Question

An observer is riding on a bicycle and moving towards a hill at $18km{h}^{-1}$. He hears a sound from a source at some distance behind him directly as well as after its reflection from the hill. If the original frequency of the sound as emitted by source is $640Hz$ and velocity of the sound in air is $320m/s$, the beat frequency between the two sounds heard by observer will be _______Hz.

Answer 1

Answer: 20

$V_S=0,V_{\text{ob }}=5m/s$
$f_{\text{direct }}=\left(\frac{320-5}{320}\right)640=630Hz$
$f_{\text{reflected }}=\left(\frac{320+5}{320}\right)640=650Hz$
$f_{\text{beat }}=650-630=20Hz$