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 to be 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 Dopplers principle $n'=\frac{v}{v+{{v}_{s}}}n$ or $n'=\left( \frac{330}{330+50} \right)\times 500$ or $n=\frac{330\times 500}{380}=434.2\,\,Hz$