An acoustic beat is a sound phenomenon that occurs when two sound waves of close frequencies are emitted simultaneously. The combination of sounds is manifested by regular variations in the pitch of the sound, called beats. These pitch variations are caused by constructive and destructive interference between the two sound waves. Acoustic beats are usually audible when the two sound waves are very close in frequency, but they can also be seen in other types of waves, such as light waves. The formula for calculating beat frequency is: beat frequency = wave 1 frequency - wave 2 frequency. Acoustic beats are used in various fields such as medicine, metrology and acoustic engineering. The discovery of acoustic beats is attributed to Christian Doppler, who published his theory in 1842.