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. 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

Answer 1

Answer: (A)

Velocity of sound towards the source

Answer 2

By using $n^{'} = n\left( \frac{v}{v - v_{S}} \right)$

$2n = n\left( \frac{v - v_{O}}{v - 0} \right)$ ⇒ $v_{O} = - v = -$ (Speed of sound)

Negative sign indicates that observer is moving opposite to the direction of velocity of sound, as shown

($n^{'} = n\left( \frac{v}{v - v_{S}} \right)$

$2n = n\left( \frac{v - v_{O}}{v - 0} \right)$ ⇒ $v_{O} = - v = -$