Speech as the medium of human communication conveys information between a speaker and a listener on several layers (Laver, 1991:22). The linguistic layer carries the semantic information encoded in the language (its grammar and phonological units) and its phonetic representation. It covers what we intend to say, i.e. the "text" of an utterance. The paralinguistic layer of communication is non-linguistic and non-verbal but tells the interlocutor about the speaker's current affective, attitudinal or emotional state. It also includes the speaker's regional dialect and characteristic sociolect. Control of the active participation of time sharing in the conversation is also conducted by this layer (Laver, 1994:21). The paralinguistic markers, however, are in part culturally determined and their respective interpretation must be learned. For example, the use of falsetto voice in English to mimic a male conversation participant's utterance counts as an accusation of effeminacy, whereas in Tzeltal (Mexico) the use of falsetto in greeting someone is a marker of deference (Laver, 1994:22). Paralinguistic communication is not so well structured as its linguistic counterpart; it is not sequential (it is obvious in which sequence the markers are produced) and relative - perception is based more on the fluctuation of a feature rather than on its actual value. The third layer, the extralinguistic behavior, in speech identifies the speaker, his/her sex, age, voice, the range of pitch and loudness as well as his/her habitual factors. In other words, it encompasses all physical and physiological (including all organic features involved in speech production) characteristics of a certain speaker.
The resulting acoustic signal is thus shaped through contributions from many different sources that overlay each other. This causes problems in formulating a clear definition of voice quality. The effects of voice quality influence all layers of communication, but the fact that listeners usually can discern these differences suggests that they are recoverable from a produced speech signal (Löfqvist, 1995:403).
Voice quality is defined by Trask (1996:381) as the characteristic auditory coloring of an individual's voice, derived from a variety of laryngeal and supralaryngeal features and running continuously through the individual's speech. The natural and distinctive tone of speech sounds produced by a particular person yields a particular voice.