Green Glue...

exactly svart, a 12" deep twin leaf wall with 2 layers of high density board and insulation per leaf will achieve an Rw or STC of about 69db. that's about the limit of lightweight constructions ability and was outlined by the bbc because of it's improved low freq results.

in my studio the wall between the control room and the tracking room is 12" overall with twin leafs. each leaf has 2 layers of 5/8" firechek and 1 layer of plasterboard internal side. each leaf has insulation held in place with wire mesh. green glue and fire mastic to all boards.

bottom plates of both walls are isolated with sorbethan type pads. top plates are isolated with flexible brackets.


ceiling was carpet/timber flooring/3 layers 5/8" -> 12" fully insulated cavity -> isolated ceiling frame, 2 layers of 5/8" + 1 layer plasteboard. green glue nd fire mastic everywhere.
above the studio are bedrooms.

there are other single leaf walls and walls with resilient channel. it depends on their location.

floors were isolated slabs.

we can record bass at levels that shake the tracking room walls at 2am in the morning and no-one in the house is disturbed and no neighbours have ever said a word.

the downside of all of that isolation is the need to have a whole heap of ventilation outcomes and lots of acoustic treatment. the third room is the drum room which has no isolation at all. we record in there during the day and the ambient noise is not a problem.

gouge how tall are the ceilings in your(nice looking) studio? they look low? the lolly column in that one picture makes me think it's in a basement?

back to our little confusion... U wanted me to look up the spec on what? concrete board? unless i missed it, you finally clued me in to the fact that you were even talking about cement board? Concrete board is meant generally to be a tile/stone backer and the materials would cost over 5x the $ for the board and galvanized fastener$, and at least quadruple the labor $(no matter what the manufacturer claims) to install and cover the same area as a single sheet of 12' 5/8" firex sheetrock, all without provable results(and we haven't even talked about the finish yet)?
   Have you ever worked with concrete board? it is very heavy, it comes in smaller more manageable pieces for a single installer to handle, or the 4x8 sheets that require a crew of guys to handle(hence the much higher labor costs). But(again despite the manufacturers claims) it's porous, crumbly, dusty, hard to cut and work with, acoustic short circuits would be ubiquitous in this stuff, hardibacker style concrete board is more rigid, dense and slightly easier to work with, but still a complete pita. I would NEVER use this in a studio for any reason, u'd be much better off performance and cost wise using wire lathe and float for a heavy monolithic seamless concrete substrate(like old style float showers), but i still doubt it would out perform a couple layers of 5/8" typex.
    "Do a skim coat for a high grade finish"? Drywall mud is NOT acceptable on concrete board, drywall mudding sheetrock is a 3 step tape, block, and finish coat process for flat wall pro's, you could use an orange peel/knock down finish in place of the 3rd step for IMO an ugly quick finish, but concrete board would require alkali taping, and a portland cement $tucco type finish and the larger costs that go along with that.
    Back to drywall, consider the idea that even with a pretty finish, if you have drywall surfaces exposed in your studio?? IMO it ain't gonna sound good... rooms take on the characteristics of the materials they're built with, bare block, concrete and even stucco to a lesser extent, sound great in measure, drywall imo sounds bad always, drywall+carpet= total sonic shit, you're better off carpeting wall to wall to ceiling to wall to floor, and "enhancing" the bad results of that room sound.. with plugs.
    Ultimately, if your going to spend the $ that's required to do what you're suggesting, you'd be much better off with metal stud 2 leaf systems, isolation, and slab cut decoupling etc. to your existing structure, or just start over and build a pro studio.
    I understood the premiss of what was being discussed here, as an effort to make an existing structure more "sound proof" at a reasonable cost? maybe i miss understood.

All this discussion is making "Quietrock" seem more and more like the best option for adding to an existing wall.

i seem to be having good results, i did a before with the bass drum slamming, i still haven't got an after, i will try to get that later tonight and give you my observations. I can tell you the difference between pounding the back of your fist against the wall before and after, shows significantly improved solidity, a much damped thud and reduced boom.

I can't compare it to green glue, what i can say is it adds more mass than green glue, the roll of liberty is extremely heavy to handle for an average man, but it sticks beautifully, i was able to place seams....seamlessly, and with 0 gap/voids, its very chewy and sticky stuff, if you let it fold onto itself, its not coming apart again, if you stick a portion to the wall and then try to peel it off, it's pulling the paint and drywall paper off the existing wall.

I highly recommend 2 installers who are willing to work smart not fast, slow your roll just a bit, set the roll on the ground against the wall and pull/unroll the sheet up toward the ceiling, align horizontally and vertically, then peel 6" of the backing and set the top 6" by smoothing with your palms, then continue to pull the backing downward slowly, and smooth onto the wall as you move downward, work middle out toward the edges(it's a feel thing), if it wanders and you're committed, just finish the section off, you will be able to see the overlap, use a straightedge and sharp utility knife and cut the overlap away. Be thorough, and i think you'll be happy with the results.

Mold problems? the structure's got bigger weather/water tight? or ventilation problems? if he has mold problems. Liberty is mostly butyl, and it's a roofing material purposefully designed to keep water out. but if a structure is weather tight, has proper ventilation, and doesn't get wet, it should not be a worry IMV, but then again i'm not near the gulf, i don't know there building codes, i do live a mile from the pacific ocean...so? 

There is a real long thread over on the other site were a lot of the studio designers weighed in on the multi-layer/density question. Its a good read. That was the only part of the site I really spent time reading. The consensus was to go for independently tested and proven methods, multi layer 5/8 inch type x drywall (for light construction). The real issue I see is why put money into something that "in theory" should work, but no one has data on the whole final assembly. The logic that 2+2=4, does not really hold true in sound transmission physics.

gouge and tonycamphd, to be honest I was confused as to what you guys were arguing about, and looked to be a big misunderstanding due to language/word use. I have learned that when talking acoustics/building with people from different areas things can get heated pretty fast due to misuse of brand/type names. Please don't see purple!

But I like Purple MC76s, ODD TAV, Sweet10 ...Rain

Oh you mean that other site!

i kid you not, i walked into a rehearsal room one time with a guy(relatively dry Socal) who hadn't been there in 2 weeks, he had built it air tight, and ventilation free, I swear there was mushrooms growing out of the carpet on the floor!! and I don't even know what to call what was in the guitar/case that he was trying to sell me...  It ruined all his stuff sadly.