Interaural time and level interaction under free-field conditions

Listeners judged the azimuthal location of sine tones presented in free field from an array of 25 small loudspeakers uniformly spaced over 180 degrees in the forward half of the horizontal plane. Probe microphones in the listener's ear canals recorded the tones, and transaural synthesis was used to reverse the interaural time difference (ITD) or the interaural level difference (ILD) across the mid-sagittal plane. Alternatively, the synthesis maintained the natural, consistent ITD and ILD. In the frequency range of 1000 Hz and below, localization judgments reflected the ITD, even as the interaural phase difference (IPD) surpassed 180 degrees with increasing azimuth or frequency, as long as the ILD and ITD had the same sign. In the reversed condition, and with small IPD, judgments reflected a compromise between ITD and ILD, with the ILD weight increasing with increasing frequency. But when the IPD exceeded a critical angle (about 120 degrees of phase) the influence of the ITD dramatically changed: Above 500 Hz, the localization judgments were consistent with ITD from a slipped cycle. At 500 Hz, judgments became chaotic. No critical angle occurred at 250 Hz. These conclusions are relevant in assessing the roles of interaural differences in sound localization. [Work supported by the AFOSR.]