A sound wave of frequency `f` propagating through air with a velocity `c`, is reflected from a surface which is moving away from the source with a constant speed `v`. Find the frequency of the reflected wave, measured by the observer at the position of the source.

Answer

`(c - v)/(c + u) f`

Explanation

`f' = f = (c - v)/(c)` Doppler effect between `S` and `W`
`f" = f'(c)/(c + v)` Dopplar effect between `W` and `O`
`= f(c - v)/(c + v)`