Consider two sounds with nearly equal frequencies. In the example shown here, there are frequencies of 10 Hz and 12 Hz. Remember superposition? When these two sounds are added together -- heard at the same time -- these two sound waves will be "in phase" and add constructively for a moment and then they will be "out of phase" and add destructively. The resulting sound wave will increase in volume and then decrease in volume. The frequency of this wavering soft and loud sound will be equal to the difference in the two frequencies. In the example shown here, that is 2 Hz.
Count the waves in this example and see that this is the case.
The beat frequency is the difference in the two frequencies that are being combined.
The beat frequency is the frequency of the increasing and decreasing sound.
As two sounds are tuned to the same frequency, the beat frequency goes to zero and the beats are not heard. This can be useful in tuning musical instruments.
(C) 2003, Doug Davis; all rights reserved