A source of sound of frequency v is moving towards a wall with speed u. Find the number of beats heard by a stationary observer on the ground when: (i) observer is between source and wall. (ii) source is between observer and wall.
(i) When observer is between source and wall, then source is approaching both observer and wall.
The apparent frequency of wave received by observer is,
The apparent frequency of wave incident on wall is,
As wall is stationary, therefore the frequency of wave reflected is same as that of wave incident in it. Thus the frequency of wave reflected by wall is v2. Now the observer receives a wave of frequency v1 from source and a wave of v2 after reflection from wall. Therefore the number of beats heard per second is,
(ii) When source is between observer and wall, then source is moving away from observer and approaching to wall.
The apparent frequency of wave received by observer is,
The apparent frequency of wave incident on wall is,
Therefore number of beats heard per second is,
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Zero.
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