# Head-Related Transfer Functions for Dynamic Listeners in Virtual Reality
###### Abstract  
In dynamic virtual reality, visual cues and motor actions aid auditory perception. With multimodal integration and auditory adaptation effects, generic head-related transfer functions (HRTFs) may yield no significant disadvantage to individual HRTFs regarding accurate auditory perception. This study compares two individual HRTF sets against a generic HRTF set by way of objective analysis and two subjective experiments. First, auditory-model-based predictions examine the objective deviations in localization cues between the sets. Next, the HRTFs are compared in a static subjective (\(N=8\)) localization experiment. Finally, the localization accuracy, timbre, and overall quality of the HRTF sets are evaluated subjectively (\(N=12\)) in a six-degrees-of-freedom audio-visual virtual environment. The results show statistically significant objective deviations between the sets, but no perceived localization or overall quality differences in the dynamic virtual reality. head-related transfer function (HRTF); spatial audio; virtual reality; binaural rendering; audio-visual integration; 6-degrees-of-freedom +
Footnote †: journal: Applied  
14 June 20217 July 20  
permitted in free-field listening [10]. Few studies investigate auditory perception during full-body 6-DoF movement. Although the listener is in translational motion, the minimum audible angle has been found to increase compared to static listening [11]. This finding hints towards an increase in localization blur during full-body motion. Such phenomena may mask any potential localization benefits granted when using individualized over generic HRTF sets. Directly comparing individual and generic HRTFs in dynamic localization tasks showed mixed results. In some papers, no effect on localization accuracy between individual and generic HRTFs is found when head-tracked rendering is used [5; 8]. At the same time, some studies show significant improvements in localization accuracy or preference for individual over generic HRTFs, even in dynamic conditions [12; 13]. However, localization is only one attribute of quality in a VR environment. One study compared the evaluation of overall audio quality either using generic or individual HRTFs in the context of 6-DoF VR critical listening via a multistimulus method. Apart from a small effect in the participants' sensitivity to elevation errors in favor of individual HRTFs, all HRTF sets yielded similar quality scores [14]. No perceptual differences were found between realistic binaural auralizations of a classroom scenario when rendered with either individual or generic HRTFs [15]. Finally, an experiment conducted using spatialized music content found a general preference of generic HRTFs over individual HRTFs [16]. Instead of arduously measuring individual HRTFs, different strategies have been developed to select the anthropometrically closest matching HRTF set from an existing HRTF database [17]. Based on an auditory model, the estimated localization accuracy has been shown to objectively increase when using anthropometric parameters to select the most suitable HRTF set [18]. However, anthropometry-based HRTF matching has been perceptually investigated in VR, where no effect on questionnaire results was found between the best match HRTFs and generic HRTFs for a free exploration task [19]. Comparing best and worst match HRTFs in a VR shooter game found a minimal impact of the HRTFs on participant performance for the most elevated target regions. Still, the benefit was limited to the early stages of the game, signaling a potential adaptation to the HRTFs as the game progressed [20]. Multiple sensory modalities help us to construct a mental representation of our surroundings [21]. In VR, audio-visual cross-modal training quickly improves source localization accuracy after short training periods [22; 23; 24]. Active tasks, such as head movements [25] or interaction with a sound source [26], are found to further increase the adaptation from our biological set to altered localization cues.