Question

With what velocity an observer should move relative to a stationary source so that he hears a sound of double the frequency of source?

• A. Half the velocity of sound towards the source
• B. Double the velocity of sound towards the source
  
• C. Velocity of sound towards the source
• D. Velocity of sound away from the source

Answer 1

The correct option is C Velocity of sound towards the source


We know that:
$n^{\prime }=n\left(\frac{v-v_0}{v}\right)$

Here,
$n=$ frequency of the source
$v=$ velocity of sound.
$v_0=$ velocity of the observer.

$n^{\prime }=n\left(\frac{v-v_0}{v}\right)$

$2n=n\left(\frac{v-v_0}{v}\right)$

$v_0=-v=-$ (speed of the sound)

Negative sign indicates that observer is moving opposite to the direction of velocity of sound. So, the observer moves relatively to the velocity of sound towards the source.

Final Answer: (a)