Vowel context effects on the spectral dynamics of English and Japanese sibilant fricatives

Abstract

Previous analyses of vowel context effects on the sibilant fricatives of English (/s, ʃ/) and Japanese (/s, ɕ/) have focused on spectral properties computed from a limited number of time points, e.g., frication midpoint or vowel onset. However, the spectra of sibilants vary temporally; thus, it is worth considering how their spectral dynamics vary across vocalic contexts. Vowel context effects were investigated with respect to the trajectories of three psychoacoustic measures computed across the timecourse of native, adult speakers' productions of word-initial, pre-vocalic sibilants. Psychoacoustic spectra were computed from 10 ms windows, spaced evenly across the frication, by passing each window through a bank of gammatone filters, which modeled the auditory system's differential frequency selectivity. From these psychoacoustic spectra, the peak frequency, excitation-drop (difference between maximum high-band and minimum low-band excitation), and the half-power bandwidth of the peak were computed. In Japanese, the peak frequency and the excitation-drop trajectories showed effects of vowel height—the trajectories for high (vs. mid and low) vowel contexts diverging from 50–75% of the fricative duration onward. In English, the excitation-drop trajectories showed similar effects of vowel height; however, the trajectories for high vowels diverged later than in Japanese. Peak bandwidth exhibited context effects only for Japanese /ɕ/, where it was lower in back vowel contexts across the first 75% of the fricative duration.