Effects of speaking rate on second formant trajectories of selected vocalic nuclei.

The effect of speaking rate variations on second formant (F2) trajectories was investigated for a continuum of rates. F2 trajectories for the schwa preceding a voiced bilabial stop, and one of three target vocalic nuclei following the stop, were generated for utterances of the form "Put a bV here, where V was /i/,/ae/ or /oI/. Discrete spectral measures at the vowel-consonant and consonant-vowel interfaces, as well as vowel target values, were examined as potential parameters of rate variation; several different whole-trajectory analyses were also explored. Results suggested that a discrete measure at the vowel consonant (schwa-consonant) interface, the F2off value, was in many cases a good index of rate variation, provided the rates were not unusually slow (vowel durations less than 200 ms). The relationship of the spectral measure at the consonant-vowel interface, F2 onset, as well as that of the "target" for this vowel, was less clearly related to rate variation. Whole-trajectory analyses indicated that the rate effect cannot be captured by linear compressions and expansions of some prototype trajectory. Moreover, the effect of rate manipulation on formant trajectories interacts with speaker and vocalic nucleus type, making it difficult to specify general rules for these effects. However, there is evidence that a small number of speaker strategies may emerge from a careful qualitative and quantitative analysis of whole formant trajectories. Results are discussed in terms of models of speech production and a group of speech disorders that is usually associated with anomalies of speaking rate, and hence of formant frequency trajectories.