Effect of speaking rate on the acceptability of change in segment duration1

An acceptability of segment duration changes in different speaking rates was studied to know perceptual characteristics for designing an objective naturalness measure in speech synthesis. Based on a series of our studies on intra-phrase positional dependency of perceptual acceptability, where listeners were more sensitive to the phrase-initial segment duration than the phrase-final one, we designed perceptual experiments using speech at three rates (fast, normal and slow) with or without a carrier sentence. The duration of each vowel at different phrase positions was either lengthened or shortened from 10 ms to 50 ms and listeners evaluated the acceptability of these changes. The results showed that the acceptability declined more rapidly as a speaking rate became faster and that the difference of acceptability declination between intraphrase positions was consistent. These results serve as fundamental data of speaking-rate dependent acceptability characteristics for designing an objective temporal naturalness.