Theory

Doppler Effect

When a source of sound and a listener are in relative motion to each other, the apparent frequency received by the listener is different from the source frequency. This apparent change in frequency observed by the listener which occurs due to relative motion between the source of sound and the listener is called Doppler Effect. The apparent change in the frequency can be explained as follows:

A. When the source moves towards stationary listener.

Let us consider the source of sound S moving with velocity V_s towards stationary listener L (V_L = 0). The source emits a sound wave with frequency fs and wavelength λ=v/f_s . v is the velocity of sound wave emitted by the source relative to the surroundings.

Consider S be the initial position of the source. Let t be the time taken by the sound wave emitted by the source to reach L from S. During this time, the source moves to point S`.