Question

A siren emitting sound of frequency $500\, Hz$ is going away from a stationary listener with a speed of $50\, m / s$, the frequency of sound heard directly from the siren is

• A. 286.5 Hz
• B. 481 Hz
• C. 434.2 Hz
• D. 580 Hz

Answer 1

Answer: (C)

434.2 Hz

Answer 2

Here: Frequency of siren $=500\, Hz$
The speed of siren $v_{s}=50\, m / s$
Using Doppler's principle
$n'=\frac{v}{v+v_{s}} n$
or $n'=\left(\frac{330}{330+50}\right) \times 500$
or $n'=\frac{330 \times 500}{380}=434.2\, Hz$