Question

A sound source $S$ is moving along a straight track with speed $v,$ and is emitting sound of frequency $v_0$ (see figure). An observer is standing at a finite distance, at the point $O,$ from the track. The time variation of frequency heard by the observer is best represented by :
$(t_0$ represents the instant when the distance between the source and observer is minimum)

• A.
• B.
• C.
• D.

Answer 1

Answer: (D)

$f_{\text{observed }}\Rightarrow \left(\frac{v_{\text{sound }}}{v_{\text{sound }}-v\cos \theta }\right)f_0$
initially $\theta$ will be less $\Rightarrow \cos \theta$ more
$\therefore f_{\text{observed }}$ more, then it will decrease.