perceptual noise substitution (speech encoding) (PNS)
Perceptual noise substitution (PNS) is the exchange of perceptual noise.
The PNS process assumes that a human ear cannot perceive a significant difference between sounds. This means that sounds can be substituted and exchanged. If a frequency band contains noise components, they can be replaced by other noise components generated or stored in the playback device.
Perceptual noise substitution is used as an enhancement in MPEG-4 and in Advanced Audio Coding( AAC) to lower the bit rate.