I am in the original design phase of my dedicated home theater project. I have the room-space mapped out, and have discussed ductwork with a 20+ years HVAC designer, so I have a very solid idea of where the ductwork must go. The problem is that no matter what I do, I will need to either lower the entire room enough to fit in a few ducts, or I can allocate some space for soffits. I am planning on building a step-tray ceiling treatment for the look anyways. One of the ducts that needs to be put in a soffit is for a set of rooms in the floor above, and not part of the HVAC for the theater room.

The question I have is:

Option 1:
Should I build a rectangular box, with holes in the box for a few ducts to pass through. Finish drywalling inside the box, then build soffits around the ductwork. What do you do to minimize the effect of the holes where the ducts pass through?

Option 2:
Do I build all the framing around the soffits with no holes in the theater space? This leaves a complex shape to drywall, and I think would be harder to frame, but perhaps it is easier to frame. *shrug* An advantage would be no holes in the box.

Option 3:
Build a rectangular box, but build the one soffit section that *must* exist to go around some ductwork that is not part of the theater HVAC. Finish the drywall in the room, then build the faux soffits to make the ceiling have that nice finished stepped-tray look.

Additional detail: I am planning on staggered studs on the common walls, RISC clip-hung ceiling dry wall, double dry wall with green glue between. I also plan on sub-contracting the drywall hanging, mudding, and finishing. The HVAC ductwork is being contracted out as well, though everything else will be DIY.

If you need drawings or pictures to visualize what I am asking, let me know.

....I'm no expert, but have just finished my "box" using dbl drywall, green glue, RSIC-1 clips and staggered studs. I would go with option 3 -- single sound shell with only the theater room HVAC ducts inside the shell, and those covered with "in-the-shell" soffits. I would avoid complex shapes with unnessesary corners and bends where possible when designing the sound shell, as those corners and bends are more difficult to make soundproof. Also, take steps in your duct work layout to reduce flanking noise both coming in and coming out of the sound shell internally through the ducts. Steps include acoustical duct liner and several 90 deg bends in both the supply and return duct system.

If you can finish the room first and then build your soffits, you'll likely have a more sound-proof room. In my room one side and the back soffet won't be build until after the ceiling and walls have been drywalled. One the other wall, I have to have a soffit built prior to drywall because of all of the existing HVAC and cabling.

I am not sure if I described the issue well enough. Instead of building a cubic space first, then putting ductwork into the room through a hole in one of the walls, I have a duct that can not be moved from where it is as it supplies a separate space in the house. It has an entrance into the theater area, and an exit about 10 feet or so down the side of the wall. In order to build the cubic space first, I would need to take down that particular duct, build the cubic theater area with two holes in the walls, then reassemble the duct. I would also have additional vents and return holes for the room's venting. But those ducts can be placed behind the walls, with only the opening for the vent piercing the cubic wall.

It may be that a temporary disassembly of the existing non-theater duct is the best method in order to follow Option #1. But I wonder if Option #3 building a "dented cube" to fit around that particular duct might give better results since that non-theater duct will not be in the theater space at all. That leads me to Option #2, building all the soffits first. I have seen a large number of builds in this forum where all of the soffits were built prior to drywall. Is it easier?

As Cathan indicated he has done, he built option 3 for his theater to get around some existing ducts and cabling. Jedi has also suggested Option 3.

I also did option #3. It probably isn't any easier or harder than option #2 (IMO, soffits are a pain either way.)

I have two soffits that surround HVAC equipment that were framed prior to drywall for sound-isolation purposes. Teh drywall guys didn't seem to ahve any problem with these. Then for a symmetrical look, I put up two more soffits inside the room after drywall (all of them are covered by GOM). These last two, I stufed with insulation and Linacoustic to serve as bass traps.

As stated, if you can build the room first, then add soffits, stages, risers, etc. you will have better results. If you have ducts, beams, etc that require framing out to incorporate them into a soffit, then these need to be protected from sound. Build the soffits, insulate lightly (don't pack) and then drywall or double drywall to protect the ducts from sound.

Your soundproofing efforts need to keep as much vibration as possible from entering the original house framing, beams and ducts.

A side note, soffit construction is generally done with 2x4s and double drywall. But this eats up a lot of ceiling height. You might consider making a double layer of plywood / drywall. The initial plywood has the structural integrity that the 2x4s provided, but in much less space. You then simply screw the single sheet of drywall to the plywood.

Good idea Ted. I am strongly leaning towards the plywood instead of 2x4 option now that I think about it. The height savings under the soffit would be nice, though not critical. The idea of using empty soffits as bass traps is appealing to me as well. I have seen this idea suggested a few times, but have seen few finished theater threads on AVS use it. I appreciate all the input. Small design changes now are much easier than trying to work into the design later.

Is there a reason why people tend to not use soffits as bass traps? This seems like a good place for them.

I suspect that the action of trapping the bass requires a resonance chamber, and this may not help you in your isolation efforts.

However, the floor riser may be a better trap. It's within the sealed up room and the concrete floor. It's also a large area, filled with absorptive material.

Ok, thanks. I have been leaning towards using corner-triangle floor-to-ceiling super-chunk type bass traps. Searching through the master acoustical treatments thread does not yield much info on soffit-mounted bass traps. My current plan is for a false wall, which is easy to hide corner bass traps behind.

I've seen a lot of conversation on the topic, so I don't think it's probably that rare. Here's a picture of mine; you can see the side wall soffit double-drywalled, while the rear sofit is built inside of the room. I put 2" of linacoustic inside (all I had left) and then filled the rest with fluffly pink. The whole thing is then covered in GOM.

To save a little height, I turned my 2x4's sideways on the drywalled soffits, and ripped them into 2x2's for the bass-trap soffits.

http://i103.photobucket.com/albums/m157/kwadswor/Basement%20Finishing/Rearsoffitstuffedwithinsulationfurr.jpg

Here's a picture of one of the drywalled soffits with HVAC and electrical running through it. (Note: the corner is offset wo that the thickness of the double drywall + furring strip would end up the same as the vertical leg with the light tray screwed onto it.)

http://i103.photobucket.com/albums/m157/kwadswor/Basement%20Finishing/Electricalwiringthroughnorthsoffit.jpg

I have soffits on all four wall/ceiling intersections and all but the one in front (filled with duct work, pipes and a bunch of other stuff) is for the purpose of acting as bass traps. The face of the soffit is covered in a piece of 1 inch cloth covered Owens Corning 703 and the plan is to fill the inside (which is currently lined with 2 inch 703 on the top, bottom and back) will fluffy fiberglass or other material. Two questions:

(1) what is the best material to stuff the soffit with? and (2) how dense do I pack it with whatever I pack it with?

I have attached a couple of photos but the finished soffit is temporarily covered with drywall until I get the answers to the above.

Thanks