24" OC is fine but the 5 ft span for 2x4 might be a stretch, they could sag in the middle. Have you room to put another ladder in close to the center to nail the 2x4s up into?

 

Great advice. I'll look into that.

 

You'll have a lot of meat with 2x4s turned flat under there. But in general you try to keep 1/2" drywall on 16"o.c. and go to 5/8" drywall with 24"o.c. to reduce potential for sag.

Usually I like 1x's under ducts (with mid-support as Neal suggested) to maximize headroom, or even eliminate under-duct support if around 16" wide, regardless of 9' ceilings or not.

 

By "meat" are you referring to the fact that there will be extra 3.5" of stud taken up underneath? I intend to connect the ladders with the stud in-between the ladders NOT underneath. I'll toenail them in since that would be my only option. Also not clear on the 1X comment? You are suggesting using 1 X 1 vs 2 X 4? Thanks for your input.

 

Yah in-between is the way here. "Meat" I was referring to if the flats are spaced 24"o.c. you'd reduce the drywall span by 2" compared to normal, and with a full 3.5" to screw to, that could minimize any sag wanting to happen in 1/2"GB. But 16 vs. 24 is your call.

I usually use 1x4 on bottom was a FWIW.

You can build soffits as big or tight as you want. Keep in mind they do affect the proportions of a space and successful ones IMO don't follow every jog, nook and cranny in the ductwork/pipes.

 

It's a 5 year old thread. Information given was good. Keep in mind sound transmits through solid installation. I would not expect double sheetrock to do much without uncoupling it from the framing.