Dependence of binaural masking level differences on center frequency, masker bandwidth, and interaural parameters

Thresholds for sinusoidal signals masked by noise of various bandwidths were obtained for three binaural configurations: N0S0 (both masker and signal interaurally in phase), N0S pi (masker interaurally in phase and signal interaurally phase-reversed), and N pi S0 (masker interaurally phase-reversed and signal interaurally in phase). Signal frequencies of 125, 250, 500, 1000, 2000, and 4000 Hz were combined with masker bandwidths of 5, 10, 25, 50, 100, 250, 500, 1000, 2000, 4000, and 8000 Hz, with the restriction that masker bandwidths never exceeded twice the signal frequency. The overall noise power was kept constant at 70 dB SPL for all bandwidths. Results, expressed as signal-to-total-noise power ratios, show that N0S0 thresholds generally decrease with increasing bandwidth, even for subcritical bandwidths. Only at frequencies of 2 and 4 kHz do thresholds appear to remain constant for bandwidths around the critical bandwidth. N0S pi thresholds are generally less dependent on bandwidth up to two or three times the (monaural) critical bandwidth. Beyond this bandwidth, thresholds decrease with a similar slope as for the N0S0 condition. N pi S0 conditions show about the same bandwidth dependence as N0S pi, but thresholds in the former condition are generally higher. This threshold difference is largest at low frequencies and disappears above 2 kHz. An explanation for wider operational binaural critical bandwidth is given which assumes that binaural disparities are combined across frequency in an optimally weighted way.