Isolation Platform for my Sota Cosmos?

Peterayer, that is way cool. Your turntable and platform look very good. I'm sure the sound is fantastic. I have an ARC preamp and it throws a deep/wide sound stage but never forward of the front plane of the speakers. It goes deep instead. My analog soundstage goes beyond the speakers and almost to the side walls (24ft across). My digital soundstage is just as deep but not nearly as wide. I also think that the better isolation reduced surface noise on my records. I can't really prove that however.

I wouldn't have thought that isolation would be a big deal with a Sota tt since it is suspended; but I was pleasantly suprised last night with a tweak that I made to my system. Years ago I had my tt sitting directly on a granite slab. After reading some threads I placed a wood board between my tt and the granite and heard a nice improvement in sound. So I knew then that there is some interaction between the base and the plinth. Well, my tech at work has a few blocks of black, dense open celled foam sitting around that was used as packing material for something that we received a while back. This stuff is very dense. It occurred to me last night that this might make for some good isolation material. These blocks are 1.75" thick and two blocks wide is just a bit larger than my granite base. I placed these foam blocks on the granite and then the wood board and tt on top of that. The foam is dense enough that it barely deflected under the load. In fact, my tt was still perfectly level. No adjustment was needed.
Right away, I could hear the difference. Images are more focused and the base is tighter; tighter like a CD. I'd even swear that the surface noise got quieter. It is a remarkable improvement. Much more than I expected to hear being a suspended table. Luckily, this dense, black foam looks good sandwiched between the granite and the wood. With so little deflection in the foam; there is no bulging of the sides. Good thing because I am not taking it out.

I posted a picture on my system page for those interested in how the foam looks sandwiched between my turntable and shelf. I'm still enjoying the improvement it made to the clarity of the bass and the more focused images.

I had better isolation under my tt years ago and drifted away from that. Somehow, years ago I thought a granite base would be advantageous. I then learned some time ago that the tt sitting directly on granite was not a good thing. Now I have learned over again that the more isolated my tt is from the granite the better. I have listened exclusively only to vinyl the past few weeks having a rediscovery how good analog sounds. The bass is once again crisp and deep; as good as or better than digital. Imaging is tack sharp. This past Sunday night I was listening to Dark Side of the Moon. I was delighted to experience the holigraphic sound stage of this album again after several years. The oval pattern of the sounds moving left to right and back at the beginning of the record was very apparent; as well as the oval pattern in the footsteps on the next track.

Peterayer, the suspension Fn on your tt is likely in the 1-3 Hz range. At least my Sota is around that. So you can maybe measure it with a stopwatch since it is slow enough. Just excite your plinth and try to count as many cycles as you can while running a stopwatch. Then just divide the no. of cycles you counted by the time on the stopwatch. What Geoffkait says is possible. If the Fn's of both the tt and the platform are the same, then they can excite each other and if they get 180 degrees out of phase, then be ready to catch your tt. :) When you excite your plinth by pushing down on it, just watch to see what the platform does. Maybe it is dampened so that it does not respond to the tt mass moving up and down.