Critical bandwidth

In phonetics, the bandwidth at which the singal threshold ceases to increase is called the critical bandwidth.

Comments
When a listener tries to detect a signal in a noise background, the listener is assumed to make use of a filter with a centre frequency close to that of the signal. Only the components in the noise passing through the filter will have any effect in masking the signal. Increases in noise bandwidth result in more noise passing through the auditory filter, provided the noise bandwidth is less than the filter bandwidth. Once the noise bandwidth exceeds the filter bandwidth, further increases in noise bandwidth will not increase the noise passing through the filter.

Link
Utrecht Lexicon of Linguistics