Hi,

Beat frequencies a side effect of wave interference. Sounds are composed of waves; sound waves can interfere with each other.  If the waves have different frequencies, they will interfere "constructively" and "destructively" in an alternating pattern, creating "beat frequencies".

For example, if you add a wave oscillating at 445 Hz with one that is at 450 Hz, the resulting frequency ill be an average of the sum of the two waves: (445 Hz + 450 Hz)/2 = 447.5 Hz.Also, its beat frequency will be 60 Hz, which would sound like a very low-pitched hum instead of a fluctuating volume.

Hope this helps.