Speakers behind stud wall, soffit construction - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 3 Old 02-07-10, 10:42 AM Thread Starter
Join Date: Aug 2009
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Speakers behind stud wall, soffit construction

I'm preparing to strip the drywall and plywood from a shear wall at the front (17') of my room (16' long x 17' wide x 9' high). This is in preparation for a new room within a room dd + gg walls & ceiling joists. Because this shear wall is structural, supporting the junction of 16' and 14' floor joists and second floor above, I can only have limited openings in it. There is, however about 18" - 24" of clearance behind the shear wall to an irregularly sloped granite boulder three to six feet high. Poured concrete stem walls let into the boulder support the exterior walls. The joists thus form a sort of variable height crawl space sloping down towards the rear of the shear wall. The granite is covered with plastic sheeting against seepage.

I am thinking of incorporating some of this space into the sound room. This would be along the lines of a sound stage let into the wall to accommodate my speakers behind the shear wall. The chamber would be integral with the new "inner room" front wall, but it would be penetrated by whatever studs or posts are required for shear wall integrity. The "stage" opening would thus be about 6 feet high and about ten feet wide, BUT the fewer studs that need to be eliminated the better for the structure of the shear wall.

I will want to leave as many studs in place as I can (16" on center) but will remove one or two at the center, replacing them with a header supported by double studs, for access to the chamber, and to provide unobstructed center space for the CC and sub. I could repeat that for the 100s as well (remove one or two studs and double the others to support a header), essentially supporting the opening with four posts. See questions beow.

I'm planning an acoustic "transparent" screen over the opening and understand that the speaker faces must be about 12" behind the screen to avoid air movement. I have Paradigm Pro Studio 100s sides and CC). My sub Paradigm 15" will be at the center, but below screen bottom. The CC will be at screen center, and the 100s about 7' apart, assuming an 8' wide screen for now. I assume the chamber and studs or posts should be flat black? Not sure about the granite.

This entire chamber approach is a lot of work compared to snugging the speakers against the wall, but gains me a couple of feet of room depth compared to mounting the screen on the wall and building a stage into the room for the speakers.

1. What can I expect to hear on account of the remaining studs? Should I try to remove more than the minimum I'm planning? Should I wrap them with 1" of 703 or just paint them black?

2. What diffraction problems will the front edges of the chamber likely create? Should they be wrapped with insulation before covering them with fabric? Would slanting the chamber side walls be advised (narrower at the rear, 8' than front 10')?

3. What treatment for the surface of the remaining front wall peripheral to the chamber opening?
I was thinking super chunk bass traps in the front corners, floor to soffit. Should I also do this in the rear corners of the chamber?

4. What precautions should I consider in constructing the chamber to avoid booming (the chamber floor will be filled with sand, like a conventional stage). I could lay masonry to extend the rear wall of the chamber rock face from its current level (4-5') up to the level of the chamber opening. The chamber "ceiling" and sides must be conventional framing, though.

5. I need suggestions on soffiting. I really won't need it for chasing wiring or HVAC at the front wall. Would I be better off with a super chunk approach, or at least a light frame filled with conventional insulation and covered in fabric?

6. I will need to chase 6" HVAC plus wiring around the remaining three walls. The HVAC is only for this room, so I'm not concerned about sound leakage, per se. Should I make a solid enclosure for the HVAC or can I go with a light frame insulated soffit covered in fabric.

7. I'm not clear on the low frequency benefits of denser fiberglass at 4" - 6" in soffits as opposed to conventional (squishy) insulation. What about the recycled blue jean cotton batts versus 703? Blue jeans seem happier than spun glass, but will my ears hear the difference?

Thanks, Sandbag.
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post #2 of 3 Old 02-07-10, 11:56 AM
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Re: Speakers behind stud wall, soffit construction

Wow. Lots of questions. Let's start with the front wall.

The front wall in a multi-channel setup should be basically 100% dead.

Second, leaving that stone open will allow moisture more easily into the room.

Any chance of a sketch and/or some pics so we can get a better feel for something this unusual?


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post #3 of 3 Old 02-08-10, 08:11 AM
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Re: Speakers behind stud wall, soffit construction

sandbag wrote: View Post
7. I'm not clear on the low frequency benefits of denser fiberglass at 4" - 6" in soffits as opposed to conventional (squishy) insulation. What about the recycled blue jean cotton batts versus 703? Blue jeans seem happier than spun glass, but will my ears hear the difference?

Thanks, Sandbag.
Are you looking to create a trap or simply lower cavity resonance? If the latter, just use standard fiberglass.

Ted White

The Soundproofing Company
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bass trap , soffit , sound stage , speaker chamber

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