Question

A siren emitting a sound of frequency $800\, Hz$ moves away from an observer towards a cliff at a speed of $15\,m\,s^{-1}$. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is : (Take velocity of sound in air = $330 \, ms^{-1}$)

• A. 800 Hz
• B. 838 Hz
• C. 885 Hz
• D. 765 Hz

Answer 1

Answer: (B)

838 Hz

Answer 2

The correct option is(B): 838 Hz.

$n' = \frac{v}{v - v_s} n_0$
$n' = \frac{330}{330 - 15} (800) = \frac{330 \times 800}{315} = 838 \, Hz$