A siren emitting a sound of frequency $800 ext{ Hz}$ moves away from an observer towards a cliff at a speed of $15 ext{ ms}^{-1}$. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is (Take, velocity of sound in air $= 330 ext{ ms}^{-1}$) [NEET 2016]

Question

A siren emitting a sound of frequency $800	ext{ Hz}$ moves away from an observer towards a cliff at a speed of $15	ext{ ms}^{-1}$. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is (Take, velocity of sound in air $= 330	ext{ ms}^{-1}$) [NEET 2016]

Aicharya · Accepted Answer

The given situation can be shown as below. [An observer stands behind a source moving at $15	ext{ ms}^{-1}$ towards a cliff.] Frequency of sound that the observer hear in the echo reflected from the cliff is given by $f' = \left(\frac{v}{v - v_s}ight)f$ where, $f =$ original frequency of source, $v =$ velocity of sound and $v_s =$ velocity of source. So, $f' = \left(\frac{330}{330 - 15}ight) 800 = 838.09	ext{ Hz} \approx 838	ext{ Hz}$

A siren emitting a sound of frequency $800\text{ Hz}$ moves away from an observer towards a cliff at a speed…

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