Remote Masking and the Binaural Masking-Level Difference

Abstract

The improvement in detection performance known as the binaural masking-level difference (BMLD) is usually measured as the difference between signal levels that correspond to 75% correct obtained when both signal and masking noise have zero interaural phase difference (NoSo) and when the noise is in-phase but the signal has a 180° interaural phase difference (NoSπ) (Colburn and Durlach 1978). One of the principal models for the BMLD is Durlach’s equalization and cancellation model (Durlach 1963). One result, difficult to accommodate within the context of this model, is the near disappearance of the BMLD in remote masking when the signals have a frequency remote from an narrowband masking noise (Zwicker and Henning 1984). The decrease in the BMLD is not caused by reduced masking, which remains substantial at signal frequencies where the BMLD has become very small. Figure 1 [from Fig. 5 of Zwicker and Henning (1984)] illustrates the awkward result. The black (NoSπ) and white (NoSo) symbols in the left panel show median thresholds for detecting 600-ms tones of different frequency. (Thresholds in quiet are shown near the 10-dB level.) The continuous masking noise, centred on 250 Hz, had a bandwidth of 10 Hz and a noise-power density of 50 dB SPL. The corresponding median BMLD, shown in black in the right panel, was about 25 dB when the signal was centred in the noise, but fell abruptly when the signal lay more than a few tens of Hertz from the noise. Because some 30 dB of remote masking occurs, sizeable BMLD’s would be expected to result if the stimuli to the ears were, in effect, subtracted in the NoSπ condition. The grey symbols (left panel, NoSπ eff) show thresholds when the centre frequency of the masking noise was shifted to correspond to that of the signal and the level of the noise adjusted to produce the same masking in NoSo as when the noise was centred on 250 Hz. When the signal phase was then