When two waves of slightly different frequencies but similar amplitudes are produced simultaneously, the loudness increases and decreases periodically. This phenomenon is called the beat. For example: When two tuning fork of same or slightly different frequencies are struck on a pad and their stems are held in a table, a series of rise or fall of sound is observed.
The beats are formed due to the superposition by the interference of waves. The time interval between two successive loud sounds is called beat period and the number of beats per second is called beat frequency. If f1 and f2 are the frequency of two sound waves producing beats, then the beat frequency is given as:
fb = f1 – f2
So, number of beats produced per second is equal to the difference of frequencies of two superposing waves.
The persistence of hearing of human ear is 0.1 second. That means human ear can detect the repeated sound if it is 10 times per second. If the sound is repeated more than 10 times in a second, our ear cannot distinguish the beats as it exceeds our persistence of hearing.