Beats in sound
Beating occurs when two tones with a small pitch difference are played at the same time. The waves of both tones then add up and influence each other. At some moments they amplify each other and at other moments they weaken each other. In the figure below, two tones (f1 and f2) with a small frequency difference are shown. In the bottom wave both tones are added up (f1 + f2). The occurring beating by the alternating amplifying and weakening of the combined signal is clearly visible. The frequency of the beating is exactly equal to the difference of the two tones (f2 - f1).
Beating of octaves
When two tones that lie approximately an octave apart from each other are played at the same time beating can also occur. In that case beating occurs from the addition of the higher octave and the first overtone of the lower octave. These are again, just like with 'ordinary beating', two frequencies which lie near at each other.