Question

A whistle sends out 256 waves in a second. If the whistle approaches the observer with velocity 1/3 of the velocity of sound in air, the number of waves per second the observer will receive

• A. 384
• B. 192
• C. 300
• D. 200

Answer 1

Answer: (A)

384

Answer 2

Wave number = $\frac{1}{\lambda}$

but $\frac{1}{\lambda'} = \frac{1}{\lambda}\left( \frac{v}{v - v_{s}} \right)\text{and}\mspace{6mu} v_{s} = \frac{v}{3}$

$\therefore$ (W.N.)’ = (W.N.) $\left( \frac{v}{v - v/3} \right) = 256 \times \frac{v}{2v/3}$

= $\frac{3}{2} \times 256 = 384$