Etienne Hendrickx

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

Binaural reproduction aims at recreating a realistic audio scene at the ears of the listener using headphones. In the real acoustic world, sound sources tend to be externalized (that is, perceived to be emanating from a source out in the world) rather than internalized (that is, perceived to be emanating from inside the head). Unfortunately, several studies report a collapse of externalization, especially with frontal and rear virtual sources, when listening to binaural content using non-individualized Head-Related Transfer Functions (HRTFs). The present study examines whether or not head movements coupled with a head tracking device can compensate for this collapse. For each presentation, a speech stimulus was presented over headphones at different azimuths, using several intermixed sets of non-individualized HRTFs for the binaural rendering. The head tracker could either be active or inactive, and the subjects could either be asked to rotate their heads or to keep them as stationary as possible. After each presentation, subjects reported to what extent the stimulus had been externalized. In contrast to several previous studies, results showed that head movements can substantially enhance externalization, especially for frontal and rear sources, and that externalization can persist once the subject has stopped moving his/her head.

Effects of interaural differences on the loudness of low-frequency pure tones

The loudness related to a sound may vary according to the localization of its source. This phenomenon is described as directional loudness and has been mainly observed for high-frequency sounds and for sources located in the horizontal plane. Because of the acoustic shadow of the head, the left and right ear pressures are modified depending on the source azimuth and the global loudness resulting from a summation process may vary accordingly. But directional loudness has also been reported to occur at 400 Hz, where shadowing effects are usually rather small. It might therefore be suspected that directional loudness effects could be influenced by other parameters involved in the localization process. In a previous study, a small but significant increase of loudness with increasing interaural time difference (ITD) was shown for low-frequency pure tones (200 and 400 Hz) at a low loudness level (40 phon). The present study aimed at getting insight into the potential cause and extent of this effect by assessing the loudness of similar pure tones lateralized with headphones by applying an interaural level difference (ILD) in addition to an ITD and by measuring the effect of ITD at the hearing threshold. It showed significant effects of both ILD and ITD on loudness, and no interaction between these factors. As the effects added even when the factors were contradictory, it supports the hypothesis that the effect is caused by the ITD itself and is not related to the localization process. However, the ITD effect was not significant at the hearing threshold.

Contributions of head-related transfer function choice and head tracking to virtual loudspeaker binaural rendering

This presentation will provide an overview of recent and ongoing studies regarding evaluations of sound fields using virtual loudspeaker binaural synthesis. Of specific interest is an identification of perceptual attributes affected by Head-Related Transfer Function (HRTF) choice beyond basic localization error and the sensitivity of listeners to head tracking with regards to latency and externalization judgments. A list of perceptual attributes, created using a Consensus Vocabulary Protocol elicitation method, and validated through listening tests, resulted in eight valid perceptual attributes for describing the perceptual dimensions affected by HRTF set variations. Employing prescribed head movements, sensitivity to head tracker latency showed small but significant differences between single and multichannel audio source scenes. A similar protocol was employed to comparing the sense of externalization as a function of head rotation with and without head tracking. In contrast to several previous studies,...

Effect of the interaural time difference on the loudness of pure tones as a function of the frequency

Significant loudness variations with source azimuth (i.e. directional loudness) are generally accounted for by at-ear pressure modifications. An effect of the interaural time difference (ITD) was also reported in previous studies by the authors: the loudness of pure tones (200 and 400 Hz) significantly increased when the stimuli were presented with an ITD of 772 μs, corresponding to an azimuth of 90°. The present study aims at observing this effect for higher frequencies, including frequencies at which ITD is no longer useful as a localization cue. The effect of ITD on the loudness of pure tones was thus studied at 500, 707, 1000, 1404 and 2000 Hz. Results show that the effect of ITD on loudness is not significant above 500 Hz, even for frequencies where ITD is still a localization cue. The effect observed at 500 Hz is still in agreement with the results reported by previous studies as the loudness of a pure tone significantly increases when its ITD is 772 μs.

Ventriloquism effect with sound stimuli varying in both azimuth and elevation

When presented with a spatially discordant auditory-visual stimulus, subjects sometimes perceive the sound and the visual stimuli as coming from the same location. Such a phenomenon is often referred to as perceptual fusion or ventriloquism, as it evokes the illusion created by a ventriloquist when his voice seems to emanate from his puppet rather than from his mouth. While this effect has been extensively examined in the horizontal plane and to a lesser extent in distance, few psychoacoustic studies have focused on elevation. In the present experiment, sequences of a man talking were presented to subjects. His voice could be reproduced on different loudspeakers, which created disparities in both azimuth and elevation between the sound and the visual stimuli. For each presentation, subjects had to indicate whether the voice seemed to emanate from the mouth of the actor or not. Results showed that ventriloquism could be observed with larger audiovisual disparities in elevation than in azimuth.

The Influence of Stereoscopy on the Sound Mixing of Movies: A Study on the Front/Rear Balance of Ambience

There is a range of opinions in the cinema industry about the appropriate influence of stereoscopy on the sound mixing for movies. The present study focuses on the perception of ambience. Eight sequences—in their stereoscopic and nonstereoscopic versions, with several different sound mixes—were presented to 44 subjects. For each presentation, subjects had to judge to what extent the mix sounded frontal or “surround.” The goal was to verify whether stereoscopy had an influence on the perception of the front/rear balance of ambience. Results showed that this influence was weak, which was consistent with a preliminary experiment conducted in a mixing auditorium where subjects had to mix the front/rear balance of several sequences themselves.

Should a movie have two different soundtracks for its stereoscopic and non-stereoscopic versions? A study on the front/rear balance

Few psychoacoustic studies have been made on the influence of stereoscopy on the sound mixing of movies. Yet very different opinions can be found among scientific, esthetical or technical communities. Some argue that sound needs to be mixed differently for stereoscopic movies, whereas others pretend that image has actually caught up with sound, that was already “three-dimensional” and should not therefore be affected by stereoscopy. In the present experiment, expert subjects were asked to achieve surround sound ambiance mixings for eleven short sequences presented in both stereoscopic and non-stereoscopic versions. The results suggest that the influence of stereoscopy on the front/rear balance strongly depends on the content of the sequence and only appears in a few specific situations.