Minimal spectral contrast of formant peaks for vowel recognition as a function of spectral slope

In four experiments we investigated whether listeners can locate the formants of vowels not only from peaks, but also from spectral "shoulders"--features that give rise to zero crossings in the third, but not the first, differential of the excitation pattern--as hypothesized by Assmann and Summerfield (1989). Stimuli were steady-state approximations to the vowels [a, i, e, u, o] created by summing the first 45 harmonics of a fundamental of 100 Hz. Thirty-nine harmonics had equal amplitudes; the other 6 formed three pairs that were raised in level to define three "formants." An adaptive psychophysical procedure determined the minimal difference in level between the 6 harmonics and the remaining 39 at which the vowels were identifiably different from one another. These thresholds were measured through simulated communication channels, giving overall slopes of the excitation patterns of the five vowels that ranged from -1 dB/erb to + 2 dB/erb. Excitation patterns of the threshold stimuli were computed, and the locations of formants were estimated from zero crossings in the first and third differentials. With the more steeply sloping communication channels, some formants of some vowels were represented as shoulders rather than peaks, confirming the predictions of Assmann and Summerfield's models. We discuss the limitations of the excitation pattern model and the related issue of whether the location of formants can be computed from spectral shoulders in auditory analysis.