The masking-level difference in low-noise noise.

In experiment 1 NoSo and NoS pi thresholds for a 500-Hz pure tone were obtained in a low-fluctuation masking noise and a high-fluctuation masking noise for six normal-hearing listeners. The noise bandwidth was 10 Hz. In agreement with previous investigations, the NoSo thresholds were lower in low-fluctuation noise than in high-fluctuation noise. For three listeners, NoS pi thresholds were similar for the two types of noise, while for the other three listeners, Nos pi thresholds were higher for low-fluctuation noise than for high-fluctuation noise. In experiment 2, the masker was created by amplitude modulating a 500-Hz pure tone by a 0-10-Hz low-pass noise. The degree of masker fluctuation was controlled by adjusting the average modulation depth (100%, 63%, 40%, and 25%). The signal was a 10-Hz-wide noise centered on 500 Hz. Results were similar to those of experiment 1: for the NoSo conditions, signal detection improved with decreasing degree of fluctuation, and for NoS pi conditions, the results were subject dependent. For three listeners, NoS pi thresholds were again similar in the two types of noise, while for the other three listeners, NoS pi thresholds were again higher in low-fluctuation noise than in high-fluctuation noise. The results showed that a high degree of masker fluctuation sometimes facilitates NoS pi detection. It is possible that the binaural detection mechanism utilizes the relatively good signal-to-noise ratios that occur in the low power or "dip" regions of fluctuating masker waveforms.