AUDITORY MODELS OF FORMANT FREQUENCY DISCRIMINATION FOR ISOLATED VOWELS

Thresholds for formant discrimination of female and male vowels are si gnificantly elevated by two stimulus factors, increases in formant fre quency and fundamental frequency [Kewley-Port et al., J. Acoust. Soc. Am. 100, 2462-2470 (1996)]. The present analysis systematically examin ed whether auditory models of vowel sounds, including excitation patte rns, specific loudness, and a Gammatone filterbank, could explain the effects of stimulus parameters on formant thresholds. The goal was to determine if an auditory metric could be specified that reduced variab ility observed in the thresholds to a single-valued function across fo ur sets of female and male vowels. Based on Sommers and Kewley-Port [J . Acoust. Soc. Am. 99, 3770-3781 (1996)], four critical bands around t he test formant were selected to calculate a metric derived from excit ation patterns. A metric derived from specific loudness difference (De lta Sone) was calculated across the entire frequency region. Since ana lyses of spectra from Gammatone filters gave similar results to those derived from excitation patterns, only the 4-ERB (equivalent rectangul ar bandwidth) and Delta Sone metrics were analyzed in detail. Three cr iteria were applied to the two auditory metrics to determine if they w ere single-valued functions relative to formant thresholds for female and male vowels. Both the 4-ERB and Delta Sone metrics met the criteri a of reduced slope, reduced effect of fundamental frequency, although Delta Sone was superior to 4-ERB in reducing overall variability. Resu lts suggest that the auditory system has an inherent nonlinear transfo rmation in which differences in vowel discrimination thresholds are al most constant in the internal representation. (C) 1998 Acoustical Soci ety of America.