Temporal properties of spontaneous speech - a syllable-centric perspective

Temporal properties of the speech signal are of potentially great importance for understanding spoken language and may provide significant insight into the manner in which listeners process spoken language with so little apparent effort. It is the thesis of this study that durational properties of phonetic segments differentially reflect the amount of information contained within a syllable, and that syllable prominence is an indirect measure of linguistic entropy. The ability to understand spoken language appears to depend on a broad distribution of syllable duration, ranging between 50 and 400 ms (for American English), which is reflected in the modulation spectrum of the acoustic signal. The upper branch of the modulation spectrum (6-20 Hz) reflects unstressed syllables, while the lower branch (< 5 Hz) represents mostly heavily stressed syllables. Low-pass filtering the modulation spectrum reduces the intelligibility of spoken sentences in a manner consistent with the differential contribution of stressed and unstressed syllables to understanding spoken language. The origins of this phenomenon are investigated in terms of the durational properties of phonetic segments contained in a corpus of spontaneous American English telephone dialogues (SWITCHBOARD). Forty-five minutes of this material was manually annotated with respect to stress accent, and the relation between accent level and segmental duration examined. Statistical analysis indicates that much of the temporal variation observed at the syllabic and phonetic-segment levels can be accounted for in terms of two basic parameters: (1) stress-accent pattern and (2) position of the segment within the syllable. Segments are generally longest in heavily stressed syllables and shortest in syllables without stress. However, the magnitude of accent's impact on duration varies as a function of syllable position. Duration of the nucleus is heavily affected by stress-accent level-heavily stressed nuclei are, on average, twice as long as their unstressed counterparts, while the duration of the onset is also significantly sensitive to stress, but to a lesser degree. In contrast, stress has relatively little impact on coda duration. This pattern of durational variation suggests that the vocalic nucleus absorbs much of the impact of stress accent and potentially sets the register for interpreting the phonetic segments contained within the syllable. Moreover, the data imply that linguistic entropy is not uniformly distributed across the syllable-the onset and nucleus convey more information than the coda. (C) 2003 Elsevier Ltd. All rights reserved.