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Discussion Starter #1
Happy Holidays to all. New kid on the block so I thought I would jump in with a question that has been bothering me for over a month now.
My situation: I built a basement home theater over a year ago trying to keep a acoustical/$$ pricepoint reasonable. I had held the best I could to recommended room size ratios and ended up with a 21'3" x 13'6 x 7'10 size. Front-Right-Back walls are block with 2x4 studs face screwed into the block (for decoupling from the joists) 1/2" rigid foam board between the studs and block with 3 1/2" batt insulation between. Floor is concrete (large braided wool area rug coming). Ceiling is 2x2x5/8 armstrong acoustical tiles with zero clearance.
The question is what should I do with the left wall? Right now it is just 2x4 and 1/2" drywall. As you might imagine, it has quite a bit of 'life' considering the other 3 are pretty solid, and is causing some problems.
1.Do I try to beef it up on the back side with drywall/osb? What to insulate with? I have concerns of building up the backside and trapping low frequencies inside the wall also with only 1/2" drywall to the sound side.
2.Or I could also brace the 2 x 4's with an additional one and treat in between them leaving the back side open to 'bleed' out the sound to the adjoining utility room to avoid trapping.
3. Lastly I could leave it live considering the opposite wall is rigid, and just treat the reflection(s) as required
As you can tell I'm stuck. I feel if I guess wrong I will be taking one step back so I would sure appreciate some feedback on this. What would you do?
Thanks
 

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Welcome to the Shack Scott.

There's nothing wrong with a bit of life in the wall. In fact, it will actually provide more midbass absorption than the others due to it's being able to flex.

I'm a bit confused though. You're bolting stud walls to concrete to decouple them from the joists above - but then you're using ceiling tiles instead of drywall which will let the sound escape much more easily - kind of defeating the purpose of the decoupling.

What is it exactly you're trying to accomplish with beefing up this last wall?

Bryan
 

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Discussion Starter #3
Hi Bryan,
Thanks for the welcome and the input. The ceiling tiles came into play with the upstairs kitchen plumbing and HVAC located in the joists. 1/2 way through the project I was 'informed' of a future kitchen island. So I made the best I could (I felt) out of the situation by using a .50 absorption tile with tegular edges. I felt I might gain dispersion from the edgework. In addition I used a 'snap' framing system which allowed me to keep the tiles fixed in place. The first reflection is insulated between the joists in addition. As you pointed out, not ideal but the wife likes it so therefore I am still moving forward:T
Back to 'the wall' - I see the point on leaving the wall with some life. Are your thoughts then to insulate with batt insulation and rock the backside and call it good?
Thanks Scott
 

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My walls are loose at a certain area slightly above ear level. I had the problem that my left wall had a problem with resonance because there were small rooms on the other side of it. I used some thicker insulation in them to help tame some additional high frequencies since the mids were sounding a bit better than they were. Then I put a thicker 5/8" drywall on the other side of the wall (not in the HT). This had to be hung more tightly than the other side inside the HT. This way the 1/2" drywall and the 5/8" drywall do not move flex at the same time causing the wall to act like a drum. I found that hanging things on the wall or placing shelves near the wall in the next room also made some audible difference as adding more mass seemed to help.
 

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Discussion Starter #5
Thanks Steven,
Your situation was just about on par with what mine is. Overall I'm pretty happy with the rooms performance before treatment. My weak link seems to be the high frequency also.
When you say "thicker insulation" does that mean you forced 5 1/2" insulation into the 3 1/2" cavity? Maybe I missed something.
Scott
 

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Thanks Steven,
Your situation was just about on par with what mine is. Overall I'm pretty happy with the rooms performance before treatment. My weak link seems to be the high frequency also.
When you say "thicker insulation" does that mean you forced 5 1/2" insulation into the 3 1/2" cavity? Maybe I missed something.
Scott
I used R19 instead of the R14 yes. I have another part of the left wall (two rooms lobby and equipment room on the other side) that is filled with R14 between the lobby and HT. The other side of the HT that is the back half of the HT is 1/2"/R14/1/2" and sound goes right through it so I have to turn up my surround some. There is shevling beside the wall in the lobby also. I need to secure the drywall on the other side of the room which we had left loose because I had thought cabinets were being installed there. The are not doing the cabinets so I will be screwing in some drywall screws there soon behind the cabinet and hope that helps with the resonance problem there as well. If not I'm not sure what wold help to fix it except maybe trying to place some absorption on the wall.

I took REW with the RTA and monitored some movies playing while in the next rooms to see how it compared to being in the HT. I checked each room and area above the theater to compare each walls performance. It depends on what STC rating you are trying to achieve but it is often best I think to leave some gap inside so it is not to tight.

I also sealed up all the gaps with some expanding white foam sealant and used caulk to tame some vibrations in the studs when they moved next to one another.
 

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Discussion Starter #7
Thanks for the detailed explanation Steven. Backing up to Bryans' question of "what exactly are you trying to accomplish?" , the answer was 'I really don't know'! I just needed to get the 'boom' out of it when I took my fist and hit the of it against the wall. ;-)
Anyway I like your plan of packing the R-19 into the cavity capped by 5/8" drywall. That makes sense. The STC isn't so much of an issue. The furnace is about 2 feet off the back side though so the 5/8" drywall is also a smart move for a firestop.
I was thinking about your shelving issue with the light wall. Depending on the size and configuration of your shelving, you may be able to get away with putting up a faced piece of finish grade 3/4" plywood. If you anchor the plywood running screws behind the shelf locations, you could trim out the edge and stain or paint to match. The plywood would definately stabilize the wall and give you more options to the location of the shelves, if you can make it work.
Thanks again Scott
 

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Yes it does seem that anchoring the shelving to the wall would add some more stability and some OSB would add some needed mass to the flimsy backing on the shelves. The shelves I am using are free standing but they are close enough to the walls. I will see about anchoring the OSB and shelf layer by placing some L connection type things and anchoring that into the drywall with drywall anchor and screws then. I think I have some already I can use saving me from having to make a trip into town. I could anchor at the bottom or tops of inside the shelving depending on the height and location of things on the shelves. To do that also I will place just a thin layer of insulation between the OSB and the wall then as I don't want to worry about small contact points on the OSB layer rattling the wall as it moves. I will give it a try that sounds like it could work.
 

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I bought the wood but never got around to messing with the wall. Since moving two of four subs to the back of the room I haven't quite had the chance to hear the problem with the wall again. Could not seem to find much problem with Master and Commander and I would have expected to hear a problem with the wall and pressure against it with that. I will just need to keep trying to confirm it is still an issue.

Did you settle on a type of wall yet?
 
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