There appear to be two philosophies in building soffits that I find in these threads. One goes for soffits designed to enhance the looks of a theater and the other appears to have been designed to effect sound quality. While appearances are important to me (+wife factor) I'm really going for sound quality.
I spent a lot of time in the acoustics thread and came away believing that corners, including wall to ceiling, create sound problems, particularly for bass. If I build my soffits with a solid bottom have I increased my problems as compared to building an open frame and wrapping the soffit in GOM. The interior could be used to place an angled piece of OC 703 - 2".
Does anyone have a design that incorporates this approach or am I way off base?

Lots of people use their soffits as bass traps. Do a search for Sandman's theater construction thread, it has some very detailed pics (huge thread, specifically see posts #199 & #561, though the whole thing should be read by anyone looking at building a serious home theater)

Thanks for the tip on the Sandman's theater. That guy does beautiful work! I looked at the use of insulation in the soffit but I'm still not clear as to why the finished soffit doesn't simply create a new corner with all of the inherent problems that you started with. Does finishing the soffit out of MDF do something different from sheetrock? Would fabric pulled over a frame be even more effective?
The rest of the theater construction has given a lot of attention to isolating the sound in the room. This includes 2 layers of rock with GG, two layers of R30 above the ceiling, quiet AC and all pipes wrapped. Now I'm working on treatment of the sound that is bottled up inside the theater. I'll be adding base traps in the corners and absorption and diffusion panels to the walls and ceiling. I'm just tring to figure out if soffits will enhance the listening experience and if so, how to get the most out of them. They won't be needed for electrical, AC or pipes. It's all about the sound.

Lots of people use their soffits as bass traps. Do a search for Sandman's theater construction thread, it has some very detailed pics (huge thread, specifically see posts #199 & #561, though the whole thing should be read by anyone looking at building a serious home theater)

Didn't he end up with too much absorption and complain that his bass was anemic?

If you build a soffit with solid bottom and sides, you've complicated the room modes issue somewhat, but you haven't done much to help any bass standing wave issues.

If you build it as an open frame covered with GOM and filled with insulation, you'll have nice bass traps all around the ceiling. Rather than an angled piece of 703, you might consider just filling the space with cotton or fiberglass insulation. If you want, you could use 703 on the faces and fill behind with insulation.

One thing to be careful of: there is such a thing as too much broadband absorption. I can't tell you how much is too much, but if you look at the soffit work in Sandman's thread, you'll notice that in places the soffit insulation (cotton, I think) is covered with kraft paper (the paper backing on insulation). This is to make sure those areas are reflecting mid-high frequencies and not absorbing them. The bass goes through kraft paper just fine.

So: bass traps in soffits are a reasonable thing. As to how much absorption, what type, and where, that's a much bigger topic. I punted and hired bpape to work with me on that part for my theater.

I looked at the use of insulation in the soffit but I'm still not clear as to why the finished soffit doesn't simply create a new corner with all of the inherent problems that you started with.

By leaving the soffit open and just creating it with fabric and a stick frame there really is not a new corner. The true corner is within the soffit and covered with a lot of insulation...in other words a bass trap.

Consider talking with one of the acoustic guys and seeing what they can offer in terms of room treatments/designs to maximize your space.

In the vertical corners of the room most of the base traps I see used are angled across the opening. Is there a benefit to creating this same angle inside the soffit and either placing additional insulation behind the angled piece or putting a flat piece on the top and back of the soffit to create the triangle affect that most corner traps have? At most I will have 6 inches of vertical but a full 24" depth.

You might check out the studiotips forums for more info on the angled corner bass traps. It's my impression that the point of those is that they are very easy to create. By just using rigid fiberglass across the front, they use the existing wall as their structure. The middle of the trap is deep enough to ensure that the trap is effective for relatively low frequencies.

Making a square bass trap in a corner would take more structure, more framing. Making a square trap that covered the same distance away from the corners (the same cross sectional area when looking into the corner at an angle) would also take twice as much insulation volume as an angled corner trap. However, I think it would be just as effective a bass trap (more actually, as the diagonal corner would be deeper and could lower the effective frequency of the trap)

In your case, if you are making a soffit, you already have the framing. Just fill sections of it with insulation until you have enough bass traps.

Well, I have a huge amount of soffit with light tray and mine sound fine. Of course the undersides are fabric frames with Linacoustic and all the walls have fabric panels with the usual acoustic treatment.

Pics are here (http://nightskypictures.com/theater/theater.htm) although the last pic is a bit old so does not show the wall panels. More pics once our permanent seating shows up!

The pictures of the Galaxy theater were great. I envy the amount of ceiling height with which you had to work. It appears that you ended up with the best of both worlds, aesthetically pleasing and acoustically complimentary. Curious- do the fabric frames on the bottom of the soffit simply attach to a solid soffit bottom or do they actually cover the open area of the soffit? Were you working with 1" or 2" linacoustics?
I will also begin to search some of the studio tips forums as suggested by Paul in his posting.

Curious- do the fabric frames on the bottom of the soffit simply attach to a solid soffit bottom or do they actually cover the open area of the soffit? Were you working with 1" or 2" linacoustics?

They attach to the solid soffit bottom. Screwed in from the top on the inner edge and from the bottom (covered by wall panels) on the outer edge. Layer of drywall above, then framing. The beaded corners of the drywall were firred down so as to be flush with the panels.

The wall panels are not shown on the page yet but are now up. They are 2 inch on screen wall behind shadow box and screen and 1 inch behind lower panels as well as under soffit panels (just polyester batting behind top panels, of course). The linacoustic was spray adhesived to the drywall and the frame fitted around it and screwed to the wall in about 10 spots around each panel. We had a continuous layer of OSB behind the drywall so we could screw anywhere on the wall. This is key as you are not limited by stud location (and in our case it was ICF foam).

FYI, I would not reccomend linacoustic and frames under soffits or ceilings for anything wider than about 14 inches - it will sag. Solid panel and glued fabric would be better for wide horizontal panels.


I will post some new pictures next week when we get the seating delivered.