Slab of Granitehttp://www.pianoworld.com/forum/ubbthreads.php/topics/1815651/KeyNo, you need something with mass to absorb the frequencies. Maybe you could find someplace to cut a half-inch granite slab for you?

This is structural noise and you need to isolate that. You need what some drummers use. I could describe it at length but there's loads of stuff out there on the web if you look for it. It will be an effort to make this thing but it will cure 95% of the problem...

You need a bid board (big and rigid enough to carry the piano and your stool when in the playing position - a big strong plywood sheet would do it). You need to cut holes into this board (quite a few holes - at least a dozen, maybe more) and the holes need to be big enough for tennis balls to locate into but not slip through. The idea is that the board on the tennis balls isolates the key thumping that generates the structural noise in your house. So you could then carpet this board so it looks ok and then you would step up onto this little platform to play the FP. You could make the board just big enough for the piano but then the piano would be a couple of inches higher than your seating position and that would probably cause issues with pedalling which is why the board ideally needs to be big enough to accommodate your bench too.

I know it's a pain to do this but all the other ideas about just using more carpet etc will not really cure this problem - you need balls to isolate that noise!http://www.pianoworld.com/forum/ubbthreads.php/topics/1815651/Key

Mount a board to the wall to support the piano, rather than placing it on the floor. The floorboards are vibrating like a drum...vibrating the wall will transmit fewer vibrations to your neighbors. Just make sure it's an outside wall :-) This is sometimes done with subwoofers and works pretty well. Just mount it close to the floor and maybe add some wood under your bench and you'll never notice while playing.Homasote is a soundproffing board sold at HD and Lowe's. I used it once to build a soundproofing box for a large pump. It worked well. However...those thumps are low-frequency and you'll need something like foam rubber to dampen them out. If you make some calls to local recording studios they may have good suggestions for materials.Another possibility is to mount your piano on spikes or cones to minimize its coupling to the floor. Another subwoofer trick. These can be found at hifi shops.http://ths.gardenweb.com/forums/load/apt/msg1217134715236.html?18

draper matanti vibration pads

I had a similar situation once. I put towels, wrapped up into pads, and used that as a buffer between the keyboard and stand and the floor. Solved the problem right away and the keyboard was still sturdy enough to practice on.

The more barriers and space you can get in, the better. If you really wanted to, put the whole keyboard and your playing spot up on a huge piece of wood to create a false floor.

I would think a few towels would do it though, although I don't know exactly how your set up.