Perception of F2 trajectories in synthetic vowels was investigated. Perceptual boundaries (50% response points of identification functions) of 20-step/U-I/continua with various shapes of F2 trajectories were determined and compared with the boundary for vowels with steady-state F2. In experiment 1, the vowels were synthesized in either/j-j/or/w-w/context resulting in parabolic trajectories, then the stimuli were split into halves resulting in quadratic F2 trajectories. All stimuli were 200 ms long. For the/wVw/, and/wV/stimuli, the boundaries were at lower stimulus numbers than for the stimuli with steady-state F2, indicating that the nonlinear F2 trajectories were perceived as having frequencies beyond the extreme values actually synthesized in the stimuli. This type of signal processing has been termed "perceptual compensation." For the /jVj/ stimuli, there was only a trend for perceptual compensation of F2 trajectories. For the/Vw/,/Vj/, and/jV/stimuli, the boundaries were at stimulus numbers corresponding to frequencies of the relatively steady-state vowel-like segments. In experiment 2, the quadratic F2 trajectories of the/wV/stimuli were changed to linear trajectories and F1 and F3 trajectories were either quadratic, linear, or steady state. The results indicated that the shape of F1 and F3 trajectories had no effect on the boundaries. For the linear F2 trajectories, as for the quadratic F2 trajectories in the/wV/ stimuli, the boundaries were at lower stimulus numbers than for the stimulus with steady-state F2, indicating presence of perceptual compensation. In the experiment 3, the F1 and F3 were steady state and the linear F2 trajectories had three different values of frequency difference, delta F, between the initial and final frequencies. The perceptual compensation was found for the 200-ms stimuli with large delta F up to 500 Hz over 200-ms stimulus duration, and perceptual enhancement of final frequencies was found for the stimuli with delta F = 280 Hz.

The relationship between relative intensity of transition segments and identification of diphthongs has been investigated. In the first experiment, synthesized stimuli were used. The stimuli differed in the amount of attenuation of the transition segment which ranged from 0 to 15 dB. It was expected that [diphthong in text] responses would be obtained for stimuli with attenuated transitions. The stimuli were tested in quiet, noise, and reverberation with ten normal-hearing and seven hearing-impaired subjects. For the stimulus with the most attenuated transition, the normal-hearing subjects gave no [diphthong in text] responses and the hearing-impaired subjects gave only 20% [diphthong in text] responses in quiet. However, in noise, both groups of subjects gave 70% [diphthong in text] responses and in reverberation, the normal-hearing subjects gave 95% and the hearing-impaired subjects gave 90% [diphthong in text] responses. Generally, less transition attenuation was needed for the hearing-impaired than for the normal-hearing subjects to give [diphthong in text] responses. These findings indicated that identification errors in noise and reverberation for naturally produced diphthongs might be related to the intensity of their transition segments. In the second experiment, naturally produced diphthongs [diphthongs in text] from the Nábĕlek et al. [J. Acoust. Soc. Am. 92, 1228-1246 (1992)] study were spectrally analyzed. There were 30 different tokens for each diphthong. The results of the analyses indicated significant correlations between the number of identification errors for these diphthongs made by either normal-hearing or hearing-impaired subjects and the relative intensities of the F2 transition segment. In both noise and reverberation there were fewer errors for the diphthong tokens characterized by high intensity F2 transitions.

Boundary locations for identification functions were tested for /i-eh/ and /u-i/ continua. In the /i-eh/ continuum, F1 varied in 14 steps from 360 to 580 Hz, and in the /u-i/ continuum, F2 varied in 19 steps from 1000 to 2000 Hz. There were five different types of stimuli in each continuum with (1) steady-state formants, (2) one formant changing linearly over time, and (3) two formants changing linearly. The single changing formant was F1 in the /i–eh/ continuum and F2 in the /u–i/ continuum. The two changing formants were F1 and F2 in both continua. The trajectories had upward or downward directions. Identification functions were obtained with ten normal-hearing subjects. Directions of boundary shifts for the /i–eh/ continuum indicated a perceptual emphasis of the initial segments of F1 and F2 trajectories. The directions of boundary shifts for the /u–i/ continuum indicated a perceptual emphasis of the final segment of F2 trajectory when only F2 changed over time. However, when changes over time in F1 were added, boundaries for these stimuli were not significantly different from the boundary for the stimuli with steady-state formants. Differences in perception of F1 and F2 trajectories in two tested continua will be discussed. [Work supported by NIH.]

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