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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 = 330 ms–1)
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- 765 Hz
- 800 Hz
- 838 Hz
- 885 Hz
Correct Option: C
According to Doppler's effect in sound 
Apparent frequency,
| n' = | n0 | |
| v - vs |
| = | (800) = | = 838 Hz | ||
| 330 - 15 | 315 |
The frequency of sound observer hears in the echo reflected from the cliff is 838 Hz.
