Head-Related Transfer Functions in the Near Field

Although researchers have long recognized the unique properties of head-related transfer functions HRTF in the near field within 1 m of the listeners head, virtually all of the HRTF measurements described in the literature have focused on source locations 1 m or farther from the listener. A rigid-sphere model of the head has been used to calculate the near-field HRTF, and the HRTF was measured at distances of 0.12 m, 0.25 m, 0.50 m, and 1.0 m with a KEMAR manikin and an acoustic point source. Both the calculations and the measurements indicate that the interaural intensity difference increases substantially for lateral sources as distance decreases below 1 m, while the interaural time delay remains roughly independent of distance. The high-frequency features of the KEMAR HRTFs were compressed in azimuth around the ipsilateral ear when the source was close. The elevation-dependent features of the HRTFs were not strongly dependent on distance, and the contribution of the pinna to the HRTF was independent of distance beyond a few centimeters from the ear. The measurements indicate that binaural source-distance cues, which occur only with nearby sources, may allow listeners to make judgments about the distances of nearby sound sources.