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NEET 2016 (Phase 2) · Beats

Beats question from NEET 2016 (Phase 2)

Q61NEET 2016 · PHASE 2Easy

Beats

Three sound waves of equal amplitudes have frequencies (n $-$ 1), n, (n + 1). They superimpose to give beats. The number of beats produced per second will be

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Answer: (d)

Adding the three waves: $y = A\sin(2\pi nt)\left[1 + 2\cos(2\pi t)\right]$
Amplitude $A(1 + 2\cos 2\pi t)$ repeats every 1 s; in each second $|1 + 2\cos 2\pi t|$ has two maxima (at $\cos = 1$ and $\cos = -1$ )
So 2 beats per second: the answer is (d) 2.

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