To be as brief as possible; metal rings, rubber, or something similar to rubber, does not. In even a reasonably modest system, everything you place under the LP's surface will affect its sound by affecting the resonant structure of the disc. Check out a product such as the Ringmat for a good idea on how to address mats. Some mats are more subject to static electric charges than others, some soften the sound, some harden the sound. Looking at the material will often give a clue as to what you can expect from a particular mat though cork has fooled more than one person. Try a series of 1/2" diameter, thin cork dots you can purchase at the hardware store before spending big dollars on a mat. Arrange the dots in a roughly circular pattern on the bare platter and place the LP on top of the dots. The idea here is to allow resonances within the LP's surface the ability to quickly dissipate without reflections back into the disc from the platter itself. Do not cover the area where the LP's label will be a bit thicker than the disc itself or use a slightly thinner dot in this area. You might also try a plain cork mat cut to size.

http://search.yahoo.com/search?ei=utf-8&fr=slv8-hptb5&p=ringmat%20platter%20mat&type=

IMO avoid Sorbothane mats and acrylic mats only work well when you can clamp the LP to the mat. Whatever material you choose should be the same relative thickness as the OEM mat that came with your table unless you have the ability to adjust the VTA (height) of the tonearm base.

I've tried a few options over the years- obviously it depends on your specific TT and what the platter is made of. Mine, a Well-Tempered Classic has a 1" acrylic platter and sounds pretty good on it's own. However about two years ago I added the Funk Firm Achromat Universal Record Mat and was pretty blown away how much more detail and tightened up sound I got. It seemed well worth the $100 to me and the engineering behind it made sense.

Check it out here

The acrylic platter you have should be a good system. Why do you want to change?

I have the groovetracer subplatter on my table, which to me sounds a lot better than the stock (sound seems to have more weight and focus), if that makes sense. The groovetracer is acrylic, (the original one was glass, if I am not mistaken).In the case of the Groovetracer, the underside of the platter is recessed to match Rega's distance from the top of the plinth to the top of the playing surface, no VTA Adjustment needed.

That's a lot of money for something you could make yourself for about $8 or less.

I don't want to burst your bubble here, but a mat is not the biggest issue with the Marantz table.