Two sound waves of similar amplitude and similar frequencies, if allowed to interfere, produce a phenomenon called beats, If the frequencies of the waves are
and
then the beat frequency is
and is heard as a sound that increases in intensity then decreases periodically.
The waves 1 and 2 are alternatively in and out of phase at i, ii, ii and iv. Notice also in the above diagram that the amplitude or maximum displacement of the result of adding the two waves,
is the sum of the two amplitudes
and
– constructive interference – and the minimum is zero – destructive interference.
The beat period is