Influence of flattening of head-related transfer functions in low-frequency region on sound localization

Abstract

It is clear that applications such as virtual auditory displays can be achieved by synthesizing head-related transfer functions with high accuracy. However, in practice, their detailed spectral shapes over the entire frequency range are not likely to be essential to sound localization. For example, low-frequency sound has a tendency to diffract around the head and torso of the listener, which leads to low-frequency characteristics of head-related transfer functions (HRTFs) being largely independent of the position of the sound source. This may imply that HRTFs involve few localization cues in the low-frequency region. We have sought to clarify whether removing spectral cues from HRTFs affects horizontal localization [K. Watanabe et al., Acoust. Sci. & Tech., 32(3), 121–124 (2011)]. In this paper, the low-frequency characteristics of the HRTFs of both ears were flattened below a certain frequency, termed the ``boundary frequency,'' to investigate the influence of the low-frequency component of HRTFs. These flattening procedures were simultaneously applied to the HRTFs of both ears while the interaural level and time differences of the original HRTFs were retained. A localization test using such partially flattened HRTFs was carried out, and the results showed that the flattening of the HRTFs did not significantly affect sound localization at boundary frequencies of 0.5–2 kHz, except for the source direction of 60°. Those boundary frequencies differed depending on source direction. The low-frequency region below these boundary frequencies may be ignored, and some data reduction can be applied here without significant influence on sound localization.