A boy is walking away from a wall towards an observer at a speed of $1\, metre/sec$ and blows a whistle whose frequency is $680 Hz.$ The number of beats heard by the observer per second is (Velocity of sound in air $= 340\, metres/sec$)
Diffcult
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(d) Observer hears two frequencies
$(i)$ ${n_1}$ which is coming from the source directly
$(ii)$ ${n_2}$ which is coming from the reflection image of source
so, ${n_1} = 680\left( {\frac{{340}}{{340 - 1}}} \right)$ and ${n_2} = 680\left( {\frac{{340}}{{340 + 1}}} \right)$
$ \Rightarrow $ ${n_1} - {n_2} = 4 \, beats$
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