I reread most of the threads you linked to above, and the decision tree is very dependent upon the location of the planned space. Most of their discussion covered the value of a floating floor on top of an existing slab-on-grade. My use case is driven by a 2nd floor location, a spouse that is very intolerant of added noise in the household, very different sleep-wake schedules (I am a night owl), and my desire to watch action adventure/bass heavy movies late at night. All these factors pointed to a maximum isolation design is crucial for my enjoyment of the theater and it all starts with the floor!
As I mentioned, we are still in the engineering phase. Low frequency isolation performance is the main driver for our designs at this point. Typically, anything you do to contain <100Hz will also do a great job at all the freqs > 100Hz. The goal is to match the STC performance of the floor, walls and doors. Over-engineering any one area just adds cost without adding to overall STC performance. Nyal modeled both a 1.5" and a 3" Gypcrete floor and the heavier floor had nearly 10dB better isolation in the 50Hz region.
We originally considered a 4" tall Kinetics Noise Control KIP pad
, but these would need to be custom manufactured, and our guess was that this would be prohibitively expensive eg >$5k just for the KIP pads. The KIP pads are the active component of the KNC RIM system
. After additional modeling by Nyal, we settled on a 2" KIP + 2x4 stringers to support the Gypcrete. This actually gave a slightly larger airspace beneath the floor once the pads are compressed, and this seems to be an important aspect for bass isolation. The change from the 4" KIPs to the 2" KIPS increased the natural frequency somewhat, but Nyal reports there should not be a significant loss of isolation in the lower octaves since we kept nearly the same airspace. The heavier Gypcrete floor also requires a a stiffer KIP, which also increases the natural frequency slightly, but this is more than offset by the additional mass and improved isolation.
Preliminary ball park quotes suggest the cost of the floor alone will be ~$6K for just the KIPs and the Gypcrete. This does not included the cost of the subfloor underlayment, adhesives, perimeter foam isolation, acoustic caulking, additional cost of stronger floor joists, structural engineer, etc....