The binaural spatializer and transaural filter were combined into a single program which runs in realtime on an SGI Indigo workstation.
Listening to the output of the binaural spatializer via the transaural system is considerably different than listening over headphones. Overall, the spatializer performance is much improved by using transaural presentation. This is primarily because the frontal imaging is excellent using speakers, and all directions are well externalized. The drawback of transaural presentation is the difficulty in reproducing extreme rear directions. As the sound is panned from the front to the rear, it often suddenly flips back to a frontal direction as the illusion breaks down. Most listeners can easily steer the sound to about 120 degrees azimuth before the front-back flip occurs. It is easier to move the sound to the rear with the eyes closed.
Elevation performance with transaural presentation is not as good as with headphone presentation. However, because the sounds are more externalized with the speakers, changing either the azimuth or elevation induces more apparent motion than with headphone presentation. Many listeners reported that changing the elevation also caused the azimuth to change. For instance, starting the sound directly to the right and moving it up often causes the sound to move left towards center before it reaches overhead.
All the performance evaluation discussed is completely informal. It would be useful to have an efficient procedure for evaluating the performance of such systems, one that does not require lengthy training sessions or experimentation.