Question

A whistle produces 256 waves per second. If the velocity of sound towards the observer and its magnitude is one-third the velocity of sound in air then the number of wave received by the observer per second will be :

• A. 192
• B. 200
• C. 300
• D. 384

Answer 1

The correct option is D 384

Let velocity of sound be $v.$

Given: Velocity of source, $v_{source}=\frac{v_{sound}}{3}=\frac{v}{3}$

Also $n=256$ waves per sec OR $Hz$

Using Doppler effect when observer is at rest and source moves towards the observer:

$n^{\prime }=n[\frac{v_{sound}}{v_{sound}-v_{source}}]$

$n^{\prime }=256[\frac{v}{v-\frac{v}{3}}]⟹n^{\prime }=384$