Stress and rate: Differential transformations of articulation

Early theorizing in speech production considered variations in syllable stress and speaking rate to be identical transformations of motor activity e.g., Lindblom, J. Acoust. Soc. Am. 35, 1773-1781 (1963). More recent work, however, suggests that modulation of stress and rate have different acoustic and perceptual effects, and thus may have independent "signatures" in production. The experiments reported here examined these hypotheses. In the first experiment, electromyographic activity from two muscles, one primarily related to vowel production and the other to consonant production, was examined during utterances whose stress and rate characteristics were systematically varied. The results support the hypothesis that stress and rate modulations have differential effects on muscle activity. The second experiment addressed the same basic question but examined a broader set of muscles. In all cases, the changes in duration and peak amplitude of muscle activity for altered stress were not equivalent to the changes for altered rate. Decreases in stress produced decreases in peak amplitude and duration of vowel-related muscle activity that were consistent across speakers. In contrast, activity patterns associated with rate contrasts varied considerably across muscles and speakers.