### 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) $885 Hz$

## Show Answer

#### 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 $838 Hz$.