A listener is at rest and a police siren is moving away from the listener at 60 ms - 1 . What frequency does…
A listener is at rest and a police siren is moving away from the listener at 60 ms - 1 . What frequency does the listener hear, given that the velocity of sound in air is 340 ms - 1 and frequency of siren is 500 Hz?
Solution
Sol. In this case, observer is at rest and source is moving away from observer.
Therefore, observed frequency,
$f' = \left(\frac{v}{v+v_s}\right) f = \left(\frac{340}{340+60}\right) 500$
$= \frac{340}{400} \times 500 = \frac{1700}{4} = 425\ \mathrm{Hz}$
Answer: 425 Hz