This is all the info I have on it. Check with AJ for more specific info.
Skip
BASIS TURNTABLE ENTHUSIASTS: NEW SUPERBELT AVAILABLE!
A.J. Conti, the designer of all Basis products, scrutinizes even the smallest component parts of each Basis product, considering each parts contribution to the final sound of the overall product. Many audio enthusiasts and other designers might consider some of these studies and experiments too much effort spent on seemingly trivial details. However, the Basis philosophy is that each detail of every component makes a difference; only with full investigation can it be determined just how large that difference is. The turntable drive belt is one such detail. The accuracy and true roundness of the Basis platters and pulleys, along with the smoothness of the Basis Signature bearings, resulted in a situation where the belt itself became the limiting factor in speed stability. As a result, in mid-2004 Basis Audio and our belt supplier spent dozens of hours in an effort to pursue the state of the art in turntable drive belts. Thousands of dollars were spent on new production fixturing and Basis designed and built a special belt inspection fixture which measures the thickness of the drive belt at all points to within one-ten-thousandth of an inch precision.
As a result, the new Basis belts are the finest belts Basis has ever offered. The variation in thickness along the length is the tightest specification in the industry; we know, we have measured them all! To take the ultimate turntable belt one step further we are announcing our Basis Superbelt, an extreme tolerance, ultra-high- performance belt. This belt is very difficult to make due to the extremely close tolerances in thickness and the high damping factor material. At Basis we sort through 20 to 25 belts of our supplier's already sorted "best effort" to yield 5 belts that will meet our incredibly stringent standards for our Superbelts. Belts not meeting these standards are then destroyed.
The improvements of the Superbelt result in greater isolation from motor noise, greater speed stability, and lower speed drift. Sonically this means greater background "blackness", a greater sense of your speakers "disappearing", more coherent soundfield echoes and cues, lower grain, along with the expected greater sense of solidity of sustained notes of piano and other instruments. In a high-resolution system the differences heard when changing to a Superbelt are easily identifiable.