Masking

When any sounds that contain similar frequencies are played simultaneously, the weaker sound tends to have those overlapping frequencies covered – ‘masked’ – by the frequencies from the stronger sound (especially in a dense mix). The frequencies of the weaker sound are still there; they are just not discernable over the more dominant sound with the same frequencies. Masking is a fact of life in audio, and not entirely a bad thing, either. Often when sounds are masked you tend to not miss them. An example of practical applications where masking plays a big part are audio CODECs, which often identify and eliminate masked sounds from an audio file in order to make the overall file size smaller.