VOWEL BOUNDARIES FOR STEADY-STATE AND LINEAR FORMANT TRAJECTORIES

Locations of boundaries and slopes of identification functions were te sted for /I-epsilon/ vowel continua with steady-state and linearly cha nging formant trajectories. In experiment 1, the boundaries and slopes for arbitrarily selected trajectory directions were determined for te n normal-hearing and ten hearing-impaired subjects in three listening conditions: Quiet, noise, and reverberation. The boundaries did not de pend upon the group of subjects or the listening condition. A boundary shift was found for stimuli with Fl changing in a downward direction relative to boundaries for stimuli with either only Fl or with both Fl and F2 changing in an upward direction. The slope of the identificati on function for stimuli with Fl changing in a downward direction was s hallower than the slopes for stimuli with steady-state formants or sti muli 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 trajecto ry directions found in the natural vowels /I/ and /epsilon/, Fl changi ng in an upward direction and F2 in a downward direction, were determi ned for nine normal-hearing subjects in two listening conditions, quie t and reverberation. The boundary for stimuli with both Fl and F2 chan ging in directions characteristic for natural vowels was shifted relat ive to the boundary for stimuli with steady-state formants. The direct ions of the boundary shifts in experiments 1 and 2 indicated a percept ual emphasis on the initial sections of changing Fl and F2. Sound qual ity of the end-point /I/ and /epsilon/ stimuli depended upon Fl and F2 trajectories. For both vowels, the best quality judgments were found for the stimuli with natural F1 and F2 trajectory directions. The qual ity judgments were weakly correlated with the slopes of identification functions, with better quality judgments being associated with steepe r slopes.