First of all, what is 7.1? Well, 7.1 is a speaker layout and sound format used for surround sound. It expects seven normal speakers and one subwoofer (bass) speaker, and these are set out roughly in a circle around the listener, with three at the front, two at the sides and two at the back. Quite a few computers and Hi-Fis now support this layout. But, it's not 3D. Because all the channels are at the same height, it's hard to make sounds appear to come from above or below.
3D7.1 is a speaker layout that is designed to work with the kit needed for a 7.1 system. All you need to do is move your speakers into new locations, and use software (like Rapture3D) that knows how to handle them. 3D7.1 was invented by Simon N Goodwin at Codemasters and there's an interesting paper on it at http://www.codemasters.com/research/3D_sound_for_3D_games.pdf.
If you're worried about the trouble of moving your speakers into these locations because you also want to listen to films, you might be interested to know that 3D7.1 is designed so that 5.1 material played directly over a 3D7.1 system will play successfully. 7.1 played directly over a 3D7.1 system would sound pretty odd as the two rear channels would appear in the "rear centre high" and "front centre low" speakers, but there aren't that many films mixed in 7.1 yet, and you probably have an option to downmix the 7.1 to 5.1.
If you want to measure out the angles and elevations accurately, they're specified here in degrees, angle measured horizontally, anticlockwise from the front centre (around to the left) and elevation is measured upwards (positive) or downwards (negative).
|7.1 Speaker Position||3D7.1 Speaker Position||Angle||Elevation||Used in 5.1|
|Front Left||Front Left High||51||+24||Yes|
|Front Right||Front Right High||309||+24||Yes|
|Front Centre||Front Centre||0||0||Yes|
|Subwoofer||Subwoofer|| || ||Yes|
|Rear Left||Rear Centre High||180||+55||No|
|Rear Right||Front Centre Low||0||-55||No|
|Side Left||Rear Left Low||129||-24||Yes|
|Side Right||Rear Right Low||231||-24||Yes|