Tori of confusion: Binaural localization cues for sources within reach of a listener

To a first-order approximation, binaural localization cues are ambiguous: m any source locations give rise to nearly the same interaural differences. F or sources more than a meter away, binaural localization cues are approxima tely equal for any source on a cone centered on the interaural axis (i.e., the well-known "cone of confusion"). The current paper analyzes simple geom etric approximations of a head to gain insight into localization performanc e for nearby sources. If the head is treated as a rigid, perfect sphere, in teraural intensity differences (IIDs) can be broken down into two main comp onents. One component depends on the head shadow and is constant along the cone of confusion land covaries with the interaural time difference, or ITD ). The other component depends only on the relative path lengths from the s ource to the two ears and is roughly constant for a sphere centered on the interaural axis. This second factor is large enough to be perceptible only when sources are within one or two meters of the listener. Results are not dramatically different if one assumes that the ears are separated by 160 de g along the surface of the sphere (rather than diametrically opposite one a nother). Thus for nearby sources, binaural information should allow listene rs to locate sources within a volume around a circle centered on the intera ural axis on a "torus of confusion.'' The volume of the torus of confusion increases as the source approaches the median plane, degenerating to a volu me around the median plane in the limit. (C) 2000 Acoustical Society of Ame rica. [S0001-4966(00)04803-7].