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Spatial Sound for the wearable jacket

The human brain uses sound as an omni-directional sense of location, whilst the eyes have a higher spatial resolution, they only do so across a small area within the angle of vision. In contrast, sound can be positioned by the brain coming from any angle and elevation. This makes it an ideal resource for constructing user-interfaces as several streams of information can be presented in parrallel and the user can conciously decided which stream to listen to without any physical interface to the jacket. In order to allow this, headphones must be worn by the user, this prevents cross-over of sound intended for one ear reaching the other which will destroy the spatial effect. This effect can be heard in the following two samples which both feature two speakers, and show how (in this case bin-aural sound) can be deciphered by the brain to focus on the seperate streams (try them with headphones).
Monoaural example   Binaural example

Psycho-acoustic spatial cues

The brain uses two main cues to detect the azimuth of sound sources. These are the ITD (Inter-aural Time Delay) or phase difference and the ILD (Inter-aural Level Difference).

ITD Calculations

As sound propagates through space omni-directionally (rather than in rays as light does) it can follow curved paths as it defracts around surfaces e.g the human head. This means that the sound path to the ear furthest from the sound source contains a curved component as it rounds the head. This can be seen in the simple case which is illustrated in the diagram below:

Sounds are not always on the X axis though, so the case for arbitary sound locations is shown in the next diagram, this shows the calculation for finding the line of incidence from an arbitary point and then the curved path:

For the complete case where the direction angle on the head can be arbitary the calculation becomes more complex and is shown in the next diagram:

ILD Calculations

Approximations for use on the ADS Bitsy


This page last updated April 23, 2002
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