<hardware> (Aureal 3-Dimensional?) A technology developed by Aureal that delivers sound with a three-dimensional effect through two speakers. Many modern sound cards and PC games now support this feature.

A3D differs from the various forms of surround sound in that it only requires two speakers, while surround sound typically requires four or five. It is sometimes less convincing than surround sound but is supposedly better in interactive environments. For example, PC games in which sounds often move from one speaker to another favour A3D, while pre-recorded video favours surround sound.

http://a3d.com/.