Seriously, I'm not making fun of you, it's just that that much isolation requires a "total package" approach - if your ductwork is plain old house construction, you may need to replace it with ducting that is designed specifically for sound attenuation. High isolation requires multiple bends and absorption INSIDE the ducts, as well as a much larger cross section. This is because the noise that is generated by moving air is proportional to the 4th or 5th POWER of the air velocity. So a slight reduction in air velocity (larger diameter duct) results in a large decrease in sound generated. Also, sound WILL "swim upstream" almost as easily as "downstream" - think about it - typical air velocities for ductwork are around 300 feet per MINUTE - sound travels in air at approximately 1130 feet per SECOND. So upstream sound may only travel at 1125 feet per second, instead of 1130...
One possible "fix" for your ducts - if your ducts run parallel to the ceiling joists, it may be possible to get your "mass-air-mass" construction ALL between the duct and the studio - I would need to see a drawing showing the duct cross-section as related to the ceiling framing to know if this is possible.
Your floating floor will have only a couple inches of air gap, so won't attenuate possible flanking noise as well as one with 4-5 inches of gap. The insulation in the floor isn't there for heat, it's there to break up sound waves and lessen the coupling between the floated floor and the concrete. Once sound gets into the concrete, you're screwed - it will conduct all over the foundation and framing, and re-radiate multiple places in the house. This is why successful floated floors are at least heavy wood (like 2" or more just for sheathing) or concrete.
Where you're floating two separate floors, first I would make them thicker and heavier and with a larger air gap if possible - second, I would make each floor different in mass/materials than the other. This is because any sound that flanks through one floor into the concrete will be less likely to share sympathetic resonance with the other floor if they have a different resonant frequency.
Remember, the ultimate goal of sound isolation is to get all surfaces resonant at sub-audible frequencies - this can only be done with mass and air space. The closer you get to this goal, the less coupling between rooms.
If walls are floated on a floated floor, there is no need to float the wall on the floor - the function of "floating" is to isolate inner leaves from structure in order to minimise flanking noise. That would be accomplished by the floor suspension, as long as the walls aren't allowed to have hard physical contact with structure. There are sway braces available that handle this last part. Also, light gauge steel studs or Resilient Channel will further isolate the inner wallboard leaf from structure.
Doors and windows are another source of disappointment in sound proofing - doors need to be hermetically sealed when closed, and double doors with a sound lock (think foyer) are necessary for anything more than STC 45-50. Overly makes one door that's rated at 55 dB if I remember correctly - it only sells as a complete frame and door and seal package, and it costs just over $5,000. To get close to that, you need tandem doors, each with a good drop threshold and well-made seals.
"Last question...really....Back to the soffits. Can/should I substitute a "hat channel" for the RC that is supporting the cleat/vertical face of the soffit?
Also, I am using metal stud walls, so can I skip the RC onthe wall/soffit intersection and screw my soffit cleat to the metal stud?" -
Hey, that last question was two questions -
First, you could do that - note, however, that I drew the RC's pretty close together - if it works out that the normal RC position is relatively close to the soffit, I would just add an extra RC right where the cleat needs to be. The extra weight of the soffit won't be any more than if you continued horizontally with 2 feet of double drywall. In addition, the hat channel isn't as resilient as the single legged RC (be careful here though - at least one company (Dietrich) has two different grades of RC, only one of which is rated for ceiling use)
Screwing the cleat direct to steel stud - Yes, you can do that - I'm hoping you're using the light 25 gauge studs, otherwise RC will be needed. Just remember to make sure you caulk everything thoroughly with REAL acoustic rated caulk - even over the net, the large tubes of that aren't much more expensive per unit than the small, WRONG ones at Home Depot.
As I said previously, if you really want to isolate ceiling from wall, you could insert a strip of RC where the outer corner cleat is drawn, then caulk thoroughly. This would break the hard path between the vertical soffit wall and the horizontal soffit bottom.
Be sure and download the USG handbook while you're at it, lots of valuable info in that - see the "stickies" section.
Don't get "gunshy" about asking questions, I've been doing this long enough to know you're nowhere NEAR through asking questions
... Steve