AVS Forum Special Member
Join Date: Feb 2009
Location: Southern Sweden
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To start with I apologize in advance for where my English is lacking.
My situation is as follows:
I have a small free standing building made out of lightweight expanded clay aggregare (LECA) blocks with currently a single layer of gypsum both for walls and ceilings with fibreglass insulation behind. Sizes are of little importance as this will have to go.
Note: it was meant for storage, not audio, when it was built 12 years ago.
I had an acoustician at home doing the internal acoustics design, so that's fine (albeit a bit stumped since the room size will have to change) but unfortunately we both forgot the fact that we might disturb the surroundings, so when I had built most the cinema and tried it out for real (the Marsian rising out of ground in War of the World at around thx reference level) my neighbours told me it was disturbing.
I have downloaded some different free softwares for my iPhone and with a rough estimate from that, my current building isolates about 30/35dB in the bass range. The major leakage looks to be below 250Hz and then falling off to about 1kHz. Could perhaps detect more up there had I played louder inside.
I believe to have identified at least two problems:
1) the LECA block are untreated and from the manufacturers paper, then don't seem to dampen sound very much in that state. I will put concrete on the surface once I have removed the inner walls. The outside has wooden panel in a quite good state which I do not feel like disturbing beforehand, so treating that side will have to wait until the wood has to be replaced some distant past
2) Above the blocks there most likely isn't anything besides the single gypsum ceiling and some fibreglass insulation before the open air of where the roof sticks out (is that called suffit too?) Above there's a full wooden floor for the attic, which is unaccessible for work from above.... it's more or less full, so I'll have to find a way to close these gaps from below.
(I hope #2 is the biggest issue and that it won't be too hard to do something about. )
I will of course try to set some kind of measurement standard and get a baseline before I do things and again when everything is torn down (come spring) and then see as I go what improvements are made.
Now for the re-construction I have some ideas and some concerns and some things I find very different information about...
It seems like one should build a room within a room for best results, so I'm thinking about that. I could imagine going for other kinds of cement block and mason up another wall. It'll cost more and take much longer, but it's an option. On the other hand the same space can be used for a larger air cavity with insulation. I don't feel very worried about higher frequencies, they didn't seem that bad now and whatever I do will improve that substantially, so my concerns are in the lower side.
I have been getting quite different recommendations about how to build the inner walls, typically:
*AVS standard seems to be gypsum+green glue+gypsum
I myself could imagine going for MDF(+MDF)+gypsum(+gypsum) possibly with green glue or Swedish equivalent.
Some say that I you cannot use too many screws or too small boards otherwise the resonance frequency will go up and bass dampening get worse. Others even want to use stringers behind to make the frame less likely to move.
Studs seem to be 2x4 on all American sites, I don't know if that's building code or function. 45x70mm would be normal here but some people recommend 45x45mm (2x2) instead - could that be lowering resonance frequency?
I went through a long list of walls on a NRC document and I do feel that wooden studs looks like the outperform metal ones on the low bass, my analysis could be off though...
Another AVS standard seems to be decoupling the entire wall with clips and channels (which don't exist over here, the closest thing found would be Auralex RC-8 which can "only" suspend two boards), but I've had some people I generally trust in acoustics scream at me for suggesting that, saying that will also make bass isolation worse. So I'm a bit confused what's correct... or if there are two diverging philosophies that both work but cannot be combined?
Have found some clips that you can mount wooden studs on instead of hat-channels - it may be possible to import enough of those if I'll be finally conviced that's the way to go...
I guess that's the main question... some minor ones:
I have also stumbled on a couple of odd materials, sand and lead.
* My first idea with building a new concrete block wall was go fill the cavities of the blocks and between this wall and the old with sand, and that seemed like a good idea until I though of possible moisture issues, so I'm less intrigued by the idea now. The LECA blocks are hydroscopic, so the sand would have access to moisture at times.
* Regarding lead - it seems only to be 0.5mm thickness that would be available in rolls that can be handled and besides being expensive, perhaps it's much to thin to have any effect at all. Also wouldn't know where it would be best applied... sandwiched in the wall or as innermost layer... or even doing an otherwise "not allowed" mounting in the center of the cavity (thinking that it would be most useful when being asked to move)
For reference -the link to the threater project is in the signature.