The paper describes a method for obtaining spherical sets of head-related transfer functions (HRTFs) based on a small number of measurements in reverberant environments. For spatial upsampling, we apply HRTF interpolation in the spherical harmonics (SH) domain. However, the number of measured directions limits the maximal accessible SH order, resulting in order-limitation errors and a restricted spatial resolution. Thus, we propose a method which reduces these errors by a directional equalization based on a spherical head model prior to the SH transform. To enhance the valid range of a subsequent low-frequency extension towards higher frequencies, we perform the extension on the equalized dataset. Finally, we apply windowing to the impulse responses to eliminate room reflections from the measured HRTF set. The analysis shows that the method for for spatial upsampling influences the resulting HRTF sets more than degradations due to room reflections or due to distortions of the loudspeakers.
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