Preliminary results

In order to experiment with head tracking in the context of transaural 3-D audio, we are currently using a Polhemus tracking system. This system returns the position and orientation of a sensor with respect to a transmitter (6 degrees of freedom). The sensor can be easily mounted on headphones or a cap to track head position and orientation. The head position and orientation can be used to update the parameters of the 3-D spatializer and transaural audio system. Results are preliminary at this time.

The strategy used to update transaural parameters based on head position and orientation obviously depends greatly on the head model used for the transaural filter. We used the simple head model given in equation 11. The following points were observed:

• For front-back motions, the symmetrical transaural filter can be used, and the interaural delay can be adjusted as a function of distance between the speakers and the listener. This has been tested and seems to be effective.
• For left-right motions and head rotations, the symmetrical transaural filter is no longer correct. The general form of the transaural filter (equation 7) may be used instead, but at much greater computational cost. It may be better to abandon the simplified IIR model and use an FIR implementation based on a more realistic head model [21].
• To compensate for head rotations, the general form of the transaural filter (equation 7) was implemented with the simplified head model (equation 11). The resulting dynamic filter compensated for the changing path lengths between the speakers and the ears in order to keep the cancellation signals aligned properly. However, this filter did not function effectively. In part this was due to the audible artifacts from the various linearly interpolated delay lines used in the implementation. However, it may be that small time adjustments are in fact unnecessary, judging from the insensitivity to small head rotations with a static transaural system. This is also suggested in the literature on equalizing dummy-head recordings for loudspeaker reproduction [14].

Using the static, symmetrical transaural system described earlier, the head tracking information was also used to update the positions of 3-D sounds so that the auditory scene remained fixed as the listener's head rotated. This gives the sensation that the source is moving in the opposite direction, rather than remaining fixed. There is a good reason for this. Using a static transaural system, the position of rendered sources remains fixed as the listener changes head orientation (provided that the change in head orientation is small enough to maintain the transaural illusion). This is contrary to headphone presentation, where the auditory scene moves with the head, even for small motions. Thus, the transaural presentation doesn't require compensation for small head motions, and if the compensation is provided, it is perceived as motion in the opposite direction. We hoped that this form of head tracking would provide dynamic localization cues to improve rear localization, but this is inconclusive. Despite the fact that head orientation should be decoupled from the positions of rendered sources, it may be important to compensate for listener position, in order that the listener can walk past virtual sources and sense the direction of the source changing.