Effects of speaking rate on the vowel length distinction in Japanese

The issue of acoustic invariance was explored with the question of whether an absolute or relative duration value can be found to reliably classify the Japanese phonemic short and long vowels produced across different speaking rates. The stimuli were Japanese disyllabic non-words (Experiment 1) and real words (Experiment 2). Four native speakers produced these words in a carrier sentence at slow, normal, and fast rates. The duration of accented and unaccented short vowels in CVCV contexts and contrasting long vowels in CVVCV and CVCVV contexts, as well as word duration, was measured. Rate changes affected the duration of long vowels more than short vowels in such a way that the duration difference between short and long vowels was greater for slow than normal, and for normal than fast speech. In contrast with the absolute durations, the ratios of long-to-short vowels and three-mora (CVVCV or CVCVV) to two-mora (CVCV) words were less affected by rate changes. Furthermore, the proportion of the vowel to the total word duration was found to distinguish the two vowel length categories across three rates with high accuracy. The results support the view that "relational" acoustic invariance exists that remains stable across speakers and rates. (C) 2004 Elsevier Ltd. All rights reserved.