Home Theater by complete acoustic newbie

[daduet]

I have included a SketchUp 3D of the home theater that we are building in a new home. The house is a few weeks away from being ready to drywall.

The room is not a very regular shape. It is 16' wide x 26' long with a 10' ceiling height, with a rectangular portion that barrel vaults from 11' to 12'-6", and there is also an octagonal bay window on the long (west) side with a vaulted (octagonal) ceiling. (Attic space above all but the vaulted bay.)

There is a 6' wide pass-through window into the gameroom in the center of the south wall (with pocket doors to close the pass-through window if desired).

The east, west, and south walls have some arched soffits (about 4" deep) protruding from the wall which will be "supported" by wooden half-columns.

The north exterior wall is double thickness, to allow for full insulation, in the outer 6 inch wall, and then speakers will be mounted behind the acoustically transparent screen in the inner 6 inch wall. There is a decorative "procenium arch" mounted a foot from this wall to "frame" the screen.

The interior walls are double 2x4 walls with an air space between them. The exterior walls are 2x6 with spray foam insulation, and then a veneer stone (with air space) as the exterior finish.

The floor is 18" web trusses with 1-1/8" sturdi-floor tongue & groove sub-floor with carpet in all areas except the "stage" area (there for aesthetics) which I thought should be done with hardwood flooring. (Kitchen & Family Room below with 5/8" drywall ceiling.)

I am wondering what can be done to assure good sound quality within the room. I am not terribly worried about sound leaving/entering the room, as I am fairly certain that I have a pretty good handle on that, and we don't tend to blast the volume. I am wondering more about the quality of the sound within the room from reverb, etc... Should anything be done to try to make the raised seating area across the back of the room (which goes completely from side-to-side and is 7-1/2" high) into a bass trap? Same question for the stage (19-1/2" high)? Do I just fill the voids with insulation for this, cutting holes in the face to allow the sound to "enter", or is the specific insulation used very important? Just from the dimensions of the room, does it look like a have an especially difficult sound problem, or should it be fairly easy to tame. Visual aesthetics of the room are extremely important. I hope I have provided adequate information, and it is in an acceptable format. I read the rules, and tried to get it correct.

[BriHar]

Not particularly familiar with the special needs of home theaters myself, this site is geared more toward recording studios and control rooms, however maybe someone here can indeed help you.

What I thought I would offer is a link to a specialty forum on the Home Theater Shack site which specifically deals with your questions. If you're not familiar with the site, then I think you'll be glad you've found it -
http://www.hometheatershack.com/forums/ ... acoustics/

[Soundman2020]

The interior walls are double 2x4 walls with an air space between them. The exterior walls are 2x6 with spray foam insulation, and then a veneer stone (with air space) as the exterior finish.

Maybe I'm totally misunderstanding what you are saying, but to me that sounds like at least a 3-leaf wall, and maybe as much as a 6-leaf wall! If so, why????? Why do you need a pair of interior walls, plus also an exterior wall?

Maybe you could model that in SketchUp, to show exactly how you are planning to build your walls. How many sets of studs you have, where the drywall goes on each set of studs, how many layers of drywall, etc. I have a feeling there is something major wrong with this plan...

Also, you say "the exterior walls are 2x6 with spray foam insulation". What do you mean by "spray foam insulation"? What type of foam? Open cell or closed cell foam? How will it be sprayed, and what precautions will be taken to prevent flanking bridges?

The floor is 18" web trusses with 1-1/8" sturdi-floor tongue & groove sub-floor ... Kitchen & Family Room below with 5/8" drywall ceiling.)

How are you going to damp the resonance in that cavity? What is the isolation plan for the floor? Right now it is tuned to 26 Hz, and will not isolate well until at least 77 Hz. In most home theaters, there's an awful lot of LFE stuff going on below 77 Hz! That is going to go straight through and be heard clearly in downstairs.

Should anything be done to try to make the raised seating area across the back of the room (which goes completely from side-to-side and is 7-1/2" high) into a bass trap?

I's say that your first priority should be to prevent it from acting as another resonant chamber, which it will be if not built right. But yes, it sure could be used as bass trap space, although 7 1/2" isn't very much for bass trapping.

Do I just fill the voids with insulation for this, cutting holes in the face to allow the sound to "enter",

That would make it into a Helmholtz resonator, tuned to a frequency set by the dimensions of the hole and the depth of the cavity. You might or might not want to do that, depending on the room response.

or is the specific insulation used very important?

Not really. For bass trapping you need something with a density of less than 50 kg/m3 if you are using mineral wool insulation, or less than 30 kg/m3 if you are using fiberglass insulation. (Contrary to popular myth, higher density is NOT better for bass trapping: in reality, LOWER density is better for bass. Higher density is better for high frequency absorption.)

Just from the dimensions of the room, does it look like a have an especially difficult sound problem,

Not too bad, but it could be much better. The modal spread is rather "lumpy", and there's an issue at 56 Hz, where your 2,1,0 tangential and 0,0,1 axial modes line up perfectly. There's another issue around 66 Hz with two almost superimposed modes, and another at 71 Hz, where three modes are practically right on top of each other (one oblique, one tangential, one axial). To make matters worse, all three of those issues are in the region that will not be isolated by your current floor concept.

or should it be fairly easy to tame.

Shouldn't be too hard to tame, firstly with standard treatment for small rooms, plus perhaps tuned devices for those specific issues, but I'd suggest you look into those other issues that I mentioned first: A multi-leaf wall is a bad idea and will not isolate well, and resonant cavities under your floor are also a bad idea. Both of those have the potential to be big problems.


- Stuart -