The purpose of this experiment was to study the effects of changes in speaking rate on both the attainment of acoustic vowel targets and the relative time and speed of movements toward these presumed targets. Four speakers produced a number of different CVC and CVCVC utterances at slow and fast speaking rates. Spectrographic measurements showed that the midpoint format frequencies of the different vowels did not vary as a function of rate. However, for fast speech the onset frequencies of second formant transitions were closer to their target frequencies while CV transition rates remained essentially unchanged, indicating that movement toward the vowel simply began earlier for fast speech. Changes in both speaking rate and lexical stress had different effects. For stressed vowels, an increase in speaking rate was accompanied primarily by a decrease in duration. However, destressed vowels, even if they were of the same duration as quickly produced stressed vowels, were reduced in overall amplitude, fundamental frequency, and to some extent, vowel color. These results suggest that speaking rate and lexical stress are controlled by two different mechanisms.

Mechanisms of speech rate control are discussed from the point of view that unidimensional models are inadequate to explain the extensive physiological and acoustical effects observed during changes in speech rate. Evidence is presented to demonstrate that changes in speech rate can result in changes in segmental duration, articulatory displacement, atriculatory velocity, and intrasyllabic coarticulation, and that these changes represent nonlinear transformations produced by a reorganization of speech motor strategies.

Duration and formant frequency measurements of the five diphthongs, /o•,a%au, e%ou/, were made for three conditions of speaking rate. Results indicate that onset target position and second-formant rate of change are fixed features of the diphthong formant movement, while offset target positions are variable across changes in duration. Other measurements show that increased diphthong duration due to the presence of a following voiced consonant is accomplished by longer steady-state targets for/or, a•, au/and longer glide durations for/e% ou/. The data provide a basis for an articulatory description of diphthong production.