# soundproofing - air cavaty size



## panduro (Oct 3, 2009)

Slowly working on the ht plan - small dedicated room in an apartmentbuilding.


reading as much as time permits is getting my head spinning with different questions.

this Question is about air cavatie size.



http://www.soundproofingcompany.com/soundproofing101/triple-leaf-effect/


So the above article states that small air cavaties is worse than no cavaties. 

but how small is a small air cavati?

Im going to build extra layers on my existing brick walls.

so of that article i understood that putting extra leafs of cld walls directy on the existing wall(no new studwall) would degrade performance compared to the original wall.

So i suppose i should make a new studwall, put the cld wall on that and space the Extra wall away from the original wall to get a bigger air cavaty.

In my small room i can live with loosing 20cm/8" on each wall, but would rather loose less.

But how far should i space the new studwall out from the original brickwall?

best regards

panduro


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## MrAngles (May 1, 2012)

What's on the other side of the brick? If it's just a solid brick wall from one side to the other, building a new wall about an inch away from the brick will be great, allowing room for insulation between the studs, and decoupling from one side of the wall to the other (one side being the new stud wall with drywall, the other being the brick wall. It will be similar to the double stud wall shown in this article.


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## panduro (Oct 3, 2009)

Hi Mr. angles, 

thanks for replying.

two walls would be plaster - bricks - insulation - bricks(one outer wall and one inner thats just build like an outer wall).

one wall would be plaster - bricks - plaster.

last wall is a normal old interior wall- plaster - wooden boards and framing - and plaster again.

yes i have read that acticle, good reading, but it only states that when building double walls, one should space them as far apart as possible.

So if i understand you correct, i should build a single stud wall and space the stud wall 1 inch from the brick wall.
just a regular 2X4 studwall with soft insulation - clips/channel - fiber gypsum - green glue - fiber gupsum?

thank you for spending the time to answer my questions 

panduro


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## MrAngles (May 1, 2012)

panduro said:


> Hi Mr. angles,
> 
> thanks for replying.
> 
> two walls would be plaster - bricks - insulation - bricks(one outer wall and one inner thats just build like an outer wall).


If you already have a cavity with insulation between two layers of brick, I would not introduce another cavity. You would need to either take out the inner brick layer and replace it with a stud wall with two layers of gypsum with green glue, or just remove the existing plaster and hang the two layers of gypsum and green glue directly on the brick. What you don't want is a cavity between two layers of brick and a cavity between brick and gypsum. Personally I would leave these walls for last. I'm sure they have enough mass as it is, plus they have insulation for absorption and they are probably fairly decoupled from each other. They may be pretty well soundproofed as it is.



panduro said:


> one wall would be plaster - bricks - plaster.


For this wall, since it currently doesn't have a cavity you'd want to remove the plaster from the bricks, then frame a stud wall an inch from the brick wall with insulation between the studs and hang two layers of gypsum with green glue on the stud wall. The stud wall would be *decoupled* from the brick wall, the gypsum would provide *mass*, green glue would provide *damping* and insulation would provide *absorption.*



panduro said:


> last wall is a normal old interior wall- plaster - wooden boards and framing - and plaster again.


You'd want to remove the plaster from this wall and either decouple by framing a new stud wall an inch or so away from the existing wall, or hang clips and channel on the existing wall, and hand double drywall with green glue. You do not need to use clips/channel on a double wall, only one method of decoupling or the other are necessary.


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## panduro (Oct 3, 2009)

hi and thanks alot for answering.

About the double bricklayerd walls:
my statement of how the wall was built is proboply wrong, an aquentance of mine who is a builder say's that
most apartment buildings in my area that was built in the same "period" has no insulation between the bricklayers, only an empty air cavaty.


1) why should i remove the plaster? 

2) why wouldnt i want 2 air cavityes? thought the article stated that one big air cavaty is better than two smaller, but its is only to a certain size the two smaller ones wil have a negative effect compared to no air cavaty?

Im not allowed to remove a bricklayer because both walls are load carrying wall. 

Im sure that youre right that those two walls are the lest of my problems and really dont need that much attention. exept for flanking noise and acoustics.

Interior wall:
Putting the clips/channel,gypsum directly on the wall, woudnt i make the same mistake as with the concrete wall(from the article) with gypsum straight on it(too small and air cavaty)? 

Double wall:
would using both double wall and channel/clips make it less soundproof or is it only not needed when soundproofing? would also like to do cld walls because of its low freq acoustic abilityes.


best regards

panduro


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## MrAngles (May 1, 2012)

panduro said:


> hi and thanks alot for answering.
> 
> About the double bricklayerd walls:
> my statement of how the wall was built is proboply wrong, an aquentance of mine who is a builder say's that
> ...


Plaster is nearly worthless in terms of soundproofing, and kind of a pain to deal with in general. I'm not sure how you plan to hang the drywall on the brick walls in the first place, but I can only imagine that it would be easier to have the drywall as close to the brick as possible, but if you're not worried about it, having plaster in between the drywall and the brick won't hurt anything from a soundproofing standpoint as far as I know.



panduro said:


> 2) why wouldnt i want 2 air cavityes? thought the article stated that one big air cavaty is better than two smaller, but its is only to a certain size the two smaller ones wil have a negative effect compared to no air cavaty?


What you're trying to avoid is the "triple leaf" effect. The more air cavities you introduce within any given wall depth, not only does the resonance point increase (meaning higher frequencies will pass through the wall easier), but you will likely have multiple resonance points due to the cavities not being exactly the same depth.

Ultimately this is the simplest explanation though, and all that really matters:











panduro said:


> Im not allowed to remove a bricklayer because both walls are load carrying wall.
> 
> Im sure that youre right that those two walls are the lest of my problems and really dont need that much attention. exept for flanking noise and acoustics.
> 
> ...


Maybe I'm misunderstanding how your wall is currently set up. I thought you described it as something like this:








I recommend that you remove the plaster and either hang double drywall with green glue on clips and channel







or create a double stud wall







. Either way you have one large air cavity with insulation for absorption and double drywall with green glue that is decoupled from the other side of the wall.




panduro said:


> Double wall:
> would using both double wall and channel/clips make it less soundproof or is it only not needed when soundproofing? would also like to do cld walls because of its low freq acoustic abilityes.


Using both will not make the wall less soundproof, it just won't make any difference, so it's a waste of money.


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## panduro (Oct 3, 2009)

thanks again for answering 

the plan was to mount the channel directly on the brick wall and install the drywall on it. 

But according to that article that size aircavaty will worsen the stc compared to no drywall, if my wall was a concret wall that is. But if i understand it correctly, its the small cavatys high resonance point that ruins the low freq soundproofing. 

so im thinking of either mounting the new 2 x 4 studwall directly to the brickwall with screws, then mounting channel, drywall etc.

or as you suggested with the single layer brickwall and the interior wall, spacing the 2x4 studwall one(no more than 3inches) inch out from the wall.



> The more air cavities you introduce within any given wall depth
> 
> Read more: http://www.hometheatershack.com/forums/newreply.php?do=newreply&noquote=1&p=596023#ixzz2NBFigomP


But i am not changing the cavaty number for that given dept of the original wall, im adding at least 5 inches(8inches max) to the wall dept which will include some size off cavaty. 

But as you said, then i will introduce an extra resonans point. is there a way to calculate the the resonans point of a cavaty?

as for how the interior wall originaly was built. its not a studwall like those shown, some kind of frame with woodenbords(like thin floorplanks) hmm imagine somebody laying a floor in that frame and putting 1,5" of plaster on each side... sorry for the explanation, having issues with my english :sad: 

if i remove all that plaster there is nothing to keeps the boards in the frame from moving and that is needed to keep the plaster on the other side of that wall well supportet. to make it worse i cant knock down the wall without having to get a new kitchen as thats is whats on the other side. it not a modular kichen, its built onto the wall, so i cant take it down and put it up again. need more speakers - not new kitchen 


Best regards

panduro


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## MrAngles (May 1, 2012)

panduro said:


> thanks again for answering


No problem!



panduro said:


> the plan was to mount the channel directly on the brick wall and install the drywall on it.
> 
> But according to that article that size aircavaty will worsen the stc compared to no drywall, if my wall was a concret wall that is. But if i understand it correctly, its the small cavatys high resonance point that ruins the low freq soundproofing.
> 
> ...


I have no idea how to calculate a resonance point. But yeah, if it's a choice between channels on the brick or a new stud wall, the new stud wall spaced away from the brick is the way to go, and cheaper than buying clips and channel.



panduro said:


> as for how the interior wall originaly was built. its not a studwall like those shown, some kind of frame with woodenbords(like thin floorplanks) hmm imagine somebody laying a floor in that frame and putting 1,5" of plaster on each side... sorry for the explanation, having issues with my english :sad:
> 
> if i remove all that plaster there is nothing to keeps the boards in the frame from moving and that is needed to keep the plaster on the other side of that wall well supportet. to make it worse i cant knock down the wall without having to get a new kitchen as thats is whats on the other side. it not a modular kichen, its built onto the wall, so i cant take it down and put it up again. need more speakers - not new kitchen
> 
> ...


I'm still having difficulty picturing your wall construction, but I'm imagining a frame that should be able to stand on it's own, with "lath" nailed horizontally across the studs to support plaster like this:








If that's the case, you should be able to remove the lathe and plaster from one side, leaving the studs alone. If the lath actually is providing some amount of structural support to the studs for some reason, you can add some cross members between the studs for support as you are pulling the lathe and plaster off. Whatever you do, you don't want to add another cavity and layer of drywall to the existing wall. If you absolutely can't take the plaster down, you would be better off hanging a couple layers of drywall with Green Glue directly on top of the plaster. It wouldn't be ideal, but it would give you better results than a triple leaf.


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## panduro (Oct 3, 2009)

MrAngles said:


> I'm still having difficulty picturing your wall construction, but I'm imagining a frame that should be able to stand on it's own.


.
i think you are dead on about the wall construction... 
good point, when you put it like that, it doesnt seem realistic that my wall wont support itself with out one side of plaster. so yes i suppose i can take one side of the wall down.
it seems like a good ideer to do the cross braces as you suggested.

just got : Home Recording Studio, Build it Like the Pros in the mail today, so i will return when ive read it a couple of times, hopefully with a bit more knowlegde, so i can return with better questions 


best regards

panduro


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## WilliamSmith (Aug 23, 2019)

you should not rely on one company now in 2019 many companies are working for soundproofing techniques.


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