Rapid changes in loudness, known as beats, occur when two tones very close in frequency are heard at the same time. The number of beats is equal to the difference in two nearly similar frequencies.

 

This "line drawing" shows how two patterns of slightly different spacing show the phenomena of beat interference. The pattern is best seen when you look from a distance of about a meter.

 

 

When two tuning forks of similar frequencies are played simultaneously, the frequency of the beats, beat frequency, is the different between the two frequencies.

 

beat frequency = |f₂ - f₁|

 

While the pitch at which those beats are heard is the average of the two frequencies.

 

beat pitch = ½(f₂+f₁)