Humans localize in the horizontal plane by utilizing interaural time differences (ITDs) at low frequencies (1.5 kHz) and interaural level differences (ILDs) at high frequencies. The physical interaural differences become complex functions of azimuth and frequency when sound interacts with the head. For example, ILDs demonstrate non-monotonic azimuthal angle functions for 1–4 kHz, which is a result of destructive interference (i.e., acoustical bright spots). The shift from ITD to ILD cues were previously thought to explain relatively poor human sound localization performance at these frequencies; however, it is unclear if other factors contribute to the frequency dependence in performance. Therefore, free-field sound localization was measured in 20 young normal-hearing listeners using narrowband noises between 0.25–8 kHz. The frequency dependence at larger (45°) source angles coincided and were explained by the non-monotonic ILD-azimuth functions. Smaller (45°) source angles also showed relatively poor sound source localization at all frequencies besides 750–1000 Hz and 4000 Hz. This suggests that not all frequencies are equally important for human horizontal-plane sound localization, and may mean that auditory spatial maps are based on physical interaural cues from only two frequency channels. The results have implications for refining the duplex theory of sound localization.
AlQasem et al. (Wed,) studied this question.
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