The result of combining two sounds less than 30 Hz or so apart in frequency together, beating is the alternate reinforcement and cancellation of amplitude in the combination sound (over around 30 Hz in difference results in an rougher “out of tune” sound, rather than distinct beats). Most easily heard when the original sounds are of equal volume, the frequency of the beats will be the difference in frequency between the two signals. Beats are common in most musical instruments, and are often used for tuning; when the instruments are in tune, beating disappears. When complex sounds are combined, beating occurs between various partials in the signals – listen to a piano for a good example of this. Beating is not restricted to musical instruments, it can occur between any two signals or sounds. A good example: We were recently subjected to an extended airplane trip in a small twin engine prop-driven plane. The pilot never did manage to completely “tune” the two engine’s speeds to where they were completely beat-free…