A railway engine whistling at a constant freq | vedclass

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Question

(D) According to the Doppler effect,the apparent frequency $(n')$ heard by a stationary observer when a source of sound moves with a constant velocity

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(D) According to the Doppler effect,the apparent frequency $(n')$ heard by a stationary observer when a source of sound moves with a constant velocity $(v_s)$ is given by:$n' = n \left( \frac{v}{v \mp v_s} \right)$where $v$ is the speed of sound in air and $n$ is the actual frequency of the source.As the engine approaches the observer,the apparent frequency is $n_{app} = n \left( \frac{v}{v - v_s} \right)$,which is constant and greater than $n$.As the engine moves away from the observer,the apparent frequency is $n_{rec} = n \left( \frac{v}{v + v_s} \right)$,which is constant and less than $n$.Since the speed of the engine is constant,the frequency remains constant at a higher value while approaching and constant at a lower value while receding.Therefore,the graph shows a sudden drop in frequency as the engine passes the observer.

$A$ railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency $(n)$ of the sound heard by the observer is plotted against time $(t).$ Which of the following best represents the resulting curve?

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