Bandwidth of spectral resolution for two-formant synthetic vowels and two-tone complex signals

Spectral integration refers to the summation of activity beyond the bandwidth of the peripheral auditory filter. Several experimental lines have sought to determine the bandwidth of this "supracritical" band phenomenon. This paper reports on two experiments which tested the limit on spectral integration in the same listeners. Experiment I verified the critical separation of 3.5 bark in two-formant synthetic vowels as advocated by the center-of-gravity (COG) hypothesis. According to the COG effect, two formants are integrated into a single perceived peak if their separation does not exceed approximately 3.5 bark. With several modifications to the methods of a classic COG matching task, the present listeners responded to changes in pitch in two-formant synthetic vowels, not estimating their phonetic quality. By changing the amplitude ratio of the formants, the frequency of the perceived peak was closer to that of the stronger formant. This COG effect disappeared with larger formant separation. In a second experiment, auditory spectral resolution bandwidths were measured for the same listeners using common-envelope, two-tone complex signals. Results showed that the limits of spectral averaging in two-formant vowels and two-tone spectral resolution bandwidth were related for two of the three listeners. The third failed to perform the discrimination task. For the two subjects who completed both tasks, the results suggest that the critical region in vowel task and the complex-tone discriminability estimates are linked to a common mechanism, i.e., to an auditory spectral resolving power. A signal-processing model is proposed to predict the COG effect in two-formant synthetic vowels. The model introduces two modifications to Hermansky's [J. Acoust. Soc. Am. 87, 1738-1752 (1990)] perceptual linear predictive (PLP) model. The model predictions are generally compatible with the present experimental results and with the predictions of several earlier models accounting for the COG effect.