitting sound at a frequency of 500Hz. As we drive away from the bank at 22 m/s, what is the frequency that we hear? In this case, the wavelength is still the same as we computed in the case above, 0.686m. The bank is not moving, so the waves are neither compressed nor spread out. The velocity of the waves relative to the car is different, however. Since the car is moving away from the bank, it is as if the car is trying to outrun the sound waves coming from behind. The relative velocity, therefore, will be the speed of sound minus the speed of the car:

v' = 343 - 22 = 321 m/s

Lastly, we use these numbers to calculate the observed velocity:

f' = v' / L' = 321 / 0.686 = 468 Hz

When the observer is receding from the source, he or she hears a lower frequency than normal. Summary If the source of sound is moving, we will have a different wavelength than normal. It will be less than normal if the source is approaching and greater than normal if the source is receding. The amount that it will be greater or less by is always the distance that the source travels during one period. A moving source does not affect the velocity of the sound waves. If the observer is moving, the sound will appear to be moving at a different velocity relative to the observer. The velocity of sound for the moving observer will be 343 m/s plus the speed of the observer if the observer is approaching the source and 343 m/s minus the speed of the observer if the observer is receding. The motion of the observer does not affect the wavelength. If both the source and the observer are moving, then there will be both a different wavelength and a different velocity. Once the new wavelength and the new velocity are know, the velocity is divided by the wavelength to give the observed frequency.

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