MASS - AIR - MASS

Hey all, I know this subject has been discussed a zillion times on here, and I think I understand it, but today I came up with a question relating to it that someone can maybe answer for me. I will be building my studio using the room in room construction. The exterior wall will be a double layer of OSB on 2X6X8 studs,,,then a 3 inch air gap,,,then 2x4x8 interior walls with 2 layers of green glued 5/8 drywall on that. Thats the jist of how I am building the walls (obviously with insulation, stucco, paint etc as well) but here is my question....' Understanding this type construction of "Mass - Air - Mass" thus having only 2 leafs, what would happen if when building this, you built a brick or block wall in the air space between the exterior and interior wall?? I know this would be then considered then a 3rd leaf (not good) but I was wondering if there was a material that had a density (like bricks, stone or block) that would be the ultimate "stopper" of the sound thus the leaf's wouldnt then be an issue. The brick wall would handle everything (and the outer and inner shell would just then be additional). I am asking this because today I spent all day removing about 500 bricks from a patio area where my studio will be going. There is also another easy 500 bricks to go yet. So the wheels were turning for a use for these bricks. They wont match the house to use on the outside, so i was just wondering>?>?>?? :lol: Again, I understand you typically dont want a 3rd leaf in your building, but if it were of a very dense material would that make a difference with the "leaf" theory?? or no?? Just wondering :?: And I am waiting till Spring to frame, but we decided to do the pad now. Today I broke ground and possibly next week the footers, foundation and pad will be done. Thanks AJ

The problem with such a "stopper" middle leaf is that it would divide the air pocket between the leaves into two smaller halves, and in fact the total of both halves would be less than before. Don't underestimate the value of the air between your leaves. It may not seem like it's worth much (air is free, right? Well, except for the square footage you lose...), but I can assure you it's worth a lot. A reduction in the air gap causes a worsening of the resonance in the the opposing leaf... And that resonance may actually amplify the noise that was stopped by the first leaf. --Keith :mrgreen:

Put the brick on the outside. Or take the money and double up on the sheetrock and perhaps GG. I think you have wasted too much space in between the walls. If you have 2x6" +3" space + 2x4" you have 13 inches of air space between the walls. Somewhere I read more than 12" and you pass the point on diminishing return.

packrat wrote:

I think you have wasted too much space in between the walls. If you have 2x6" +3" space + 2x4" you have 13 inches of air space between the walls. Somewhere I read more than 12" and you pass the point on diminishing return.

I'm the one who often writes about the diminishing return after 12", and I have to respectfully disagree with the notion that he's wasting too much space with 3" between frames. First of all, lumber dimensions are nominal, so 2x4s are 3/½" and 2x6s are 5½". Thus, the gap is actually 12", not 13“. And in this case, he's got plenty of square footage to throw at his very lofty (no pun intended! ;-)) isolation goals. I never said there's no point to air gaps wider than 12". I have said, basically, that the last inches account for less isolation improvement than the first inches. I wouldn't object to his even adding a couple of inches if he wanted to. --Keith :mrgreen:

Yeah man, if you've got the space, then the luxury of being able to have large airgaps is priceless. I agree with Keith in that you should definately NOT divide your airgap with another leaf.

Thanks guys and I wont use the bricks. I'll build a patio with them instead. :wink: I just thought there was some physical properties or physical law (you know one of those 2~x56>76-23Q = 2 leafs type things) :lol: that would explain this phenomenon. I know the mass air mass design works, and I want it to make sense on how in my head, but even if it dosent, I know thats the best way to go. If I never would have found this forum, I would have never believed it. Before coming here, I was one of those who would have made like 25 layers of walls thinking that was the best. :( Man would I have been mad when it didnt work!! :twisted: I owe you guys all many beers for all your help through all this!!!! :lol: My walls will be Mass (5 1/2" exterior wall) Air (3") and Mass (3 1/2" Interior wall) which will give me a total of 12" of space. the bricks,,,well,,,I see a patio or sidewalk in their future. :D As always,,,Thanks!! AJ

Actually, I think a 25 leaf system would work really well... But it would chew up a lot of real estate and it would not be the most efficient use of resources. ;-) Let's just say that, ideally the number of leaves should be... Equal to 2, or equal or greater than 25. :lol: --Keith :mrgreen:

Monty Python said it best - (slightly screwed up for these purposes) - "thou shalt count to two - neither shalt thou count to one, nor three - four is right out. " Been a while, this was a poor mind-fart from the "holy hand grenade" skit... Steve