A siren emitting sound of frequency `800Hz` is going away from a static listener with a speed of `30 m//s`. frequency of sound to be heard by the listener is: `("velocity of sound" = 330 m//s)`

Options

A. 481.2 Hz
B. 286.5 Hz
C. 733.3 Hz
D. 644.8 Hz

Answer

C

Explanation

The apparent change in the frequency of the source due to relative motion between source and observer is known as Doppler's effect. The perceived frequency `(n')` when listerner is static and source is moving away is given by
`n' =n((v)/(v+v_S))`
Where `n` is frequency of source `v` is velocity of sound and `v_S` is velocity of source.
putting `v=330m//s`, `v_S=30 m//s, n=800Hz`.
`n' =800xx((330)/(330+30))`
`n' =733.3Hz`
In the limit when speed of source and observer is much lesser than that of sound `v_1`, the change in frequency becomes independent of the fact whether the source is moved or the detector.