Chapter 2 described the general handling, processing and visualisation of audio vectors: sequences of samples captured at some particular sample rate, and which together represent sound. This chapter will build upon that foundation, and use it to begin to look at speech. There is nothing special about speech from an audio perspective – it is simply like any other sound – it's only when we hear it that our brains begin to interpret a particular signal as being speech. There is a famous experiment which demonstrates a sentence of sinewave speech. This presents a particular sound recording made from sinewaves. Initially, the brain of a listener does not consider this to be speech, and so the signal is unintelligible. However after the corresponding sentence is heard spoken aloud in a normal way, the listener's brain suddenly ‘realises’ that the signal is in fact speech, and from then on it becomes intelligible. After that the listener cannot ‘unlearn’ this fact: similar sentences which are generally completely unintelligible to others will be perfectly intelligible to this listener.

Apart from this interpretative behaviour of the human brain, there are audio characteristics within music and other sounds that are inherently speech-like in their spectral and temporal characteristics. However speech itself is a structured set of continuous sounds, by virtue of its production mechanism. Its characteristics are very well researched, and many specialised analysis, handling and processing methods have been developed over the years especially for this narrow class of audio signals.

Initially turning our back on the computer and speech processing, this chapter will consider the human speech production apparatus, mechanisms, and characteristics.