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 `2m//s`. If the speed of sound be `330 m//s`, what will be the beat frequency heard by the observer?

Options

A. `8`
B. `4`
C. `6`
D. `1`

Answer

B

Explanation

Observer is moving away from siren 1 and towards the siren 2.
Hearing frequency of sound emitted by siren 1
`n_1=n((v-v_0)/(v))=330((330-2)/(330))=328Hz`
hearing frequency of sound emitted by siren 2
`n_2=n((v+v_0)/(v))=330((330+2)/(330))=332Hz`
Hence, beat frequency `=n_2-n_1=332-328=4`.