A study of human hearing and the biomechanical processes involved in hearing, reveals several nonlinear steps, or stages, in the perception of sound. Each of these stages contributes to the eventual unequal distribution of subjective features against purely physical ones in human hearing.

Put simply, what we think we hear is quite significantly different from the physical sounds that may be present (which in turn differs from what would be captured electronically by, for example, a computer). By taking into account the various nonlinearities in the hearing process, and some of the basic physical characteristics of the ear, nervous system, and brain, it is possible to account for the discrepancy.

Over the years, science and technology has incrementally improved the ability to model the hearing process from purely physical data. One simple example is that of A-law compression (or the similar μ-law used in some regions of the world), where approximately logarithmic amplitude quantisation replaces the linear quantisation of PCM: humans tend to perceive amplitude logarithmically rather than linearly, and thus A-law quantisation using 8 bits sounds better than linear PCM quantisation using 8 bits. It thus achieves a higher degree of subjective speech quality than PCM for a given bitrate.

Physical processes

The ear, as shown diagrammatically in Figure 4.1, includes the pinna which filters sound and focuses it into the external auditory canal. Sound then acts upon the eardrum where it is transmitted and amplified by the three bones, the malleus, incus and stapes, to the oval window, opening on to the cochlea.