I'm installing a new ceiling in the kitchen of the bottom unit of a multi-family home. The house is quite old and uses 2x8 (actually 2"x7.5", not 1.5"x7.5", due to era of lumber) over a 12 foot span, which experience more deflection than is ideal but is certainly acceptable. This does create more transfer of impact noise (foot steps, dropping things, etc). I have isolation clips for the ceiling furring (http://www.kineticsnoise.com/arch/isomax.html) but, now that we're not installing recessed lighting, I'm considering adding a second, separate set of joists to hang the ceiling from.
What is the minimum joist depth and maximum spacing needed for new joists that support only a ceiling (since there are separate existing joists carrying the load of the floor)?
My calculations for two layers of 5/8" drywall/blueboard and some mass-loaded vinyl come to just over 5 lbs/sqft. There will be additional weight in veneer plaster and a trivial amount in light fixtures. All span calculators I can find accept a minimum of 10 lbs/sqft of live load and an additional 10 lbs/sqft of dead load. This situation seems to me to be around 6-7 lbs/sqft of dead load and essentially no live load over a 12 ft span. Trying to figure out if I can get away with 2x4s or need 2x6s or can cut 2x6s down to 2x5s, etc and whether I can space them every 24" or need to stick with 16". Not so much a matter of cost as of space for other sound proofing materials and considerations. Deflection concern in this case is to avoid veneer plaster cracking.
To clarify, this image is roughly what I'm planning for, where the red joists are to be added and whose dimensions I'm trying to determine.