The role of interaural level differences in the localization of low-frequency sine tones

Abstract

Five human listeners reported the azimuthal locations of low-frequency sine tones presented in free field, either by a 180-degree loudspeaker array or by virtual reality. The virtual sources were synthesized using cross-talk cancellation based on signals continuously monitored in the listener's ear canals. The experiment tested the duplex model of sound localization, especially the role of interaural level differences (fixed ILD = 0, 6, or 12 dB) for frequencies of 750 Hz or less. Trials with real sources, baseline virtual sources, and virtual sources with manipulated ILDs were always combined in all experiments. Experiments showed that the interaural time differences (ITD) dominated most fixed opposing ILDs, as previously reported, only when interaural phase differences (IPD) were less than 90 degrees. When IPDs exceeded 90 degrees, the ITD lost its influence, and localization was dominated by ILDs. Localization judgments never followed IPDs across a 180-degree boundary, except when a zeroed ILD caused judgments to become chaotic. Within the 90-degree IPD limit, judgments for fixed ILD appeared to follow the ITD better than the IPD. Abnormal interaural conditions (e.g., ITDs and ILDs of opposite sign) led to a notable increase in front-back confusions. [Work supported by AFOSR grant FA9550-11-1-0101.]