What frequency sound would be observed by observer at rest is source is moving away from him at speed of sound? And Why so?
This can be derived from the doppler frequency shift equation with reference to a stationary observer.
f = f'(v/v+v')
- v is the speed of sound in a medium
- v' is the speed of the source
- f' is the source frequency
- f is the apparent frequency
if the speed of the source (v') equals the speed of sound (v) then the equation folds down to:
f = f'(v/2v) = f'/2
Half the original frequency. If you listen to the sound that is happening 1 second later, it arrives with an additional delay of 1 second due to the sound propagation. So the phase moves forward at half the original speed.