Measurement of near-field head-related transfer functions: Dense measurements in the upper hemisphere

This article reports on measurements of the near-field head-related transfer functions (HRTFs) of a head and torso simulator (HATS) conducted at NHK Science and Technology Research Laboratories. The measurements included 27 source distances ranging from 0.20 m to 1.50 m at 0.05 m intervals, and 865 directions at each distance. It is desirable to use a sufficiently small sound source that has a wide frequency range such that it approximates a point source with omnidirectional characteristics for the measurements of near-field HRTFs. This article introduces the measurement system constructed in an acoustic anechoic chamber at NHK Science and Technology Research Laboratories and the compact loudspeaker manufactured to meet these requirements. The measurements reveal that, in addition to the overall decrease in the HRTF magnitudes with distance, a sharp attenuation of the low-frequency magnitude could be seen on the ipsilateral side of the sound source, and peaks and notches observed in the spectra varied in their depth and frequencies. These tendencies are consistent with previous reports on the near-field HRTFs.