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.
Highlights
The primary acoustic cues for distance perception in the far field are sound intensity, direct-to-reverberant energy ratio (DRR), and spectral cues [1, 2]
The data are separated with respect to the between-subjects factor head tracking and the within-subjects factor normalization, resulting in four subsets: Head Tracking – Loudness Normalization (HTNorm), Head Tracking (HT), Static – Loudness Normalization (STNorm), and Static (ST)
This indicates that head movements had no significant influence on distance estimation of nearby virtual sound sources. (b) Participants did not accurately rate distance of the normalized stimuli (HTNorm and STNorm), suggesting that they could not exploit the non-individual binaural distance cues with the applied loudness normalization method. (c) As expected, participants rated according to the nominal distance if the 6 very distant 5 distant 4 rather distant 3 medium 2 rather close 1 close 0 very close
Summary
The primary acoustic cues for distance perception in the far field are sound intensity, direct-to-reverberant energy ratio (DRR), and spectral cues [1, 2]. Broadband ILDs obtained from HRTFs (Head-Related Transfer Functions) measured with a dummy head can increase about 10 dB to an order of 20 dB [4] or 23 dB [6] for a lateral sound source at a distance of 0.12 m or 0.25 m respectively. Because of these drastic changes in ILDs across the whole spectrum, it is assumed that ILDs play an important role for distance estimation in the near field [2]
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