Audio Engineering Society

Chicago Section

Meeting Review, January 25, 2007

other meeting reports

1/25/07 Meeting Highlights
by Nick Kettman

An enthusiastic crowd of about 40 local audio professionals attended Dan Mapes-Riordan's presentation titled "Auditory Illusions: Psychoacoustics in Action," given at the January 25th meeting of the Chicago Section. Mr. Mapes-Riordan began by giving a brief overview and history of the expansive field of psychoacoustics, including a description of a few of the currently "hot topics". He then gave an explanation and demonstration of three auditory illusions: the Franssen Illusion, the Clifton Effect, and the McGurk Effect.


Mr. Mapes-Riordan defined psychoacoustics as "the scientific study of how organisms perceive acoustic waves". The field of psychoacoustics has historical foundations in psychology. Early psychocousticians, including Fechner and Helmholz, performed experiments of a psychophysical nature on sensation and the perception of sound and vision. Their fundamental approach was to determine the relationship between the physical and psychological domains.


Modern psychoacoustics, on the other hand, is a collection of numerous and diverse subfields, including neuroscience and auditory scene analysis, for example. The study of neuroscience involves imaging the brain to determine which areas are used for various auditory tasks. Auditory scene analysis is the study of the brain's ability to segregate concurrent sounds into separate "streams" or "objects". This ability is linked to what are called segregation and fusion cues. These cues are types of temporal or spectral differences and similarities that can be used to group sounds. They include harmonicity, location (ITD, or inter-aural time delay, and ILD, or inter-aural level differences), onsets and offsets, and modulation (AM or FM).


During the second half of his presentation, Mr. Mapes-Riordan demonstrated three auditory illusions. Such illusions can arise when there are conflicting segregation or fusion cues, with a resulting condition in which the psychological perception is very different from the physical stimuli. The Franssen Illusion produces large localization errors due to the ineffectiveness of ITD and ILD cues in determining the location of the source of a sinewave in a reflective room. The Clifton Effect is a result of the breakdown of the precedence effect. The McGurk Effect results when a visual cue contradicts an accompanying auditory cue.