Effects of speaking rate on f0 trajectories: Evidence from Thai lexical tones

This paper investigates the effect of speaking rate on tone production in Thai through two production experiments involving 44 speakers. Our analyses reveal clear trends: as speech rate increases, Thai speakers predominantly adjust the rate of fundamental frequency (f0) change to maintain tonal contours, with minimal alterations to contour shapes and values. Additionally, speakers tend to elevate the f0 onset value for most tones, positioning the contour within a higher f0 space at faster rates. While effects resembling "truncation" and undershoot of f0 contours occur in specific tonal combinations at faster speaking rates, we suggest that they are best attributed to global retiming of laryngeal commands governing f0 production. The strategies observed in laryngeal adjustments during faster speech rates are, thus, reminiscent of those observed in supralaryngeal articulation, suggesting the existence of a unified mechanism where rate effects impact both laryngeal and supralaryngeal articulation.