How to treat large soffit?

[Dwight Angus]

Hello

In my Home theater there is a soffit that extends the length of the room at 25 ft on one side of the room and the width is 4 ft.Low frequencies cause some echo coming from the soffit and need some treatment. This is the main HVAC trunk. The soffitt has already been closed in and finished in drywall so my treatment options are limited to the surfaces in the room. I also have a door that opens into the room at one end of the soffitt where the top of the door is 2 inches below the soffitt so treatment thickness at one end will be an issue.

Questions:

Do I treat just the underside of soffit or the underside and depth (depth is 9 1/2 inches)of the soffitt as well?
What type of treatment is recommended?
How to finish around the door opening if the treatment is thicker than 2 inches?

Other info:
Room size- 25 ft long by 16 ft wide (the rear width is 12 ft)
Room surface is 5/8 inch drywall including ceiling.
Extra soffits could not be installed to balance the look of the room as there are 2 windows (front wall and sidewall). I have covered both with scrape pieces of drywall, vapour barrie and covered with panels for appearance.

Thanks
Dwight

[bpape]

Nothing you're going to put on the outside (that's reasonable thickness) is going to fix this - sorry. My suspicion is that you're getting a resonance from since it's hollow and uninsulated. The best thing you can do is to try to lower the resonant frequency and try to make it less efficient. I'd try to put another layer of drywall on it. This will accomplish both.

Bryan

[Dwight Angus]

Thanks Bryan. My better half not agreeing to doubling up of the drywall. Can I treat the underside and depth over the majority of the soffitt in say 4 inch absorptive material except for the door area? Not sure what that would looK like but is that an option?

[bpape]

It won't stop the ringing if that's what's happening. It will give you some broadband absorbtion but then you'll be out of symmetry left to right. It's a tradeoff.

Bryan

[eugovector]

Would it help to blow in some insulation? That would only require a few smalls holes and a simple patch job.

[bpape]

Absolutely if there's room for anything significant. However, he said that any destruction of the existing drywall wasn't doable so I disregarded any ideas of insulating inside - which is what should happen.

Bryan

[Dwight Angus]

Thats a great idea and probably doesn't require major surgery to the room. This maybe the solution.

Thanks Eugo

[Seb]

If you know what frequencies are causing the problem then there is no reason absorption wouldn't work. If the problem is "boominess" from the soffit cavity or "ringing" of the plasterboard then that is a different issue which still may be solved with heavy absorption to the underside as this would provide some damping of the plasterboard. A few extra screws may also help reduce the reaction of the plasterboard.

[bpape]

Sorry, but insulating on top isn't going to fix it sufficiently. Extra screws will only change (MAYBE) the frequency of the ringing.

Bryan

[Dwight Angus]

Seb

The problem is boominess. Treating the underside with heavy absorption would help however there is a door opening into the room where the top of the door is 2 inches below the soffit. Applying absorption thicker then 2 inches will be problematic.

[Seb]

So apply it 2-inches where you have to and thicker elsewhere. Do you want it better or want to be precious about how it looks - and it won't look that bad.

[Dwight Angus]

It would certainly help and the tapered look would not bother me however I have a WAF to deal with.