Auditory localization of nearby sources. Head-related transfer functions

TLDR

Researchers have long recognized the unique properties of the head‑related transfer function (HRTF) for nearby sources, yet most measurements focus on distances of 1 m or more. This study aims to calculate and measure HRTFs for sources between 0.12 m and 1 m. The authors used a rigid‑sphere head model and measured HRTFs with a KEMAR manikin and acoustic point source. The results show that as distance falls below 1 m, interaural level differences for lateral sources rise markedly (even at low frequencies) while interaural time differences stay roughly constant; high‑frequency responses depend on source direction relative to the outer ear, elevation cues are largely distance‑independent, and the pinna’s contribution remains unchanged beyond a few centimeters, indicating that binaural cues are crucial for distance perception of nearby sounds.

Abstract

Although researchers have long recognized the unique properties of the head-related transfer function (HRTF) for nearby sources (within 1 m of the listener’s head), virtually all of the HRTF measurements described in the literature have focused on source locations 1 m or farther from the listener. In this study, HRTFs for sources at distances from 0.12 to 1 m were calculated using a rigid-sphere model of the head and measured using a Knowles Electronic Manikin for Acoustic Research (KEMAR) and an acoustic point source. Both the calculations and the measurements indicate that the interaural level difference (ILD) increases substantially for lateral sources as distance decreases below 1 m, even at low frequencies where the ILD is small for distant sources. In contrast, the interaural time delay (ITD) is roughly independent of distance even when the source is close. The KEMAR measurements indicate that the direction of the source relative to the outer ear plays an important role in determining the high-frequency response of the HRTF in the horizontal plane. However, the elevation-dependent characteristics of the HRTFs are not strongly dependent on distance, and the contribution of the pinna to the HRTF is independent of distance beyond a few centimeters from the ear. Overall, the results suggest that binaural cues play an important role in auditory distance perception for nearby sources.