You're asking about beats in sound, which are the periodic variations in loudness that occur when two sound waves of slightly different frequencies are played together.

Here's how to understand the frequency of beats:

* The beat frequency is the difference between the two original frequencies:

* Beat frequency = |f1 - f2|

Example:

If you have two sound waves with frequencies of 440 Hz and 442 Hz, the beat frequency would be:

|440 Hz - 442 Hz| = 2 Hz

This means you would hear two beats per second.

Why Beats Occur:

When two sound waves with slightly different frequencies interfere, they periodically align in phase (constructive interference) and then out of phase (destructive interference). This creates the alternating loud and soft sounds we call beats.