A study of human hearing and the biomechanical processes involved in hearing reveals several non-linear 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 in reality (which in turn differs from what might be recorded onto a computer, given the imperfections of microphones and recording technology). By taking into account the various non-linearities in the hearing process, and some of the basic physical characteristics of the ear, nervous system, and brain, it becomes possible to begin to account for these discrepancies between perception and physical measurements.

Over the years, science and technology has incrementally improved our ability to understand and model the hearing process using 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 (pulse coded modulation): humans tend to perceive amplitude logarithmically rather than linearly, thus A-law quantisation using 8 bits to represent each sample sounds better than linear PCM quantisation using 8 bits (in truth, it can sound better than speech quantised linearly with 12 bits). It thus achieves a higher degree of subjective speech quality than PCM – for a given bitrate [4].

Physical processes

A cut-away diagram of the human ear (outer, middle and inner) is shown in Figure 4.1. The outer ear 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 through the middle ear by the three bones, the malleus, incus and stapes, to the oval window, opening on to the cochlea in the inner ear.

The cochlea, as a coiled tube, contains an approximately 35mm long semi-rigid pair of membranes (basilar and Reissner's) enclosed in a fluid called endolymph [35]. The basilar membrane carries the organs of Corti, each of which contains a number of hair cells arranged in two rows (approximately 3500 inner and 20 000 outer hair cells).