Beats
See and hear what happens with tones of different frequencies are combined.
A single tone has a sound pressure that oscillates in a sinusoidal fashion. Tone 1 in the demonstration above has a frequency of 220 Hz, so the signal oscillates above and below zero 220 times every second. Tone 2 is similar but has a frequency of 208 Hz. For both tones, the amplitude of the oscillations is 0.4.
The times where the 208 Hz signal is above zero (positive) are not the same as they are for 220 Hz. As a result, when the two tones are played together, there are times when both signals are positive and add up constructively to give an amplitude of 0.8; there are other times when the signals are of opposite sign and add up destructively, cancelling to give an amplitude of zero. The figure shown above for “Both” illustrates this constructive and destructive interference.
When both signals are playing, the amplitude varies at a rate given by the difference in frequency between the two component signals — for this example, the difference is 12 Hz. This is heard as the rapid warble in the audio track. This is the beat frequency.