DOPPLER'S EFFECT
 
 
INTRODUCTION
 
  When a source of sound or a listener, or both are in motion relative to each other, the frequency and hence the pitch of sound, as heard by the listener is not the same as when the listener and source of sound are at rest. This phenomenon is referred to as 'The Doppler's effect'.
 
DEFINITION
 
The apparent change in the pitch or frequency of sound due to relative motion of source of sound and the listener is called 'DOPPLER'S EFFECT'.
 
EXPLANATION
 
  When a sounding body passes near a stationary person, a considerable change in the pitch of sound is detected. When the body is approaching, the pitch of the sound increases where as the pitch of sound decreases when the sounding body is moving away. A similar change in pitch also occurs when a moving listener passes a stationary source of sound.
All these apparent changes in the pitch of sound are referred to as 'DOPPLER'S EFFECT'.
 
DIFFERENT SITUATIONS
 
WHEN THE LISTENER IS MOVING AND THE SOURCE IS AT REST
 
WHEN THE LISTENER IS MOVING TOWARDS THE SOURCE OF SOUND IS AT REST
  Suppose the listener is moving towards a stationary source of sound. The speed of the listener is VO. Let the source emits sound waves of frequency u and wave length equal to l .
We know that:
 
l = V/u --------(1)
  Several wave crests separated by equal distance l are spread in all directions. The waves approaching the moving listener have a speed of propagation relative motion to the listener will be (V + VO).
Thus the frequency as heard by the listener is u'
 
u' = Relative velocity of sound/Wave length
 
  This expression shows that when a listener moves towards a stationary source of sound, he detects larger frequency and hence higher pitch as compared to original.
 
 
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