Ear centering in the spatial and transform domains for near-field head-related transfer functions

The head-related transfer function (HRTF) is a major tool in spatial audio. The HRTF for a point source is defined as the ratio between the sound pressure at the ear position and the free-field sound pressure at a reference position. The reference is typically placed at the center of the head. However, when using spherical Fourier transforms (SFT) and distance-varying filters (DVF) to synthesize HRTFs for point sources very close to the head, the synthesized HRTF assumes that the measurement position and the reference position are the same. Ear centering is a technique that overcomes this ambiguity. Ear centering can be performed with translation operators in the spatial domain (the unit sphere) or with DVFs in the transform domain (the SFT domain). This paper presents a review of ear centering and shows that operating in the spatial domain is computationally more efficient than operating in the transform domain. The reason behind this is that transform-domain ear centering requires DVFs that depend on two source distances, from the reference and from the ear, whereas spatial-domain ear centering requires DVFs that depend on the distance from the reference only. Moreover, operating in the spatial domain is more accurate.