When two sound waves of different frequency approach your ear, the alternating constructive and destructive interference causes the sound to be alternatively soft and loud - a phenomenon which is called "beating" or producing beats.A beat frequency is the combination of two frequencies that are very close to each other. The sound heard will fluctuate in volume according to the difference in their frequencies. One may often hear beat frequencies when objects vibrate.
fbeat = f2 − f1
where f1 and f2 are the frequencies of the two sources
A piano tuner will strike a key and then compare the note with a tuning fork. If the piano is slightly out of tune, he will be able to hear the beat frequency and then adjust the piano wire until it is at the same frequency as the tuning fork. If the piano is severely out of tune, it makes the job more difficult, because the beat frequency may be too fast to readily hear.