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This paper presents the results of formant analysis using a newly developed formant contour model. We model formant contours with a linear combination of formant target values and coarticulation functions for /wVl/ and /tVl/ words. While formant target values are estimated globally over different speaking styles, coarticulation coefficients are estimated for individual tokens. The results show that the estimated coarticulation coefficients are inherently different between clear (CLR) and conversational (CNV) speech and that the movement of articulators when producing CLR speech is faster than when producing CNV speech. On the other hand, speaking rate is not a key determinant in movement of articulators at vowel onsets. The direct measure of F2 slope is strongly correlated with estimated coarticulation coefficients, which may lead to less parameters to estimate.

We examined how much listeners can benefit from listening to “clear” (CLR) speech compared to “conversational” (CNV) speech, both spoken at different speaking rates. Vowel intelligibilities of four front vowels (/i:/, /I/, /E/, and /ei/) in background noise were measured with four speaking styles (CNV/SLOW, CNV, CLR, and CLR/FAST). Results showed only tense vowels of CLR speech had a significant difference between CNV and CLR speaking styles, after energy and F0 contour were normalized. We synthesized hybrid (HYB) speech whose formant features were equal to those of CLR speech, while all other features were taken from CNV speech. Primary conclusions from this study are (1) naturally-spoken fast CLR speech was not as intelligible as CLR speech, (2) enhancing formant frequencies to resemble those of CLR speech was effective at improving vowel intelligibility, and (3) spectral tilt and formant bandwidths were not contributing factors to the CLR speech benefit.