Beats occur when two waves of slightly different frequencies $f_1$ and $f_2$ travel in the same direction and superimpose.

• At some points, they are in phase (constructive), and at others, they are out of phase (destructive).
• This produces an 'envelope' wave whose intensity pulses (beats) periodically.
• The frequency of these pulsations is given by the change in relative phase, which is $f_{beat}=|f_1-f_2|$.