Vowel boundaries for steady-state and linear formant trajectories

Locations of boundaries and slopes of identification functions were tested for /I-epsilon/ vowel continua with steady-state and linearly changing formant trajectories. In experiment 1, the boundaries and slopes for arbitrarily selected trajectory directions were determined for ten normal-hearing and ten hearing-impaired subjects in three listening conditions: Quiet, noise, and reverberation. The boundaries did not depend upon the group of subjects or the listening condition. A boundary shift was found for stimuli with F1 changing in a downward direction relative to boundaries for stimuli with either only F1 or with both F1 and F2 changing in an upward direction. The slope of the identification function for stimuli with F1 changing in a downward direction was shallower than the slopes for stimuli with steady-state formants or stimuli with F1 changing in an upward direction. The slopes obtained from the hearing-impaired subjects were shallower than those of the normal-hearing subjects and were shallower in noise than in either quiet or reverberation. In experiment 2, boundaries and slopes for the trajectory directions found in the natural vowels /I/ and /epsilon/, F1 changing in an upward direction and F2 in a downward direction, were determined for nine normal-hearing subjects in two listening conditions, quiet and reverberation. The boundary for stimuli with both F1 and F2 changing in directions characteristic for natural vowels was shifted relative to the boundary for stimuli with steady-state formants. The directions of the boundary shifts in experiments 1 and 2 indicated a perceptual emphasis on the initial sections of changing F1 and F2. Sound quality of the end-point /I/ and /epsilon/ stimuli depended upon F1 and F2 trajectories. For both vowels, the best quality judgments were found for the stimuli with natural F1 and F2 trajectory directions. The quality judgments were weakly correlated with the slopes of identification functions, with better quality judgments being associated with steeper slopes.