A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of `15ms^-1`. Then the frequency of sound that the observer hears in the echo reflected from the cliff is (Take velocity of sound in air `=330ms^-1`)

Options

A. `800Hz`
B. `838Hz`
C. `885Hz`
D. `765Hz`

Answer

B

Explanation

Here, `v=800Hz, upsilon_(s)=15m//s, v^(')=?`
For the echo, cliff acts as the source of sound moving towards the observer as shown in Fig,
`:.` Apparent frequency,
`v^(')=(upsilonv)/(upsilon-upsilon_(s))=(330xx800)/(330-15)=(330xx800)/(315)`
`v^(')=838Hz`