Abstract
BackgroundWhen stimuli are presented over headphones, they are typically perceived as internalized; i.e., they appear to emanate from inside the head. Sounds presented in the free-field tend to be externalized, i.e., perceived to be emanating from a source in the world. This phenomenon is frequently attributed to reverberation and to the spectral characteristics of the sounds: those sounds whose spectrum and reverberation matches that of free-field signals arriving at the ear canal tend to be more frequently externalized. Another factor, however, is that the virtual location of signals presented over headphones moves in perfect concert with any movements of the head, whereas the location of free-field signals moves in opposition to head movements. The effects of head movement have not been systematically disentangled from reverberation and/or spectral cues, so we measured the degree to which movements contribute to externalization.Methodology/Principal FindingsWe performed two experiments: 1) Using motion tracking and free-field loudspeaker presentation, we presented signals that moved in their spatial location to match listeners’ head movements. 2) Using motion tracking and binaural room impulse responses, we presented filtered signals over headphones that appeared to remain static relative to the world. The results from experiment 1 showed that free-field signals from the front that move with the head are less likely to be externalized (23%) than those that remain fixed (63%). Experiment 2 showed that virtual signals whose position was fixed relative to the world are more likely to be externalized (65%) than those fixed relative to the head (20%), regardless of the fidelity of the individual impulse responses.Conclusions/SignificanceHead movements play a significant role in the externalization of sound sources. These findings imply tight integration between binaural cues and self motion cues and underscore the importance of self motion for spatial auditory perception.
Highlights
An externalized sound is one appearing to originate from a source out in the world, whereas an internalized sound appears to originate from somewhere inside the head [1,2]
An anechoically recorded signal convolved with a perfectly measured binaural room impulse response (BRIR) and played over spectrally corrected headphones ought to be perceived as externalized because if the BRIR is spectrally accurate and captures binaural difference cues in their entirety, the acoustics of the convolved signal at both ear canals would precisely match that of a real external sound
The experiment was run with two conditions: either with fullband signals so as to ensure that head and pinna related spectral cues and reverberation would be accessible by the listeners; or with signals lowpass filtered at 500 Hz to examine whether the elimination of high-frequency spectral cues affected externalization in our paradigm
Summary
An externalized sound is one appearing to originate from a source out in the world, whereas an internalized sound appears to originate from somewhere inside the head [1,2]. Sounds presented over headphones are typically internalized, whereas real-world signals tend to be externalized This difference is often attributed to the spectral attributes of the signal [3,4], the amount of reverberation present in the signal [5,6], and/or the way in which the signal source appears to move with the head [7-9]. Sounds presented in the free-field tend to be externalized, i.e., perceived to be emanating from a source in the world This phenomenon is frequently attributed to reverberation and to the spectral characteristics of the sounds: those sounds whose spectrum and reverberation matches that of free-field signals arriving at the ear canal tend to be more frequently externalized. These findings imply tight integration between binaural cues and self motion cues and underscore the importance of self motion for spatial auditory perception
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