Question

The driver of a car travelling with speed 30 m/sec towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is 330 m/s, the frequency of reflected sounds as heard by driver is :

• A. 555.5 Hz
• B. 720 Hz
• C. 500 Hz
• D. 550 Hz

Answer 1

The correct option is B 720 Hz

Frequency of sound when observer is stationary and source is moving towards observer -

$v=v_o\times \frac{C}{C-V_s}$

and

Frequency when observer is stationary and source is moving away from observer -

$v=v_o\times \frac{C}{C+V_s}$

$v=\frac{v}{v-30}\times v$

$v=\frac{v}{v-30}\times v=\frac{v+30}{v-30}\times v$

=$\frac{360}{300}\times 600=720Hz$

so (B) is the correct answer