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Asked in NEET 1992 · Beat frequency
Beats are the slow waxing and waning heard when two notes of slightly different frequency are sounded together.
Superposing cos 2π u₁t and cos 2π u₂t gives an oscillation at the mean frequency whose amplitude varies at | u₁- u₂|.
So the one essential condition is that the two frequencies differ, and differ only slightly enough for the ear to follow the variation.
Equal amplitudes only make the minima fall to exact silence; they are not needed for beats to occur.
A common phase is not needed either, since the phase difference between two unequal frequencies is changing all the time.
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