Virtual sound localization using head related transfer functions modified in the spectral modulation frequency domain
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Sound localization cues generally include interaural time difference (ITD), interaural intensity difference (IID) and spectral cues. The purpose of this study is to investigate the important spectral cues involved in so-called head related transfer functions (HRTFs) using a combination of HRTF analyses and a virtual sound localization (VSL) experiment. One of the advantages of VSL is that it allows one to manipulate different cues systematically, and sound stimuli used in VSL can be synthesized in a well controlled way. Previous investigations have shown that HRTFs vary greatly among different subjects and that performance in VSL tasks using randomly selected, non-individualized HRTFs is considerably worse than performance with individualized HRTFs. An efficient procedure for HRTF customization and subject training in a VSL task were developed in the present study to improve the VSL performance using non-individualized HRTFs. The HRTF customization procedure combined both subjective and objective evaluation methods. The overall virtual localization accuracy (in the baseline condition) of all of our subjects was higher than that reported by studies using non-individualized HRTFs. The individually customized HRTFs were used for further investigation of the spectral cues in sound localization. Previous psychoacoustical and physiological studies have both suggested the existence of spectral modulation frequency (SMF) channels for analyzing spectral information ( e.g. the spectral cues coded in HRTFs). SMFs are in a domain related to the Fourier transform of HRTFs. The relationship between various SMF regions and sound localization was tested here by filtering or enhancing HRTFs in the SMF domain under a series of conditions using a VSL experiment. Present VSL results revealed that azimuth localization was not significantly affected by HRTF manipulation. Applying notch filters between 0.1 and 0.4 cycles/octave or between 0.35 and 0.65 cycles/octave resulted in significantly less accurate elevation responses at low elevations, while spectral enhancement in these two SMF regions did not produce a significant change in sound localization. Likewise, lowpass filtering at 2 cycles/octave did not significantly influence localization accuracy, suggesting that the major cues for sound localization are in the SMF region below 2 cycles/octave. Together, these results highlight the relationship between SMF region and sound localization in the vertical plane. These results also confirm and extend previous attempts at optimizing the use of non-individualized HRTFs in VSL. Knowledge of the influence of manipulations in the SMF domain on sound localization has implications for the application of various signal processing algorithms and models of monaural and binaural hearing.