Median plane sound localization using early head-related impulse response

Previous studies reported that the outline of the spectral notches and peaks in the head-related transfer function (HRTF) in the frequency range above 5 kHz plays an important role in the perception of the vertical angle of a sound image. Moreover, the notches and peaks were reported to be generated in the pinna. These findings imply that the important information for the vertical localization is mostly included in the early part of the head-related impulse response (HRIR), because the response from the pinna arrives at the receiving point (the entrance of the ear canal) earlier than that from the torso. However, the duration of the HRIR required for accurate median plane localization is unclear. In the present study, we measured the HRIRs for seven target vertical angles in the upper median plane (0° to 180° in 30° steps) for five subjects and generated early HRIRs, the durations of which were 0.25, 0.5, 1, and 2 ms. We analyzed the amplitude spectra of the early HRIRs and performed two psycho-acoustical tests with regard to the vertical angle and the distance of a sound image. The results suggested that (1) the outline of the amplitude spectra of the early HRIRs of 1 and 2 ms was approximately the same as that of the full-length HRIR, whereas the outline of the amplitude spectra of the early HRIRs of 0.25 and 0.5 ms differed from that of the full-length HRIR, and (2) no statistically significant difference in the mean vertical localization error or in the scale value of the perceived sound image distance was observed between the full-length HRIR and each of the early HRIRs of 1 and 2 ms at any target vertical angle. These results suggest that the early HRIR of 1 ms includes information of the outline of the spectral notches and peaks with respect to physical aspect. Moreover, the results suggest that the early HRIR of 1 ms provides approximately the same vertical angle and distance of a sound image as the full-length HRIR in the upper median plane with respect to perceptual aspect.