The psychoacoustic effects of stimuli plausibility on headphone externalisation

Fazenda, BM; Newton, J

Abstract

In the world we live in, sound events are typically associated with a physical event happening within our surroundings. In this case, the event carries a degree of plausibility in that it exists in the physical world and should therefore have associated with it all the aural cues that allow our hearing system to place it somewhere in the space around us. Throughout life, one’s ability to locate such sounds is learnt through incorporating Interaural Time Differences (ILDs), Interaural Level Differences (ILDs), Head Related Transfer Functions and Room Reverberation of the signals arriving at our ears.
The field of spatial hearing is well established and the importance of the various acoustic cues for realistic auditory perception has been researched in considerable depth. When listening on headphones, these perceptual features are not present, causing sounds to have an apparent location inside the head. Much research has been devoted to counteract this effect by processing headphone audio in such a way that it contains a form of the features which are usually present in everyday listening, and thus evoke out-of-head localization or externalisation.
In the area of Human perception, seminal studies have demonstrated that there exists a multimodal integration between the senses of sight and sound that is crucial for our sense of reality. Despite this, there seems to be little attention given to the aspects of expectation and plausibility of stimuli, in eliciting out-of-head effects when the sound events are presented over headphones.
A previous study presented at RS 2013 looked at factors contributing to the percept of externalization over headphones. It was found that correctly modeled binaural cues could elicit the effect of externalization. However, an interesting find revealed that the plausibility of a stimuli existing in the space where the audition was taking place was also a significant effect in creating the percept of externalization.
In the study presented here, this effect has been researched further using a novel experimental methodology. In this case, a number of sound events have been enacted within the room where a subject (listener) is engaged in a task (playing a computer game). The subject was asked to pay attention to the events taking place, mainly by listening to the sounds of those events. The plausible sound sources in the room were acted silently (by mimicking) whilst their sounds were replayed solely over the headphones. This was carried out with 2 listener groups, each of them being played the sound events with or without binaural cues. The sound events were plausible in the sense that they were associated with objects placed and moved around the room. Additional sound events, used as experimental controls, were played over the headphones but these did not have their plausible physical counterparts in the room. The perception of externalisation was assessed through a questionnaire at the end of the test where subjects were required to indicate whether they heard the sounds within the headphones or in the room and, in the latter case, where the sound event was located.
The study thus poses the following research question:
What is the relative importance between sound event plausibility and correct binaural cues in eliciting the percept of externalization when listening on headphones?
Our motivation is to find evidence that may be used by producers and sound designers concerned with the creation of believable audio scenes in 3 dimensional audio rendering systems.

Citation

Fazenda, B., & Newton, J. The psychoacoustic effects of stimuli plausibility on headphone externalisation. Presented at Reproduced Sound 2014, NEC, Birmingham

• Presentation Conference Type: Other
• Conference Name: Reproduced Sound 2014
• Conference Location: NEC, Birmingham
• Publication Date: Feb 1, 2015
• Deposit Date: May 19, 2016
• Series Title: Proceedings of the Institute of Acoustics
• Book Title: Reproduced Sound 2014
• ISBN: 9781634395212
• Additional Information: Event Type : Conference