So far this book has dealt with individual subjects ranging from background material on audio, its handling with Matlab, speech, hearing, and on to the commercially important topics of communications and analysis. Each of these has been relatively self-contained in scope, although the more advanced speech compression methods of the previous chapter did introduce some limited psychoacoustic features.

In this chapter we will progress onward: we will discuss and describe advanced topics that combine processing elements relating to both speech and hearing – computer models of the human hearing system that can be used to influence processing of speech (and audio), and computer models of speech that can be used for analysis and modification of the speech signal.

By far the most important of these topics is introduced first: psychoacoustic modelling, without which MP3 and similar audio formats, and the resultant miniature music players from Creative, Apple, iRiver, and others, would not exist.

Psychoacoustic modelling

Remember back in Section 4.2 we claimed that this marriage of the art of psychology and the science of acoustics was important in forming a link between the purely physical domain of sound and the experience of a listener? In this section we will examine further to see why and how that happens.

It follows that a recording of a physical sound wave – which is a physical representation of the audio – contains elements which are very relevant to a listener, and elements which are not. At one extreme, some of the recorded sound may be inaudible to a listener.