question_answer

With what velocity an observer should move relative to a stationary source so that he hears a sound of double the frequency of source                                                [MP PMT 1991]

A)            Velocity of sound towards the source

B)            Velocity of sound away from the source

C)            Half the velocity of sound towards the source

D)            Double the velocity of sound towards the source

Correct Answer: A

Solution :

           By using \[{n}'=n\,\left( \frac{v}{v-{{v}_{S}}} \right)\]                    \[2n=n\,\left( \frac{v-{{v}_{O}}}{v-0} \right)\] Þ \[{{v}_{O}}=-v=-\](Speed of sound)                    Negative sign indicates that observer is moving opposite to the direction of velocity of sound, as shown