The Contribution of Head Movement to the Externalization and Internalization of Sounds

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.

Asymmetries in behavioral and neural responses to spectral cues demonstrate the generality of auditory looming bias

“…Consistent with previous studies on distance perception and sound externalization (27)(28)(29), our individualized spectral cue manipulations affected perceived distance. Natural spectral cues (C = 1, measured at a distance of 1.5 m) created externalized auditory percepts that were perceived as farthest from the listener, whereas flattened spectra (C = 0) created percepts of sounds that were very close, often "internal" to the head (29).…”

“…1 A and B) by reducing the contrast of measured high-frequency spectral cues from the natural acoustics of each individual listener. Spectral cues are particularly suitable for this investigation because (i) in contrast to gradual intensity changes, spectral cues can evoke instantaneous changes of distance percepts (27)(28)(29)(30) and thus allow precise analysis of processing latencies; (ii) spectral cue manipulations can create transitions to internal (infinitely close) auditory percepts within peripersonal space where looming bias is most prominent (31); and (iii) spectral cues can be manipulated independently of overall stimulus intensity.…”

Influence of head tracking on the externalization of speech stimuli for non-individualized binaural synthesis

“…In Brimijoin et al (2013), six subjects listened to short phrases (3 s long), reproduced in the horizontal plane at azimuths from −25°to +25°. Two kinds of transfer functions were measured in a room (RT30 = 0.35 s): individualized HRTFs and transfer functions measured from a simple pair of microphones on a bar.…”

The role of spectral detail in the binaural transfer function on perceived externalization in a reverberant environment

“…When applying spectral smoothing on the reverberant part of the BRIR, the effects of the acoustical properties of delayed versions of the sound source on externalization were studied. The experiments were carried out not allowing the listeners to move their heads, since head movements have been demonstrated to affect externalization (Brimijoin et al, 2013).…”