Cue for rear sound image localization in head-related transfer function below 4 kHz

It is known that notches and peaks at frequencies above 5 kHz in the head-related transfer function (HRTF) act as cues for median plane sound localization. However, it has also been shown that front-back discrimination of the direction of a sound image can be achieved even with only the components below 4 kHz. In the present study, we investigated the cues for rear sound image localization below 4 kHz. First, we analyzed the HRTFs for 118 ears (59 subjects) in the median plane and showed that the sound pressure around 1 kHz in the rear HRTF was larger than that in the front HRTF. This boosted band (hereinafter referred to as P0) coincided with Blauert’s directional band. Next, we proposed a hypothesis that can comprehensively explain the two types of cues that have been proposed in the past: spectral notches and peaks, and directional bands. In order to verify the effects of P0 for rear-direction localization, three preliminary psychoacoustic experiments were conducted. The results showed that eliminating P0 tends to increase localization errors at frequencies below 4 kHz. For wide-band signals, adding P0 to a previous parametric HRTF model (N1N2P1P2) tends to reduce the mean vertical localization error, and made the auditory source width approximately the same as that for the measured HRTF. These preliminary results support our hypothesis and imply that P0 acts as a cue for rear sound image localization.