Relational acoustic invariance in the single/geminate stop distinction in Japanese

Abstract

This study addressed an issue in the theory of relational acoustic invariance [Pickett et al., Phonetica 56, 135–157 (1999)]. The question was whether an invariant acoustic property exists for distinguishing Japanese single and geminate voiceless stops across different speaking rates. Four native Japanese speakers produced disyllabic words with single and geminate voiceless stops spoken in a carrier sentence at three speaking rates. Durations of sentences, words, stop closures, and vowels preceding the contrasting stops were measured. Ratios of geminate to single stop closures, geminate words to single words, closures to preceding vowels, and closures to words were calculated. The stop closure duration significantly overlapped between the single and geminate categories across rates. However, the ratio of geminate to single closure duration was unaffected by rate. Among the measures examined, the ratio of closure to word duration (0.35 as an optimal boundary) best classified all single and geminate tokens with 95.7–98% accuracy. These results suggest that, in spite of overlap in absolute closure duration between single and geminate stops, there is a relationally invariant measure that divides the two phonemic categories across rates and speakers, supporting the theory of relational acoustic invariance.