An experimental investigation on cues of auditory distance perception in the free field based on dynamic virtual experiment

Previous physical analysis and experiments with real sound source indicated that loudness and near-field head-related transfer functions (HRTFs, including interaural level difference ILD and pressure spectra at ears) are cues for auditory distance perception in the free filed. However, recent experiments conducted on the platform of virtual auditory display (VAD) have yielded inconsistent results, especially regarding the controversy over whether ILD in near-field HRTF is a crucial cue on distance perception in the free field. This work aims to settle the controversy and further investigate the contribution and interaction of various distance perception cues in the free field. Based on a platform of dynamic VAD and accurate near-field HRTFs, binaural signals under different conditions are synthesized and presented by headphone, and the contributions of distance perception cues are then identified by psychoacoustic experiments. Firstly, results indicate that the contributions of various distance perception cues depend on source direction: (1) Loudness contributes significantly to distance perception in all directions; (2) ILD contributes importantly at lateral directions. Contribution of ILD reduces as source departs from lateral to the frontal, back or high-elevation directions; (3) Spectral cue contributes weakly to distance perception. Only in the median plane where the ILD vanishes may the contribution of spectral cue be detected. Secondly, different cues interact with one another but are somewhat redundant: (1) The coordination of ILD and loudness obviously enhances distance perception at lateral directions; (2) The coordination of loudness and spectral cue enhances distance perception at rear direction to some extent; (3) ILD or loudness alone enables to moderately distance perception at lateral direction. The controversy on the results of some recent experiments is due to the fact that these experiments were conducted on the platform of static VAD, which were inclined to cause in-head localization and influence distance perception. Moreover, inaccurate near-field HRTF data, inappropriate experimental and statistical methods may also cause inconsistent results.