The main application of audio compression systems is to obtain compact digital representations of high-quality (CD-quality) wideband audio signals. Typically, audio signals recorded on CDs and digital audio tapes are sampled at 44.1 or 48 kHz and each sample is represented by a 16-bit integer; that is, the uncompressed two-channel stereo CD-quality audio requires 2 × 44(48) × 16 = 1.41(1.54) Mb/s for transmission. Unlike speech compression systems, the audio codecs process sounds generated by arbitrary sources and they cannot exploit specific features of the input signals. However, almost all modern audio codecs are based on a model of the human auditory system. The key idea behind the so-called perceptual coding is to remove the parts of the input signal which the human cannot perceive. The imperceptible information removed by the perceptual coder is called the irrelevancy. Since, similarly to speech signals, audio signals can be interpreted as outputs of sources with memory, then perceptual coders remove both irrelevancy and redundancy in order to provide the lowest bit-rate possible for a given quality.

An important part of perceptual audio coders is the psychoacoustic model of the human hearing. This model is used in order to estimate the amount of quantization noise that is inaudible. In the next section, we consider physical phenomena which are exploited by the psychoacoustic model.