A and B are two sources generating sound waves. A listener is… | JEE Main 2013 (22 Apr, Shift 1)

Q72JEE Main 2013 · 22 APR · SHIFT 1Medium

Doppler Effect

A and B are two sources generating sound waves. A listener is situated at C. The frequency of the source at A is 500 Hz. A, now, moves towards C with a speed 4 m/s. The number of beats heard at C is 6. When A moves away from C with speed 4 m/s, the number of beats heard at C is 18. The speed of sound is 340 m/s. The frequency of the source at B is:

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Answer: (c)

A approaching C: $f_A = 500 \times \frac{340}{340-4} = 500 \times \frac{340}{336} \approx 506$ Hz
A receding from C: $f_A' = 500 \times \frac{340}{344} \approx 494$ Hz
Beats: $|506 - f_B| = 6$ and $|494 - f_B| = 18$
Both hold only for $f_B = 512$ Hz.
So the answer is (c) 512 Hz.

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Doppler Effect question from JEE Main 2013 (22 Apr, Shift 1)

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