Masking
Is the affect where weaker (low frequency) signals can be made inaudible by stronger (high frequency) signals. I.E a frequency when heard raises the threshold of hearing to the point where a second tone can be made inaudible (see [see page 13, here]).
This is due to the overlap between auditory filters.
We define critical band the bandwidth over which masking operates
We use a [see page 14, spreading function] to determine the affect of a large tone on the hearing threshold, with the MP3 one being designed from hearing tests.