4.1 Introduction

Physics can tell us the mechanism by which disturbances propagate through the air. If this is our definition of sound, we have the problem that in physics there are no limits to the frequencies and levels which must be considered. Biology can tell that the ear only responds to a certain range of frequencies provided a threshold level is exceeded. This is a better definition of sound; reproduction is easier because it is only necessary to reproduce that range of levels and frequencies which the ear can detect.

Psychoacoustics can describe how our hearing has finite resolution in time, frequency and spatial domains such that what we perceive is an inexact impression. Some aspects of the original disturbance are inaudible to us and are said to be masked. If our goal is the highest quality, we can design our imperfect equipment so that the shortcomings are masked. Conversely if our goal is economy we can use compression and hope that masking will disguise the inaccuracies it causes.

By definition, the sound quality of a perceptive coder can only be assessed by human hearing. Equally, a useful perceptive coder can only be designed with a good knowledge of the human hearing mechanism. ^{[1 ]}The acuity of the human ear is astonishing. The frequency range is extremely wide, covering some ten octaves (an octave is a doubling of pitch or frequency) without interruption. It can detect tiny amounts of distortion, and will accept an enormous dynamic range. If the...