How to fur down a gypsum board ceiling?

[mjwh]

I've designed a duplex where one unit is on top of the other. Sound is a major issue. I have a 2 hour rated floor assembly between the units with 18" deep floor trusses to allow two layers of fiberglass batt insulation in the floor trusses. I have been trying to figure out how to lower another gypsum board ceiling 6" to allow for light fixtures (recess or surface mount), wiring, exhaust fans & duct, to all route below the fire rated ceiling. I feel the labor in installing the second ceiling wound be cheaper than the fire rated ceiling fixtures, fire caulking, and fire dampers on the exhaust fan ducts.. I thought this could be done easy witha 5 1/2" +/- "Z" channel that would have openings to allow the wiring to pass through, than I could attach the gypsum board to it or have the option of a resilent hat chennel & then the gypsum board ceiling for more sound proofing. It seems like I'm recreating the wheel.
Any suggestions or comments would be appreciated.
Thanks

[tad311]

You seem to be on the right track. You definately do not want to move the rated ceiling membrane below the fixtures and ductwork for the reasons you just stated. You are also correct in framing the drop ceiling with metal studs, because there is a section in the building code that requires this framing to be noncombustable. I would think you only need a 1-hour floor-ceiling assembly, but since I don't know the type of construction you are using I can't say if 1-hour is adequate. Your floor-ceiling assembly also needs to have a S.T.C. rating of 50 and an I.I.C. rating of 50, which, if you provide, should achieve the sound insulation you are looking for.