Originally Posted by **skeeter** The graph shows the period of the combined sinusoids, from which the frequency can be determined. |

As per on the definitions:

This resulting particle motion is the product of two travelling waves. One part is a sine wave which oscillates with the average frequency fa = ½(f1 + f2). This is the frequency which is perceived by a listener. The other part is a cosine wave which oscillates with the difference frequency fd = ½(f1 - f2). This term controls the amplitude "envelope" of the wave and causes the perception of "beats". The beat frequency is actually twice the difference frequency, fbeat = (f1 - f2)."

The frequecny that you showed as far as I can tell corresponds to the difference frequency fd but I cannot find any trace of the average frequency fa