On the influence of non-individual binaural cues and the impact of level normalization on auditory distance estimation of nearby sound sources

Abstract

Nearby sound sources provide distinct binaural cues, mainly in the form of interaural level differences, which vary with respect to distance and azimuth. However, there is a long-standing controversy regarding whether humans can actually utilize binaural cues for distance estimation of nearby sources. Therefore, we conducted three experiments using non-individual binaural synthesis. In Experiment 1, subjects had to estimate the relative distance of loudness-normalized and non-normalized nearby sources in static and dynamic binaural rendering in a multi-stimulus comparison task under anechoic conditions. Loudness normalization was used as a plausible method to compensate for noticeable intensity differences between stimuli. With the employed loudness normalization, nominal distance did not significantly affect distance ratings for most conditions despite the presence of non-individual binaural distance cues. In Experiment 2, subjects had to judge the relative distance between loudness-normalized sources in dynamic binaural rendering in a forced-choice task. Below chance performance in this more sensitive task revealed that the employed loudness normalization strongly affected distance estimation. As this finding indicated a general issue with loudness normalization for studies on relative distance estimation, Experiment 3 directly tested the validity of loudness normalization and a frequently used amplitude normalization. Results showed that both normalization methods lead to remaining (incorrect) intensity cues, which subjects most likely used for relative distance estimation. The experiments revealed that both examined normalization methods have consequential drawbacks. These drawbacks might in parts explain conflicting findings regarding the effectiveness of binaural cues for relative distance estimation in the literature.

Do near-field cues enhance the plausibility of non-individual binaural rendering in a dynamic multimodal virtual acoustic scene?

“…Our experiments, as well as the study by Rummukainen et al [14], might indicate that correct reproduction of intensity as the primary and strongest distance cue is, in most cases, sufficient for a plausible representation of nearby sound sources in dynamic multimodal virtual environments. In line with this, experiments on distance perception revealed that, if available, the intensity cue dominates auditory distance estimation and masks the much more subtle near-field cues [11,13]. Furthermore, a multimodal AR experience, as in the present study, provides proprioceptive and visual cues in addition to auditory cues, enhancing auditory localization and providing listeners with more information to judge the plausibility of a virtual sound source reliably.…”

“…Consistently, the synthesized near-field HRTFs show strong low-frequency ILDs for lateral directions at close distances and a significant increase in ILD with increasing frequency. Overall, the described characteristics of the synthesized HRTFs are very similar to those of measured near-field HRTFs [5,11,41], confirming that the synthesis yields correct results. In particular, the low-frequency horizontal plane ILDs and ITDs of the synthesized near-field HRTFs are nearly identical to those of measured Neumann KU100 near-field HRTFs from [5] (see Fig.…”

“…A Levene's test confirmed that the variances of the results are significantly different [F(1,30) = 4.70, p = .038]. We consider this and the absence of correlations across epochs discussed above as indication that the female speech test signal used in Experiment 2 elicited fewer perceptual differences between the near-and far-field HRTFs than the noise test signal used in Experiment 1. cues to auditory distance perception (see, e.g., Arend et al [11] for an overview). However, there is very little research investigating other perceptual aspects of near-field HRTFs, even though, especially with the emerging interest in binaural 6-DoF rendering for real-time VR and AR applications that often have limited resources, it is becoming increasingly important to determine whether simulating physically correct near-field cues is perceptually necessary.…”

“…As briefly outlined above, previous research mainly focused on distance estimation accuracy of nearby sound sources and has obtained partly conflicting results regarding the contribution of the various near-field cues to distance perception [1,2,6,10]. A recent study also investigating the influence of binaural cues on distance estimation of nearby sound sources reviews several studies on this topic and discusses the differing results [11]. Further studies in virtual acoustics that used near-field HRTFs synthesized from far-field HRTFs by applying distance variation functions (DVFs) also exclusively evaluated the influence of the synthesized near-field cues on distance estimation accuracy [12,13].…”

“…Moreover, many of the abovementioned studies tested distance estimation accuracy under unimodal (audio-only) conditions. In most of them, listeners had a passive role (i.e., they could not interact with the sound scene) and had to judge the distance of stationary or dynamic sound events, which, if the study was conducted in virtual acoustics, were even often reproduced with static binaural synthesis only (see, e.g., Arend et al [11] for an overview). To better evaluate individual auditory distance cues, these methods often attempted to eliminate other cues (e.g., the intensity cue by level normalization), resulting in unnatural stimuli.…”