Question

Two sirens situated one kilometer apart are producing sound of frequency $330$$Hz$. An observer starts moving from one siren to the other with a speed of $2$ m/s. If the speed of sound to be $330$$m/s$, what will be the beat frequency heard by the observer?

• A. 8
• B. 4
• C. 6
• D. 1

Answer 1

The correct option is B 4

Observer is moving away from siren 1 and towards the siren 2




Hearing frequency of sound emitted by siren 1
$n_1=n\left(\frac{v-v_{\circ }}{v}\right)=330\left(\frac{330-2}{330}\right)=328Hz$Hearing frequency of sound emitted by siren 2
$n_2=\left(\frac{v+v_{\circ }}{v}\right)=330\left(\frac{330+2}{330}\right)=332Hz$
Hence, beat frequency $=n_2-n_1$
$=332-328=4$