Effect of speaking rate on the acceptability of change in segment duration

The acceptability of changes in segment duration at different speaking rates is studied to find useful perceptual characteristics for designing an objective naturalness measure in speech synthesis. Based on a series of previous studies on the intra-phrase positional dependency of perceptual acceptability, we investigate three factors: (1) speaking rate, (2) position within a phrase, and (3) presence/absence of a carrier sentence using three-mora (three-syllable) phrases at three rates (fast, normal and slow) with or without a carrier sentence (Experiment 1). Seven listeners evaluate the acceptability of resynthesized speech stimuli in which one of the vowel segments was either lengthened or shortened by up to 50 ms. Moreover, to understand the observed results within a psychophysical or auditory-based framework instead of language-dependent features, we simplify and replicate the temporal structures of the speech stimuli used and investigate the corresponding three factors (Experiment 2). Ten listeners rate the difference between standard and comparison stimuli in which one of the duration was either lengthened or shortened by up to 40 ms. The speech experiment shows that the acceptability for the same amount of absolute change decreased with an increase in speaking rate, i.e., the listeners more sensitively responded to the same absolute duration change when the speaking rate was fast than when it was slow. Similarly, the non-speech experiment shows that the detectability for the same amount of absolute change increased with an increase in tempo. In addition, the speech experiment shows the differences in acceptability declinations due to intra-phrase positions at three speaking rates. Similarly, the non-speech experiment shows the differences in the detectability due to temporal positions at three tempi. These agreements between the speech and non-speech experiments suggest that the two experiments share a common perceptual mechanism in processing temporal differences. On the other hand, the speech experiment shows no consistent tendency of the acceptability declinations due to the presence/absence of a carrier sentence, while the non-speech experiment shows, in several cases, that the presence of a carrier context could lower the detectability.