Calculation of head related transfer functions in the proximity region using spherical harmonics decomposition: comparison with measurements and evaluation
Khoa-Van Nguyen, Thibaut Carpentier, Markus Noisternig, and Olivier Warusfel
This article describes a comparison between different compensation and range extrapolation methods to predict head related transfer functions (HRTFs) in the proximity of the head, and acoustic measurements of a HEAD acoustics dummy head in an anechoic room. An extensive set of left and right ear impulse responses was measured on a dense angular grid over an open sphere at eight different distances from the head, providing a total of 6920 measurement positions. In this study, HRTF data is represented in the spherical wave spectrum, turning the range extrapolation problem into an acoustic radiation problem. The radial extrapolation is calculated from a given set of HRTFs at a fixed distance, from near-field to far-field and vice versa. The prediction based on spherical harmonics is compared with methods such as the inverse distance attenuation coefficient, a nearfield compensation filter, and a geometrical approach for the selection of HRTFs. A spectral distortion measure is computed to quantify the similarity between predicted HRTFs and actual measurements. The data fit between predicted and measured HRTFs is correct for most of the different methods but the spherical harmonics method provides more convincing results especially for nearfield prediction at low and medium frequencies.