Springs under turntable

I don't use a sprung turntable, but in the days I had a Linn....the springs should be wound in different directions to cancel each other's bounce.

Yes Uberwaltz, you are right but the resonance frequency of the suspended units is so low. In Boston after the Big Dig a concrete paneled ceiling in one of the tunnels broke and collapsed on a car killing I think it was two people. Traffic though the tunnel created very low frequency rumble exciting the panels cracking them.
Again, they are so heavy there is not near enough energy produced by even the largest system to get the concrete "ringing"
That would apply to apartment buildings, office buildings and skyscrapers. Not to mention you can't play a system very loud in those buildings without pissing someone off.
Most of us I would assume (maybe I'm wrong) have reinforced concrete slabs sitting on compressed stone dust, a wonderful floor for a Media room. 

I think you just described my room Mijo, lol.

And which is why despite the too large amounts of glass it still works pretty well for me as my music room.

I would still think there maybe some slight differences but not sure how  you would start to prove it.

Lewm, exactly right. The springs have to have the exact same rate and they have to be located exactly the same distance from the center of mass. Unfortunately, this is not so easy to find without harming the turntable. The result is that is is extremely difficult to get this right at a frequency below 3 Hz. It is far easier to create an unstable system than a stable one. INHO if you want a suspended turntable, and I believe everyone does even if they don't know it, buy a well engineered suspended turntable or a MinusK platform. This of course eliminates the Linn LP12:)

I made a big mistake. It is not a resonance frequency I was talking about above but the energy or power required to move the concrete. If you could pluck the cables holding up those concrete slabs the frequency would be very high because the cables are stretched tight and they are not very long. In buildings I believe the concrete is going to be placed on steel girders. The resonance frequency of the floor would be determined by the weight of the floors and the flex of the steel girders. The only thing I could hear when I lived in a 19 story apartment building was a low frequency boom when someone upstarts jumped down on the floor. A person weighting around 150 lb is going to put way more energy into the floor than any HiFi could. I had RH Labs subwoofers at the time and the only complainers were my next door neighbors. Nobody up or down ever complained.