Individualization Feature of Head-Related Transfer Functions Based on Subjective Evaluation
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To realize a three-dimensional virtual sound image with a Virtual Auditory Display (VAD), it is important to individualize Head Related Transfer Functions (HRTFs) for listeners. The purpose of the present study was to establish a fitting method for HRTFs based on a listening test. To this end, a number of sets of virtual images were synthesized using HRTFs of different individuals. The listeners were then asked to choose appropriate virtual sound images located in the intended orbits. To achieve our fitting method of HRTFs by such a subjective evaluation, it is desirable that the same HRTFs can be chosen with stability. In this study, the process used to select a set of HRTFs subjectively was examined in detail, and the features of the individualization of HRTFs by subjective evaluation were investigated. First of all, the process to choose the best of 32 sets of HRTFs by a Swiss-style tournament was repeated ten times, and the regularity of wins in the tournament was examined. As a result, it was understood that the same set of HRTFs is not always chosen and that the individualization method has probability features. The strength of the sets of HRTFs which won the tour- nament several times was then evaluated. A round-robin comparison with the 130 sets of HRTFs in our HRTF-corpus was repeated twenty times. It was shown that a subjective evaluation itself was also a probability feature. Moreover, the percentage of winning for the set of HRTFs which won the tournament was estimated to be about 15% from the results of the round-robin comparison.