Auditory models of formant frequency discrimination for isolated vowels

Thresholds for formant discrimination of female and male vowels are significantly elevated by two stimulus factors, increases in formant frequency and fundamental frequency [Kewley-Port et al., J. Acoust. Soc. Am. 100, 2462-2470 (1996)]. The present analysis systematically examined whether auditory models of vowel sounds, including excitation patterns, 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 variability observed in the thresholds to a single-valued function across four sets of female and male vowels. Based on Sommers and Kewley-Port [J. Acoust. Soc. Am. 99, 3770-3781 (1996)], four critical bands around the test formant were selected to calculate a metric derived from excitation patterns. A metric derived from specific loudness difference (delta Sone) was calculated across the entire frequency region. Since analyses of spectra from Gammatone filters gave similar results to those derived from excitation patterns, only the 4-ERB (equivalent rectangular bandwidth) and delta Sone metrics were analyzed in detail. Three criteria were applied to the two auditory metrics to determine if they were single-valued functions relative to formant thresholds for female and male vowels. Both the 4-ERB and delta Sone metrics met the criteria of reduced slope, reduced effect of fundamental frequency, although delta Sone was superior to 4-ERB in reducing overall variability. Results suggest that the auditory system has an inherent nonlinear transformation in which differences in vowel discrimination thresholds are almost constant in the internal representation.