Brick Wall Soundproofing for Common Wall Between Properties


Postby d.blaise » Fri Nov 02, 2018 9:10 pm

Hello,

I would like to soundproof the common brick wall between my house and my neighbour. I thought about using Green Glue but I'm confused about one point :

How do I attach the drywall / green glue sandwich to the brick wall ?

Here's what I planned to do : neighbour - brick - drywall - green glue - drywall - my living room.
I'm afraid that if I attach the first layer of drywall directly to the wall it will create a common mass with the brick wall, nullifying the dampening system.

How do I decouple the first drywall layer from the wall ?

Part of this wall is a staircase in which I can only afford 20mm (3/4") of thickness. I planned on using Green Glue sandwiched between two layers of 9.5mm (3/8") drywall.
I guess I could decouple the first drywall by adding a very thin air layer but I'm afraid of resonance.

What do you think ? How could I soundproof for airborne noise a plastered brick wall with only 3/4" of thickness to afford ?

Thank you,
Dorian
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Postby DocMartin » Sat Nov 10, 2018 1:51 am

I am not an expert...

...but - soundproofing relies on (a) mass and (b) absorption to work. Also, there must be no airgaps sound can leak through or other sneak paths. If you haven't got the thickness to put in a proper system it could be tricky.

First off, rather than use standard drywall, there is an acoustic version which should be superior.

As you say, you should use some sort of airgap. I don't think resonance will be a problem as the wavelengths of sound humans can hear are probably all longer than the airgap. To be specific, resonances might occur down to a quarter-wavelength of sound. Probably all the sound you are bothered by is under 1-2kHz and a quarter wavelegth of 2kHz sound is 4cm...Your 'thin' airgap doesn't sound anything like that big. Also, 2kHz is probably on the high side for annoying noises, more like below 1kHz, at a guess.

You are right that fixing drywall (acoustic or otherwise) directly to a wall probably won't be the best solution. It will help a little as it is extra mass, but there is not much extra absorption. Green glue (had to look that up) looks like a noise absorbent (visco-elastic) material but I think an air gap would help too as sound is reflected from interfaces so for instance any that makes it through your wall into the air gap would mostly bounce off the rear of the drywall, then rereflect off the wall etc, some getting through the drywall on each reflection but some going back through the wall. Plus, if the sound in the gap meets up with some kind of damping material as it bounces back and forth, it loses energy every reflection that way too.

I believe you can get resilient mounts for this sort of thing. They take the form of two metal pieces with mounting holes, separated by an absorbant rubber material. They can carry load but there is no direct path from one side to another except going through the rubber, which attenuates sound.

I would think that you have to be careful at the edges of the drywall too, as sound gets through small air gaps and if you grout it to the adjoining walls that constitutes a sound-leak path. You could try taking it just shy of the adjoining walls and injecting the gaps with green glue or neoprene rubber sealant or even polyurethane foam? Actually, never mind the duralay, look up acoustic foam tiles - you could stick them to the back of an acoustic drywall panel and then also use suitably shaped rubber grommets to take drywall screws into battens. Just some ideas - although some of the sound deadening foams look quite expensive per square foot as it were, but they could seal any gap around the edges of the drywall panels too...
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Postby d.blaise » Sun Nov 11, 2018 5:31 pm

Thank you for you in-depth response. I don't know much about resonance and sound can be tricky to understand but, like you said, better have a thin air gap than no gap.

I live in Belgium and there is two soundproofing panels commonly sold : PanTerre and Calibel.
Basically they are what you said : mass and absorption. It's drywall with either a spongy thing or fiberglass glued on the back.

I could use that for the part of wall where thickness isn't a issue, and a homemade solution for the staircase where I can only afford 20mm of thickness.
A heavy acoustic drywall with a bit of sound absorption material (like cork?) and an air gap. Maybe I could attach the whole thing with silicon, so it's decoupled from the wall...
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Postby DocMartin » Wed Nov 14, 2018 12:52 am

I wouldn't use cork. I know what you're thinking, but I don't think it absorbs sound well enough. Something like heavy old carpet in the gap beween the wall might be better if you're trying to keep the cost down.

As for siliconing a panel to a wall, I think you'd have to apply quite a lot to be sure it would stick and that would let a lot of sound leak through? If you could find a way to put 'hangers' on the wall and 'hooks' on the back of the acoustic drywall or whatever, then you could just hook the panel into position, securely held. You'd want to make sure the hooks and hangers had some kind of resilient rubber insert in them to stop direct contact between them.

Interesting project, I think you may have to use a little ingenuity especially if you want to make your own solution!
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Postby DocMartin » Wed Nov 14, 2018 12:58 am

PS: It occurs to me that the sort of metal brackets used to hang kitchen units on walls might be suitable. They are low profile, and if you got a sheet of rubber you could cut it into little rectangles, glue it onto the bracket and then there will be a similar bracket on the back of the drywall, only upside down, that slot into the brackets on the wall.

A problem though - you have to lift up kitchen units a little then drop them down onto their brackets, but you can't do that with drywall that needs to touch the ceiling when mounted on the brackets... you couldn't lift it up enough to get it onto the brackets!
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