Detection thresholds for isolated vowels.

A series of experiments on the detectability of vowels in isolation has been completed. Stimuli consisted of three sets of ten vowels: one synthetic, one from a male talker, and one from a female talker. Vowel durations ranged from 20-160 ms for each of the sets. Thresholds for detecting the vowels in isolation were obtained from well-trained, normal-hearing listeners using an adaptive-tracking paradigm. For a given duration, detection thresholds for vowels calibrated for equal rms sound pressure at the earphones differed by 22 dB across the 30 vowels. In addition, an orderly decrease in vowel thresholds was obtained for increased duration, as predicted from previous data on temporal integration. Several different analyses were performed in an attempt to explain the differential detectability across the 30 vowels. Analyses accounting for audibility reduced threshold variability significantly, but vowel thresholds still ranged over 15 dB. Vowel spectra were subsequently modeled as excitation patterns, and several detection hypotheses were examined. A simple average of excitation levels across excited critical bands provided the best prediction of the level variations needed to maintain threshold-level loudness across all vowels.