1. A method for generating a 3D sound signal, the method comprising: (a) providing a regularized head-related transfer function (HRTF) filter; and (b) applying an input sound signal to the regularized HRTF filter to generate the 3D sound signal, wherein the regularized HRTF filter is generated by: (1) generating a plurality of sets of spatial characteristic function (SCF) samples; (2) applying a corresponding regularizing model to each of one or more of the sets of SCF samples using a corresponding smoothness factor that trades off between smoothness and localization for the corresponding set of SCF samples; (3) combining each set of SCF samples with a corresponding Eigen filter; and (4) summing the results of the combining to generate the regularized HRTF filter.