sorry fohat, but what is the use of the mat? It's just to put underneath the keyboard, right? Or does it have any special uses?
EG I have tried different types of plastic and paper and cloth underneath my Model Ms and Fs and they still make noise. But the styrofoam feet mod I told you about seemed to dampen noise a bit more than other materials. Since I do not have rubber, silicon or your EPDM mat, I have not tried enough materials and can't draw any conclusions.
Sound propagates in different ways. Dampening the keys themselves is one issue, isolating the body of the keyboard itself from its environment is something else. Imagine the loudspeaker of your sound system sitting on its shelf, or wherever - what does it rest on?
The keyboard vibrates, and if there is a hard intersection between the keyboard and the desk, that surface will act as a sounding board, too.
A folded towel under the keyboard may be the most effective sound dampener, but is kind of goofy both in look and feel.
If you like solidity (and Model M/F users surely do) then you want the keyboard to feel firm and secure while you are striking it. The shelf liner mats are probably very good at dampening, come in a variety of colors, and are cheap and lightweight. But they have a good deal of cushioning and "give" as they are being pummeled. This could be an advantage or a disadvantage, depending on your opinion.
These thick dense heavy rubber mats provide a firm and solid base, but also insulate and isolate between keyboard and deck. And they have the benefit of preventing any movement or slippage, too.
