Prosodic effects on vowel production: Evidence from formant structure

Speakers communicate pragmatic and discourse meaning through the prosodic form assigned to an utterance, and listeners must attend to the acoustic cues to prosodic form to fully recover the speaker's intended meaning. While much of the research on prosody examines supra-segmental cues such as F0 and temporal patterns, prosody is also known to affect the phonetic properties of segments as well. This paper reports on the effect of prosodic prominence on the formant patterns of vowels using speech data from the Buckeye corpus of spontaneous American English. A prosody annotation was obtained for a subset of this corpus based on the auditory perception of 97 ordinary, untrained listeners. To understand the relationship between prominence perception and formant structure, as a measure of the 'strength' of the vowel articulation, we measure the steady-state first and second formants of stressed vowels at vowel mid-points for monophthongs and at both 10% (nucleus) and 90% (glide) positions for diphthongs. Two hypotheses about the articulatory mechanism that implements prominence (Hyperarticulation vs. Sonority Expansion Hypothesis) were evaluated using Pearson's bivariate correlation analyses with formant values and prominence 'scores' - a novel perceptual measure of prominence. The findings demonstrate that higher F1 values correlate with higher prominence scores regardless of vowel height, confirming that vowels perceived as prominent tend to have enhanced sonority. In the frontness dimension, on the other hand, the results show that vowels perceived as prominent tend to be hyperarticulated. These results support the model of the supra-laryngeal implementation of prominence proposed in [5, 6] based on controlled "laboratory" speech, and demonstrate that the model can be extended to cover prosody in spontaneous speech using a continuous-valued measure of prosodic prominence. The evidence reported here from spontaneous speech shows that prominent vowels have expanded sonority regardless of vowel height, and are hyperarticulated only when hyperarticulation does not interfere with sonority expansion.