Substitution (PNS) is the exchange of perceptual noise. The PNS process assumes that a human ear cannot perceive any 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.