Control of auditory distance perception based on the auditory parallax model

Simulation of the distance of a sound image within 2 m from a listener, in the absence of reflections and a loudness cue, was investigated. To do this, a model named the “auditory parallax model”, which focuses on the role of parallax angle information involved in head-related transfer functions (HRTFs), was examined with psychoacoustical experiments. For purposes of comparison, experiments were also done on an actual sound source, sound with digitally synthesized HRTFs, and sound with the interaural time difference (ITD) and interaural level difference (ILD) synthesized by the Hirsch–Tahara model. The perceived distance of a sound image by the actual sound source monotonically increased with the physical distance of the source up to 1–1.5 m without any cues of sound pressure level and reflections from walls. The perceived distance of a sound image simulated with the auditory parallax model and that with synthesized HRTFs showed tendencies very similar to those with the actual sound source. On the other hand, for a sound image produced by the Hirsch–Tahara model, the perceived distance of the sound image increased up to around 40 cm and then became saturated. These results show that simple synthesis of the ITD and ILD as a function of distance is insufficient to explain auditory distance perception. However, the results of the experiment in which HRTFs were simulated based on the auditory parallax model showed that the cues provided by the new model were almost sufficient to control the perception of auditory distance from an actual sound source located within about 2 m. Possible reasons for the good performance of the auditory parallax model are the resemblance between the relative frequency characteristics (shape) as well as the shape of ILD as a function of frequency simulated by the auditory parallax model and those of the actual HRTFs.