# A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15 ms-1. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is :(Take velocity of sound in air = 330 ms-1) Option 1) 765 Hz Option 2) 800 Hz Option 3) 838 Hz Option 4) 885 Hz

As discussed in

Frequency of sound when observer is stationary and source is moving towards observer -

$\nu {}'= \nu _{0}.\frac{C}{C-V_{s}}$

- wherein

$C=$ speed of sound

$V_{s}=$ speed of source

$\nu _{0}=$ original frequency

$\nu {}'=$ apparent frequency

Apparent frequency,

$V'=\frac{V}{V-V_{s}}V_{0}$

$=\frac{330}{330-15}V_{0}=838 Hz$

Option 1)

765 Hz

This option is incorrect.

Option 2)

800 Hz

This option is incorrect.

Option 3)

838 Hz

This option is correct.

Option 4)

885 Hz

This option is incorrect.

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