INTENSITY DISCRIMINATION IN THE PRESENCE OF RANDOM-FREQUENCY, MULTICOMPONENT MASKERS AND BROAD-BAND NOISE

This study examined the effects of multicomponent, random-frequency ma skers and broadband-noise maskers on intensity discrimination at 1000 Hz. Maskers and signals were 200 ms, presented simultaneously. In the first set of conditions, thresholds were measured for the detection of a 1000-Hz tone in the presence of 40 or 60 dB SPL random-frequency or noise maskers, with extensive training of listeners with the random-f requency masker to assure stable effects of masker-frequency uncertain ty. The random-frequency maskers had two, six, or ten components chose n at random from a large frequency range (300-3000 Hz, excluding a 160 -Hz band around 1000 Hz). For these maskers, performance across the fo ur listeners was very similar, showing large effects of masker-frequen cy uncertainty. For noise maskers, performance matched predictions for energy-based masking. In the second and third sets of conditions, int ensity discrimination was measured at 1000 Hz for pedestals ranging fr om 40 to 80 dB SPL, first in isolation and then in the presence of the maskers. The pattern of results for intensity discrimination in quiet showed the expected near miss to Weber's Law, but poorer performance than typically observed. The addition of broadband-noise maskers had l ittle effect on performance. However, random-frequency maskers degrade d performance in nearly all conditions, with the size of the effect de pendent on the level of the pedestal relative to the masker. Consideri ng the pedestal as a tonal masker, the data were fitted with various m odels of combined masking. A simple power-law model provided excellent fits, with exponents ranging from 0.24 to 0.35 for the multicomponent maskers, but 1.0 (linear) for the noise. The results support models p ositing that the effects of individual maskers undergo nonlinear trans formation before they are added, independent of the mechanisms which p roduce these effects. Because random-frequency maskers presumably prod uce informational (uncertainty-based) masking, the nonlinearity in thi s case appears central rather than peripheral. (C) 1996 Acoustical Soc iety of America.