Dear student,
Beats are the  periodic and repeating fluctuations heard in the intensity  of a sound when two sound waves of very similar frequencies  interfere with one another. The diagram below illustrates  the wave interference pattern resulting from two waves  (drawn in red and blue) with very similar frequencies. A  beat pattern is characterized by a wave whose amplitude is  changing at a regular rate. Observe that the beat pattern  (drawn in green) repeatedly oscillates from zero amplitude  to a large amplitude, back to zero amplitude throughout the  pattern. Points of constructive interference (C.I.) and  destructive interference (D.I.) are labeled on the diagram.  When constructive interference occurs between two crests or  two troughs, a loud sound is heard. This corresponds to a  peak on the beat pattern (drawn in green). When destructive  interference between a crest and a trough occurs, no sound  is heard; this corresponds to a point of no displacement on  the beat pattern. Since there is a clear relationship  between the amplitude and the loudness, this beat pattern  would be consistent with a wave that varies in volume at a  regular rate.