Beats are formed when two sound waves of similar frequency and amplitude interfere.
They are called beats because the resulting sound is a single frequency sound that varies in amplitude or loudness periodically, typically several times a second. Musicians may use them to tune their instruments. As an instrument is tuned to a note, the beat frequency decreases. When the instrument is tuned correctly, the beats disappear.
Beats may be illustrated by adding waves of similar frequency as shown below.
If the frequencies of of the sounds creating the beats are
and
then the frequency of the sound heard is
while the beat frequency is
The beat frequency can be derived from the graphs by reflecting that the beat period is the time for one wave to undergo one more oscillation than the other. In one second, the waves will undergo andcycles respectively, so one will undergo one oscillation more than the other
times per second.
In fact the beat phenomena is not confined to sound. The Sun causes two tides a day on the Earth as the Earth rotates, and the moon causes two tides every twenty five hours. The similar periods (and frequencies) imply beats. When Sun and moon are aligned or in phase we get spring (large) tides and when they are out of phase we get neap (small) tides.