This section discusses the 3-D audio system that has been developed
for the ALIVE project at the Media Lab [4]. The audio system
can position sounds at arbitrary azimuths and elevations around a
listener's head. The system uses stereo loudspeakers arranged
conventionally (at 
degrees with respect to the listener).
The system works by reconstructing the acoustic pressures at the
listener's ears that would occur with a free-field sound source at the
desired location. This is accomplished by combining a binaural
spatializer with a transaural audio system. The spatializer
convolves the source sound with the direction dependent filters that
simulate the transmission of sound from free-field to the two ears.
The resulting binaural output of the spatializer is suitable for
listening over headphones. In order to present the audio over
loudspeakers, the output of the spatializer is fed to a transaural
audio system which delivers binaural signals to the ears using stereo
speakers. The transaural system filters the binaural signals so that
the crosstalk leakage from each speaker to the opposite ear is
canceled.
Transaural technology is applicable to situations where a single listener is in a reasonably constrained position facing stereo speakers. If the listener moves away from the ideal listening position, the crosstalk cancellation no longer functions, and the 3-D audio illusion vanishes. As discussed earlier, the ALIVE system uses video cameras to track the position of the listener's head and hands. A goal of this work is to investigate whether the tracking information can be used to dynamically adapt the transaural 3-D audio system so that the 3-D audio illusion is maintained as the listener moves.
We will first briefly review the principles behind binaural spatial synthesis and transaural audio. Then we will discuss the 3-D audio system that has been constructed for the ALIVE project. Finally we will discuss how the head tracking information can be used, and give preliminary results.