The role of head-induced interaural time and level differences in the speech reception threshold for multiple interfering sound sources
Journal of the Acoustical Society of America
Three experiments investigated the roles of interaural time differences ͑ITDs͒ and level differences ͑ILDs͒ in spatial unmasking in multi-source environments. In experiment 1, speech reception thresholds ͑SRTs͒ were measured in virtual-acoustic simulations of an anechoic environment with three interfering sound sources of either speech or noise. The target source lay directly ahead, while three interfering sources were ͑1͒ all at the target's location ͑0°,0°,0°͒, ͑2͒ at locations distributed
... ions distributed across both hemifields ͑Ϫ30°,60°,90°͒, ͑3͒ at locations in the same hemifield ͑30°,60°,90°͒, or ͑4͒ co-located in one hemifield ͑90°,90°,90°͒. Sounds were convolved with head-related impulse responses ͑HRIRs͒ that were manipulated to remove individual binaural cues. Three conditions used HRIRs with ͑1͒ both ILDs and ITDs, ͑2͒ only ILDs, and ͑3͒ only ITDs. The ITD-only condition produced the same pattern of results across spatial configurations as the combined cues, but with smaller differences between spatial configurations. The ILD-only condition yielded similar SRTs for the ͑Ϫ30°,60°,90°͒ and ͑0°,0°,0°͒ configurations, as expected for best-ear listening. In experiment 2, pure-tone BMLDs were measured at third-octave frequencies against the ITD-only, speech-shaped noise interferers of experiment 1. These BMLDs were 4 -8 dB at low frequencies for all spatial configurations. In experiment 3, SRTs were measured for speech in diotic, speech-shaped noise. Noises were filtered to reduce the spectrum level at each frequency according to the BMLDs measured in experiment 2. SRTs were as low or lower than those of the corresponding ITD-only conditions from experiment 1. Thus, an explanation of speech understanding in complex listening environments based on the combination of best-ear listening and binaural unmasking ͑without involving sound-localization͒ cannot be excluded.