Speech occurs always in some context—the context of who is present, where they are, what they are doing, and what has already been said. The context tell the hearer to some extent what the speaker is likely to say, so that he does not have to be equally on the alert for every theoretically possible message, and often does not have to catch all of what is said in order to understand. In this defining context, the speech signal does two things, not just one: it tells the hearer what utterance has been used by the speaker, but it is also shaped in part by the speaker’s voice-quality modulation.
- Hockett (1958), Pag. 115