Sound externalization in dynamic binaural listening: A comparative behavioral and EEG study.

Binaural reproduction aims at recreating a realistic sound scene at the ears of the listener using headphones. Unfortunately, externalization for frontal and rear sources is often poor (virtual sources are perceived inside the head, instead of outside the head). Nevertheless, previous studies have shown that large head-tracked movements could substantially improve externalization and that this improvement persisted once the subject had stopped moving his/her head. The present study investigates the relation between externalization and evoked response potentials (ERPs) by performing behavioral and EEG measurements in the same experimental conditions. Different degrees of externalization were achieved by preceding measurements with 1) head-tracked movements, 2) untracked head movements, and 3) no head movement. Results showed that performing a head movement, whether the head tracking was active or not, increased the amplitude of ERP components after 100 ms, which suggests that preceding head movements alters the auditory processing. Moreover, untracked head movements gave a stronger amplitude on the N1 component, which might be a marker of a consistency break in regards to the real world. While externalization scores were higher after head-tracked movements in the behavioral experiment, no marker of externalization could be found in the EEG results.

Loudness constancy for noise and speech: How instructions and source information affect loudness of distant sounds.

The physical properties of a sound evolve when traveling away from its source. As an example, the sound pressure level at the listener's ears will vary according to their respective distance and azimuth. However, several studies have reported loudness to remain constant when varying the distance between the source and the listener. This loudness constancy has been reported to occur when the listener focused attention on the sound as emitted by the source (namely the distal stimulus). Instead, the listener can focus on the sound as reaching the ears (namely the proximal stimulus). The instructions given to the listener when assessing loudness can drive focus toward the proximal or distal stimulus. However, focusing on the distal stimulus requires to have sufficient information about the sound source, which could be provided by either the environment or by the stimulus itself. The present study gathers three experiments designed to assess loudness when driving listeners' focus toward the proximal or distal stimuli. Listeners were provided with different quality and quantity of information about the source depending on the environment (visible or hidden sources, free field or reverberant rooms) and on the stimulus itself (noise or speech). The results show that listeners reported constant loudness when asked to focus on the distal stimulus only, provided enough information about the source was available. These results highlight that loudness relies on the way the listener focuses on the stimuli and emphasize the importance of the instructions that are given in loudness studies.

Does Loudness Relate to the Strength of the Sound Produced by the Source or Received by the Ears? A Review of How Focus Affects Loudness.

Loudness is the magnitude of the auditory sensation that a listener experiences when exposed to a sound. Several sound attributes are reported to affect loudness, such as the sound pressure level at the listener's ears and the spectral content. In addition to these physical attributes of the stimulus, some subjective attributes also appear to affect loudness. When presented with a sound, a listener interacts with an auditory object and can focus on several aspects of the latter. Loudness appears to differ depending on how listeners apprehend this object, notably whether they focus on the sound that reaches their ears or that is produced by the source. The way listeners focus on the auditory object may depend on the stimulus itself. For instance, they might be more likely to focus on the sound emitted by the source if the latter is visible. The instructions given by the experimenters can also explicitly direct the listener's focus on the sound reaching the ears or emitted by the source. The present review aims at understanding how listeners focus on the auditory object depending on the stimuli and instructions they are provided with, and to describe how loudness depends on this focus.

Loudness of low-frequency pure tones lateralized by interaural time differences.

Directional loudness is that phenomenon by which the loudness of a sound may vary according to the localization of its source. This phenomenon has been mainly observed for high-frequency sounds, 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. 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. This study assessed the loudness of low-frequency pure tones (200 and 400 Hz) lateralized with headphones by applying an interaural time difference (ITD) but no interaural level difference. It showed small but significant variations of loudness with respect to ITD at a low loudness level (40 phon): ITD values associated with virtual azimuths of ±60° and ±90° led to a small but significant increase in loudness (up to 1.25 dB). However, there was no such effect at a moderate loudness level (70 phon).

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.