Waves - Result Question 69
71. 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 :
[2016]
(Take velocity of sound in air $=330 ms^{-1}$ )
(a) $765 Hz$
(b) $800 Hz$
(c) $838 Hz$
(b) $885 Hz$
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Answer:
Correct Answer: 71. (c)
Solution:
- (c) According to Doppler’s effect in sound
Apparent frequency,
$ \begin{aligned} & n^{\prime}=\frac{v}{v-v_S} n_0 \\ & =\frac{330}{330-15}(800)=\frac{330 \times 800}{315}=838 Hz \end{aligned} $
The frequency of sound observer hears in the echo reflected from the cliff is $838 Hz$.
