On the role of the amplitude envelope for the perception of [b] and [w]

This study investigated the role of the amplitude envelope in the vicinity of consonantal release in the perception of the stop-glide contrast. Three sets of acoustic [b-w] continua, each in the vowel environments [a] and [i], were synthesized using parameters derived from natural speech. In the first set, amplitude, formant frequency, and duration characteristics were interpolated between exemplar stop and glide endpoints. In the second set, formant frequency and duration characteristics were interpolated, but all stimuli were given a stop amplitude envelope. The third set was like the second, except that all stimuli were given a glide amplitude envelope. Subjects were given both forced-choice and free-identification tasks. The results of the forced-choice task indicated that amplitude cues were able to override transition slope, duration, and formant frequency cues in the perception of the stop-glide contrast. However, results from the free-identification task showed that, although presence of a stop amplitude envelope turned all stimuli otherwise labeled as glides to stops, the presence of a glide amplitude envelope changed stimuli labeled otherwise as stops to fricatives rather than to glides. These results support the view that the amplitude envelope in the vicinity of the consonantal release is a critical acoustic property for the continuant / noncontinuant contrast. The results are discussed in relation to a theory of acoustic invariance.