Not all clays are equal
When I last did this (something like 20 years ago) I used a combination of Plastilina and Plast-I-Clay, and it worked just fine, and they were reasonably comparable. In any case, I did have an issue with it falling off the inside of the enclosure in hot (over 100 degree) weather, but neither seemed to be superior in that regard.
Of course, it does EXACTLY what Frank claims, and had quite remarkable effects on the overall "coloration" of the sound, i.e., the "boxy" sound and many stray resonances completely went away. If speaker manufacturers were really smart, they would use it at the factory. It would be trivial to build a double-walled enclosure with standoffs between the parts, and cast the clay in place. That way it would stay forever. I think it would be far more effective than wooden internal bracing - the panels themselves would be dead, instead of taking "ringy" panels and trying to restrain them.
I would have to say, with all due respect for Frank's experience, that I had a lot better luck using sticky-backed lead foil on the driver "baskets", than clay. It seemed much more secure, had less total volume, and had essentially the same effect on the resonances. I recently recommended this approach to a manufacturer, as it had GREATLY improved his driver by knocking down a few mid-high frequency resonances (and made it go from a very good to an exceptional speaker for a very good price) but they, as typical, they weren't interested.
Brett
