On the subjective nature of binaural externalisation

Externalisation (the sensation of hearing a sound object outside the head) on headphone reproduction is a recurrent topic in modern research for binaural sound reproduction. One of the inherent issues with testing for externalisation is its quantification. Generally, subjects are asked to indicate if a source sounds ‘more external’ or ‘more distant’ in order to determine the degree to which they are externalising. Herein lies an issue with the response’s subjective nature. It cannot be certain that what a subject believes to be external is a genuine perception of externalisation, and a source seeming further away doesn’t necessarily imply it is being localised farther from the head. There is also an issue with cohesion between subjects: it cannot be assured that two subjects that indicated a source to be more external by the same level actually perceived it to the same level as externalisation is an immeasurable response. The motivation therefore exists for a means of confirming that a subject is in fact externalising a source. This was attempted using considerations of the cocktail party effect in previous research1,2,3,4.An aspect that has received little attention in past research regards the situation in which the stimulus is presented. It follows from Brookes6 and Wersényi7, who’s findings both involved measures taken to ‘fool’ the auditory system into perceiving the auralisation as a natural event, that the stimulus being realistic in the context of the current situation is an important factor. The effect that the plausibility of a given stimulus occurring has on its externalisation is hence of particular interest.The main objectives of the study presented here are thus as follows:1. To investigate an objective measure of externalisation via binaural simulation of room acoustics and the use of a speech intelligibility performance task.2. To investigate the effect that the plausibility of a stimulus location has on its externalisation when reproduced binaurally