Economy of effort in different speaking conditions. I. A preliminary study of intersubject differences and modeling issues.

This study explores the hypothesis that clear speech is produced with greater "articulatory effort" than normal speech. Kinematic and acoustic data were gathered from seven subjects as they pronounced multiple repetitions of utterances in different speaking conditions, including normal, fast, clear, and slow. Data were analyzed within a framework based on a dynamical model of single-axis frictionless movements, in which peak movement speed is used as a relative measure of articulatory effort (Nelson, 1983). There were differences in peak movement speed, distance and duration among the conditions and among the speakers. Three speakers produced the "clear" condition utterances with movements that had larger distances and durations than those for "normal" utterances. Analyses of the data within a peak speed, distance, duration "performance space" indicated increased effort (reflected in greater peak speed) in the clear condition for the three speakers, in support of the hypothesis. The remaining four speakers used other combinations of parameters to produce the clear condition. The validity of the simple dynamical model for analyzing these complex movements was considered by examining several additional parameters. Some movement characteristics differed from those required for the model-based analysis, presumably because the articulators are complicated structurally and interact with one another mechanically. More refined tests of control strategies for different speaking styles will depend on future analyses of more complicated movements with more realistic models.