LEVEL DISCRIMINATION OF FROZEN AND RANDOM NOISE

This paper examines how the difference limen for level, AL, is affected by stimulus bandwidth and variability. The ΔL's were measured in three normal listeners using an adaptive two-interval, forced-choice procedure. The 30-ms stimuli were a 3-kHz tone and nine noise bands with half-power bandwidths ranging from 50 Hz-12 kHz. Except for the 12-kHz bandwidth, which was a low-pass noise, the noise bands were centered at 3 kHz. The ΔL's were measured for both frozen and random noises presented at 30, 60, or 90 dB SPL overall. For frozen noises, the same sample of noise was presented throughout a block of 50 trials; for the random noises, different samples of noise were used in each interval of the trials. Results show that the ΔL's are higher for random than for frozen noises at narrow bandwidths, but not at wide bandwidths. The ΔL's for frozen narrow-band noises decrease with increasing level and are similar to those for the pure tone, whereas the ΔL's for wideband noises are only slightly smaller at 90 than at 30 dB SPL. An unexpected finding is that the ΔL's are larger at 60 than at 30 dB SPL for both frozen and random noises with bandwidths greater than one critical band. The effect of bandwidth varies with level: The ΔL's decrease with increasing bandwidth at low levels, but are nearly independent of bandwidth at 90 dB SPL. The interaction of bandwidth and level is consistent with the multiband excitation-pattern model [M. Florentine and S. Buus, J. Acoust. Soc. Am. 70, 1646–1654 (1981)], but the nonmonotonic behavior of AL as a function of level suggests modifications to the model. © 1990, Acoustical Society of America. All rights reserved.