Sound absorption:More density or more air gap?

Thanks to all the advices around, i have designed the construction isolation for my studio, but before buying all the materials, i am getting some dilemna: My isolation walls are designed to include (in order from the original wall): 2" air gap + 3" rockwool (3.0 pcf) + 0.5" gypsum board //+ 2" air gap + 3" rockwool (3.0 pcf) + 0.5" gypsum board// + sheetblok + 0,5" gypsum board //. -Total wall thickness:11,5" I am wondering if instead of that double 'layer' of air/mass, one 'layer' only but with more density rockwool won't be better: 2" air gap + 4" rockwool (6.0 pcf/9pcf) + 0.5" gypsum board // + sheetblok+ 0,5" gypsum board // -Total wall thickness:7" That solution would give less thickness to the wall, but would get only one layer of air gap.Both solution should be equally cost effective, anyway. So here's my question: do you think the second solution will be more effective? Is it better to use several air gaps, or higher mass?

Nightmusic, The first question is what is that original wall? Too often the mistake is made that one adds isolation to an existing situation, where the existing isolation is interpreted as a constant. It isn't. From the moment you add something you create a new system where the existing situation makes part of. If I mount a reslient gypsumboard in front of a heavy brick wall at a small distance, I will degrade low frequent isolation of that heavy brick wall and upgrade mid and high frequent isolation. Hence I can not add the value of that gypsum board (not even partly) to this heavy brick wall. I created something new listening to its own physics. You have to avoid triple leaf systems. A search in the forum will show a lot of related topics. Analysis of the NRC/IRC documents, and data collected and measured by Brian Ravnaas shows that you get the best overall isolation for music, by using traditional low density mineral wool filling in double leaf wall systems. Hence use the traditional boards or blankets traditionally sold for that purpose. Also other studies show that. I admit there is some slight contradictary infomation available, but in general one can state that no study advices to increase density for low frequency insulation (which is the defining factor) and the previous paragraph confirms the negative effect of higher densities on low frequent isolation.

Nightmusic wrote:

My isolation walls are designed to include (in order from the original wall): 2" air gap + 3" rockwool (3.0 pcf) + 0.5" gypsum board //+ 2" air gap + 3" rockwool (3.0 pcf) + 0.5" gypsum board// + sheetblok + 0,5" gypsum board //. -Total wall thickness:11,5"

OK - big problem I see here - somewhere along the way yu lost the concept of MAM system......... That would be 2 layers of Mass encompassing one layer of Air You say you have an original wall - and from tha wal you are installing - SO that wall already has mass. You don't explain if it has mass both sides - but let's assume for a moment it's a concrete wall. Existing wall................. MASS .....................................AIR (5" total) (2" open air - 3" insulation) Inner Most Face MASS (1/2" drywall) .....................................AIR (another 5" total) (2" open air - 3" insu) ....................................MASS (1/2" drywall - sheetblok - 1/2" gypsum. That is the triple layer Eric is advising you to avoid above.......... If the original wall is a wood frame assembly- with sheathing both sides - then it would be a 4 leaf. Anyway you look at it - this is not what you want to build. OK - you have to answer - exactly what is the existing wall.

2" air gap + 4" rockwool (6.0 pcf/9pcf) + 0.5" gypsum board // + sheetblok+ 0,5" gypsum board // -Total wall thickness:7"

BTW - why are you using 1/2 drywall with sheetblok instead of 3 layers of 5/8" drywall - (which is a wholeheck of a lot cheaper than sheetblok)??????????

Both solution should be equally cost effective, anyway.

Nope - things are only equally cost effective if they produce the same results for the same costs......... you will not get a 3 leaf wall to isolate as well as a 2 leaf for the same cost. ANd I don't believe you can frame the 2nd wall for the same cost. Sincerely, Rod

Well, first of all, note that the original walls are all solid (reinforced) concrete.

If I mount a reslient gypsumboard in front of a heavy brick wall at a small distance, I will degrade low frequent isolation of that heavy brick wall and upgrade mid and high frequent isolation. Hence I can not add the value of that gypsum board (not even partly) to this heavy brick wall. I created something new listening to its own physics.

Eric, you're learning me something when you explain that sound isolation isn't just an addition to any other system existing... But well thought, I can understand for ex. that a gypsumboard alone can pronounce, by it own vibration, some band of frequencies, hence making the original wall behind vibrating more at these frequencies, than if the gypsumboard wasn't there. But in most of the cases, we got mineral wool behind board, or even sheetbloks somewhere, parts of a system that would lead low frequencies to be absorbed in the right way, right? But i also basically thought that the more density wall, the better, following the rule of mass for absorption.That is why i thought about putting 9pcf rockwool.I also intend to use the same rockwool pcf and thickness for building deep bass traps, so i may be confused from that point, using the same principle for acoustic isolation, hence thinking that the more density insulation would absorb lower frequencies, in a sound isolation system.

Existing wall................. MASS .....................................AIR (5" total) (2" open air - 3" insulation) Inner Most Face MASS (1/2" drywall) .....................................AIR (another 5" total) (2" open air - 3" insu) ....................................MASS (1/2" drywall - sheetblok - 1/2" gypsum.

Rod, your post lead me to confirm my confusion between mineral wool and mass.You consider insulation as air part, while i thought that insulation had to be as much dense as possible, leading to become a part of the mass system itself. So you both guys help me understand the principle of absorption system, which basically appeared to me as a single addition of materials.

That is the triple layer Eric is advising you to avoid above..........

I conclude that i should go for that system: Existing wall................. MASS CONCRETE .....................................AIR (5" total) (2" open air - 3" insulation) ....................................MASS (1/2" drywall - sheetblok - 1/2" gypsum.) 1) From that solution, what would be the ideal ratio between: open air-insulation-outer MASS? 2) Rod, i thought about using sheetblok, after seeing it used in most of the constructions, even in Forums advices around, etc... -I also think it would 'envelop' holes made thru drywalls/Gypsumboards (against the risk of 'sound bridges'??), as sheetblok can be heatened and act as a 'glue' to the surface. -It also appeared to me that sheetblok stuck inside 2 layers of gypsumboard would break some sound transmission, as far as it has a different density from gypsum. -I agree it is much more expensive than the solution you suggest, anyway! (Actually i intend to use a product called 'Amortson', which follows the advantages described above, plus being twice as cheap as Sheetblock).http://www.riviera-acoustics.com/catalog/product_info.php?products_id=780&cPath=28&language=en 3) Is there any reason why not using glasswool instead of rockwool for that type of insulation? Everyone talks about rockwool around, but...I used to put 4" glass wool in the ceiling of my previous home studio, and it seemed to work well ! Any comparison made? Well guys, i just hope i am approaching the final solution ... Anyway, u helping me a lot yet at that point !