A source of sound of frequency `256Hz` is moving rapidly towards wall with a velocity of `5m//sec`. How many beats per second will be heard if sound travels at a speed of `330m//sec`.

Options

A. `7.8Hz`
B. `6.7Hz`
C. `3.9Hz`
D. zero

Answer

A

Explanation

The observer will hear two sounds, one directly from source and other from reflected image of sound.
Hence number of beats heard per second
`=((v)/(v-v_S))n-((v)/(v+v_S))n`
`=(2nv v_S)/(v^2-v_S^2)=(2xx256xx330xx5)/(335xx325)=7.8Hz`