Influence of head tracking on distance estimation of nearby sound sources

Binaural synthesis serves as a powerful tool for psychoacoustic research, virtual environments or architectural acoustics. For many of those applications head tracking can be applied and the auralization be adapted accordingly to the listener’s head orientation. By this, several aspects perceiving the auditory scene are enhanced. Many studies (e.g. [1][2]) have shown that head tracking can increase localization accuracy and significantly decrease front-back confusions. As shown in [3] appropriately considering head movements can as well result in a better externalization of sound sources. However, no relationship between head tracking and externalization was found e.g. in other studies, e.g. [4]. In this paper we analyze, if distance perception of nearby sound sources is affected by head tracking. The underlying question of our research is to investigate if monaural or binaural directional cues have an influence on distance perception in the near field. Brungart et al. [5] investigated the specific properties of sound incidence from nearby sound sources in great detail. We confirmed most of these properties in own measurements [6]. One prominent feature of near-field HRTFs (HeadRelated Transfer Functions) is the increase of ILDs (Interaural Level Differences) as a function of source proximity. According to Brungart [5], especially at source distances below 0.5 m, this rise of ILDs is dramatic. Caused by frequency-dependent head-shadowing effects, the ILDs increase the more the sound source is positioned laterally to the head. This effect is stronger for closer distances; at 0.25 m the HRTFs showed ILDs of up to about 23 dB [6]. Thus ILDs might provide a relevant cue for distance perception in the proximal region. The ITDs (Interaural Time Differences) are as well maximal for sound incidence from about φ= 90° and φ= 270° in the horizontal plane. However, the ITDs are barely influenced by source distance. Another prominent effect is the low-pass filtering character of nearby sources, meaning that sound sources are getting darker in timbre as they approach the head. This effect is perceived strongest for very close distances and sound sources at the front or rear. It might as well serve as a monaural cue for distance estimation here. All these cues are altered significantly when the head is turned and thus give the listener additional information. If directional cues (monaural and binaural ones) influence distance estimation of nearby sound sources, it can be expected that head movements are relevant here. Scientific publications investigating the influence of head movements on distance estimation are rare. Simpson and Stanton [7] as well as Rosenblum et al. [8] performed experiments with a loudspeaker-based setup and investigated their influence on distance perception of nearby sound sources. Both studies revealed no influence of head movements on perceived source distance. In order to investigate distance estimation in headphonebased virtual environments, specific near-field HRTFs are required which consider the acoustical specifics of nearby sound sources. In [6] we presented measurements of such near-field HRTFs and performed a first listening experiment which investigated if those near-field HRTFs can be applied to code distance. In this experiment we considered head tracking in all tested conditions and investigated if appropriate distance estimation is still possible when natural level differences between the stimuli are missing. For level-normalized stimuli we found no correlation between the chosen near-field HRTF set and the estimated distance. These findings are supported by the results from a comparable experiment without head tracking by Shinn-Cunningham et al. [9][10] who could as well not reveal any relationship between binaural cues and distance estimation in the near field. These results are in contrast to Brungart [11] who found that distance perception is still possible when loudness differences are removed. Brungart concluded that binaural cues are relevant for distance estimation of nearby sound sources in the proximal region. Several other studies on distance perception of nearby sound sources were applied in virtual acoustic environments without head tracking: Parseihian et al. [12] compared distance perception of real and virtual sound sources. Kan et al. [13] as well as Spagnol et al. [14] used Distance Variation Functions (DVFs) in order to model the influence of nearby sound sources on the monaural and binaural cues. While Kan et al. [13] found improvements in distance estimation by using DVFs, the studies of Spagnol et al. [14] revealed that intensity cues overshadow any other cue and thus the DVFs do not allow a better distance estimation than intensity cues only. All these studies investigate if directional cues (monaural and binaural ones) have an influence on distance perception. This is the general question which motivated us to perform this study. In the experiment presented here we used near-field HRTF datasets of a Neumann KU100 dummy head [6]. These datasets, which are freely available for download, contain impulse responses, measured spherically with a high angular resolution at sound source distances between 0.50 m and 1.50 m. Additionally HRTFs on a circular grid at distances between 0.25 m and 1.50 m were capDAGA 2017 Kiel