When the source of sound and the listener have a high relative velocity between them the pitch or frequency of the sound heard by the listener goes on changing. When the distance between the two decreases the pitch increases and vice versa. This motion related frequency change is called Doppler Effect
The apparent change in frequency due to relative motion between the source and the listener is called Doppler Effect.
A station master hears a continuous low intensity sound of the horn of a diesel locomotive fast approaching the station. As it is a pass through station where the train has no designated halt, the station master shows the green flag. The pitch of the sound of the horn increases fast as the locomotive approaches the station and decreases fast as it passes though the station even though horn is emitting a sound with a constant pitch.
The relative velocity between the source of sound and the listener can be dealt in the following three ways.
1. Source moving and observer stationary
2. Observer moving and source stationary
3. Both observer and source moving.
and Vs represents the velocities of a listener and a source respectively. Let V
be the velocity of sound and n and n1 be the true and apparent frequencies of the
sound. Then if
i) The source alone is in motion towards the observer,
ii) The source alone is in motion away from the observer
iii) The observer alone is in motion towards the source
iv) The observer alone is in motion away from the source
v) The source and the observer both are in motion towards each other
Vi) The source and the observer both are in motion away from each other
vii) The source and the observer both are in motion source following the observer
viii) The source and the observer both are in motion observer following the source
ix) The source, observer and the medium all are moving in the same direction as the
x) The source and the observer are moving in the direction of the sound but the
Direction of wind is opposite to the direction of the propagation of sound,