From the technical, signal-oriented point of view, the production of speech is widely described as a two-level process (Koreman, 1996:1). In the first stage the sound is initiated and in the second stage it is filtered on the second level (Fant, 1960; Titze, 1994:136-168; Stevens, 1997:463). This distinction between phases has its orgin in the source-filter model of speech production (Fant, 1960).
The basic assumption of the model is that the source signal produced at the glottal level is linearly filtered through the vocal tract. The resulting sound is emitted to the surrounding air through radiation loading (lips). The model assumes that source and filter are independent of each other. Although recent findings show some interaction between the vocal tract and a glottal source (Rothenberg 1981; Fant 1986), Fant's theory of speech production is still used as a framework for the description of the human voice, especially as far as the articulation of vowels is concerned.
From the linguistic phonetic point of view, the production of speech is regarded as a superposition of initiation, phonation, articulation and prosodic organization processes (Laver, 1994:116). An overview of the physiological constraints on speech production will be given later in this section. The organs involved in the production of speech are depicted in the Fig.1.
Figure 1. The production of speech sounds -lungs, glottis, vocal tract (from: Borden et al.,1994:64).