FREQUENCY DISCRIMINATION OF BAND-LIMITED HARMONIC COMPLEXES RELATED TO VOWEL FORMANTS

Just-noticeable differences (jnd's) in the center frequency of bandlimited harmonic complexes were measured for normal hearing subjects. A triangular and a trapezoidal spectral envelope were used. The center frequency ranged from 2000 to 2100 Hz for the triangular envelope and from 2000 to 2200 Hz for the trapezoidal envelope. For both envelope shapes the values 100, 200, and 400 dB/oct were used for the slope. The fundamental frequency was 100 Hz for all complexes. A three-interval, three-alternative forced-choice task was used. All measurements were performed with and without roving of the signal level. Without roving, the jnd's were in most cases about a factor of 1.5 smaller than with roving. The bandwidth and the center frequency were factors having the major influence on the jnd's. For the triangular envelope, the jnd's were smallest for center frequencies that were halfway between two signal components. For the trapezoidal envelope, the jnd's were smallest for center frequencies that coincided with a signal component. For the triangular envelope, the smallest jnd's for each slope were about half the value of those for the trapezoidal envelope. In the case of the smallest jnd's for each spectral envelope shape, the jnd's were smaller when the spectral slope was steeper. Part of the. data for the triangular envelope can be explained by modulation depth discrimination data. The data for the trapezoidal envelope can be interpreted in terms of a spectral profile comparison. The remaining part might be interpreted either in terms of a temporal mechanism, or of the mentioned spectral profile comparison. It is argued that, for this spectral comparison, the ear probably combines information over a limited frequency range by comparing the spectral profiles of the two presented stimuli, rather than comparing individual component levels. (C) 1995 Acoustical Society of America.