**4. Discussion**

In the previous section, the SDR and SD computed in five domains were compared. It was shown that the larger number of PCs brings about the accurate reconstruction of HRTFs and HRIRs in all domains. As differences in domains in the SPCA of HRTFs and HRIRs, the average SDR has the largest value in the domain C, and the average SD has the smallest value in the domains L and CL for almost any given number of components. Since the HRTFs and HRIRs are used for the sound signals at the listener's both ears, it is essential to take their subjective evaluation into account. Most of the previous researches dealt with both the objective and subjective evaluation [17, 19, 27, 28]. However, three investigations can be found, in which the relation of the subjective evaluation to the SDR and SD values. Hanazawa *et al.* reported the results of a hearing experiment in which the relation between the accuracy of the interpolated HRIRs with their proposed method and the sound localization performance when the sound stimuli convoluted with them were presented [29]. They showed that around 6 dB in SDR had insignificant difference from the performance when the sound stimuli convoluted with the original HRIRs. Takane *et al.* also carried
