Question

Obtain the expression of Apparent frequency when observer moves towards the stationary observer.

Answer 1

Consider a source produces sound of frequency f. If v is the speed of sound in air, then the wavelength of the sound wave

λ = v/f

Now suppose the observer O moves towards stationary source with a speed of v_O. The speed of sound relative to observer

v' = v + v_O  

As the source is stationary, so wavelength of the sound wave remain same. Because of the increased relative speed of the sound, the frequency appears to increase . Thus apparent frequency

If the observer moves away from the stationary source, then

General Doppler effect equation: 

The apparent frequency can be written as ;