Interaural fluctuations and the detection of interaural incoherence: bandwidth effects.

One-hundred left-right noise-pairs were generated, all with a fixed value of long-term interaural coherence, 0.9922. The noises had a center frequency of 500 Hz, a bandwidth of 14 Hz, and a duration of 500 ms. Listeners were required to discriminate between these slightly incoherent noises and diotic noises, with a coherence of 1.0. It was found that the value of interaural coherence itself was an inadequate predictor of discrimination. Instead, incoherence was much more readily detected for those noise-pairs with the largest fluctuations in interaural phase or level differences (as measured by the standard deviations). One-hundred noise-pairs with the same value of coherence, 0.9922, and geometric mean frequency of 500 Hz were also generated for bandwidths of 108 and 2394 Hz. It was found that for increasing bandwidth, fluctuations in interaural differences varied less between different noise-pairs and that detection performance varied less as well. The results suggest that incoherence detection is based on the size of interaural fluctuations and that the value of coherence itself predicts performance only in the wideband limit.