# Full front wall absorbtion?



## atledreier (Mar 2, 2007)

I'm in the process of making my new home cinema in a new house. The cinema is fairly long (8meters), so I have some spare room behind the screen. My plan is to do a 2ft deep absorber that cover the entire front wall.

The absorber will be made of relatively light material (8310rayls/m). The wall is 2,4m tall and 4m wide. I am hoping this will create an acoustically 'invisible' front wall, at least down to the midbass and maybe even beyond. The rear wall will get chunk-style traps floor to ceiling. I have some Rockwool panels as well, planning on doing these as needed around the room, after I identify problems.

Are there any pitfalls with this much front wall damping?


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## bpape (Sep 14, 2006)

2' thick will go very low. If you have the space, I think it would be of more benefit on the wall behind the seating and with a thin membrane on it to keep the bass control but not overdo and kill the surround field. On the front wall, 2" 3lb would be fine.

Bryan


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## atledreier (Mar 2, 2007)

Thanks for the advice. 

I have the space in the front, but the back wall is more cluttered. Some windows, the projector and a computer setup means that wall has much less area for absorbtion. The seating position will be almost 6' from this wall, though. I will try to cover as much of the back wall as possible, but I think I'll end up with an assymetric absorbtion area and only around 50% of the wall area.


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## bpape (Sep 14, 2006)

6' is pretty close to the rear for a room that's 8m long. 

Bryan


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## atledreier (Mar 2, 2007)

Yeah, the actual structure is 8m long. The screen will probably end up around 2-3m from the front wall. I will experiment with placements before I set anything 'in stone'. I have quite a bit of stuff that needs to fit behind the screen, so room back there is very nice. 
My mains are large, dual 15" and horn MTM constructions, and I have four JBL 4645C 18" subwoofers with the 2242 drivers. Amps and stuff also needs to go behind the screen. But again, none of this is set in stone.










These two photos show where the front wall will be. The wall with the hole in it is being torn down, and the actual wall of the room is another 1,8m in.









The rest of the room, seen from the current front wall


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## atledreier (Mar 2, 2007)

Coming back to this setup now. I was running the room flipped a while, but hte new IB subwoofer means the room is flipped back. The listening position is pretty far back in the room, but that's really the only viable place I get decent bass from the mains and (hopefully) the subwoofer. 

But some changes means I can treat most of the rear wall, cost permitting. I currently have chuck style Rockwool traps in the rear corners, but I have some ~8000rayls/m material (comparable to OC701, I believe) that I can do on the rear wall. This material comes in 2'x4'x4" or 6" thickness. So, a single layer across as much are as possible, or do fewer, thicker absorbers that dig deeper? I know OC701 isn't really effective in anything less than 12" thinkness.


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## bpape (Sep 14, 2006)

Are you meaning 703 or 701? What specific issues are you having? If you have the rest of the room treated and are happy with it other than the bottom end, might consider doing a damped membrane to keep from overdoing the mids and highs and just addressing the bass in the rear - if that's where the problems are coming from and if you need the entire wall.


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## atledreier (Mar 2, 2007)

I have some panels of Rockwool acoustic panel, pretty dense, I guess equivalent to OC705. Made corner chunks of those, so all four wall-wall corners are floor to ceiling. I still have some issues, but to be honest I haven't even measured the room as it stands, since I don't have my new subwoofer ready yet. I was thinking of damping as much of the rear wall as possible, I'm a fan of well-damped rooms. The front wall is less easy to get proper thickness due to limited space behind the screen. I can only get about 6-10" thickness on the front wall.


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## bpape (Sep 14, 2006)

You'd like the rear of the room to be a bit more live in the mids and highs to keep the surround field a bit more lively and diffuse. I would highly recommend some sort of membrane on the rear.

No need to do 705. 703 in any sort of thickness over 3-4" will do just as well if not better for less money and be a better 'spring' for the membrane.


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## atledreier (Mar 2, 2007)

Taking this up again now that my IB subwoofer is finished. I liek my sound as dry as possible, especially in the bass, and I think I'll try to get the rear wall as "invisible" as possible.

I'm thinking something like this. Maybe with a thin, light membrane to keep the surround field a little livelier, if needed. The white blocks in the rear represent 400mm thick 2'x4' sheets of Rockwool, around 7k Rayls/m2










Without the absorbtion. I currently have corner chucks on this wall.


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## bpape (Sep 14, 2006)

If the rest of the room is pretty dry, then you might consider a bit denser membrane to keep the rear wall absorption a bit narrower in scope and balance out the decay time curve.


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## atledreier (Mar 2, 2007)

My main problem at the moment is a huge 24Hz peak and a wide valley from 40-70. The 24Hz is room length and would hopefully benefit from some serious absorption on the rear wall. 24hz is hard to trap, I realize that, but we're talking about over 10sq meters of coverage.

If I use a heavy membrane on part of the wall I can target that length node pretty accurately, while still keeping the broadband damping on the rest of the wall. With a light membrane on part of the wall to liven up the rear field a little. I am sitting fairly close to the rear wall though. 

http://www.acousticmodelling.com/ml...membrane&s31=4&k31=0.1&s32=2&d32=400&v32=7500


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## bpape (Sep 14, 2006)

While surface area helps, trying to get that low is all about thickness. For example, even our Soffit Trap at 17" thick and with the benefit of being in a corner only goes down to about 35hz. Something flat on a wall, think maybe 36" thick.When it's that low you're best to just move out of it.


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## atledreier (Mar 2, 2007)

I'm planning on a 400mm thick absorber. With a heavy membrane I can get a very good absorption factor right where I need it most, at 24-26Hz.


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## NBPk402 (Feb 21, 2012)

bpape said:


> While surface area helps, trying to get that low is all about thickness. For example, even our Soffit Trap at 17" thick and with the benefit of being in a corner only goes down to about 35hz. Something flat on a wall, think maybe 36" thick.When it's that low you're best to just move out of it.


If you have a whole wall that you are trying to do 2-3' thick of insulation to work as a bass trap behind a AT screen... How would you attach it to the walls?


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## bpape (Sep 14, 2006)

Well, not sure that's the best place to put it but probably the easiest way is to just lay 24" wide insulation on the floor face down and just stack in the layers.


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## NBPk402 (Feb 21, 2012)

bpape said:


> Well, not sure that's the best place to put it but probably the easiest way is to just lay 24" wide insulation on the floor face down and just stack in the layers.


The reason I am asking is I am going to have a 4-6' deep area behind a AT screen (19' wide area), and was thinking that if I made the whole back area (around the speakers) absorb it would be a excellent bass trap. We will also have a 3' wide soffit going around the room that will be stuffed with insulation, plus acoustic panels on the ceiling and walls. I don't want to over damp but the bass traps need to be the biggest, correct? Would the soffits and corners be enough for bass traps?


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## bpape (Sep 14, 2006)

That would be good though for the soffits you might want to use a light membrane to cut them off around a few hundred hz. Front wall for HT you want to absorb all the way up and with it that thick, you'll certainly not have any SBIR issues off the front wall.


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## atledreier (Mar 2, 2007)

I currently have my IB manifold outlet behind the center speaker. I have about 4' from the screen wall to the wall behind the screen. I have large front speakers. If I were to fill the void behind the screen, how would I go about it with the IB? Just leave a "tunnel" without any material from the manifold and around the sides of the Center speaker?


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## bpape (Sep 14, 2006)

Just don't cover it with insulation. maybe box around it and extend the opening forward.


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## atledreier (Mar 2, 2007)

Extending the manifold will make it a transmission line or a horn, though.... I'll experiment a little with it.


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## bpape (Sep 14, 2006)

Then make it from something porous like maybe just a frame and window screen. Just something to keep the insulation out of the opening.


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## atledreier (Mar 2, 2007)

Yep. That should work.


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