Cues for perception of the diphthong /aI/ in either noise or reverberation. Part I. Duration of the transition.

Location of boundaries (the 50% response point) and slopes of identification functions were determined for synthesized /a-aI/ vowel continua. Within each continuum, the stimuli contained a steady-state segment followed by a transition in which the frequencies of formants changed in time. Here, F1 changed in a downward direction and F2 changed in an upward direction. Total duration of each stimulus was 200 ms. The duration of the transition was increased in steps from 0 to 140 ms. Two patterns of formant transition were used: (1) formants changing in the direction of, but not reaching, target frequencies (except in the end-point stimulus), and (2) formants reaching F1 and F2 targets. The data were collected with ten normal-hearing and ten hearing-impaired subjects. The boundaries and slopes were determined for four listening conditions: quiet, noise, short reverberation (0.8 s), and long reverberation (1.1 s). The location of boundaries depended upon: (1) pattern of formant transitions, (2) listening condition, and (3) status of subjects' hearing. Generally, longer transitions were needed for formants changing in the direction of, but not reaching, target frequencies, than for those reaching F1 and F2 targets. The required transition durations were similar in quiet and noise, but were longer in reverberation. The hearing-impaired subjects generally required longer transitions to reach the boundaries than normal-hearing subjects. The slopes of the identification functions were shallower in either noise or reverberation than in quiet and were shallower for hearing-impaired than for normal-hearing subjects. In reverberation, the slopes for formants reaching targets were shallower than the slopes for stimuli with formants changing in the direction of target frequencies. The relationships between these findings and identification errors for naturally produced tokens of the diphthong /aI/ are discussed.