mass air mass

Hello, Say I have a wall made of cinder blocks and concrete and I want to improve the TL in building a second wall in the concept of a mass-air-mass separation. I would have to use the actual wall as one leaf of my two leaf separation and build another one on ONE Side of it. Now on the other side, this concrete wall is not decorated so I would like to add some board of gypsum just to be able to paint. To do this I will have to have some kind of frame again the wall to fix my boards onto. This will create a little space of air and thus will transform my mass-air-mass separation in a mass-air-mass-air-mass one. I know that creating another mass-air layer is almost useless but can it worsen the TL ? Do I need to avoid any air on that leaf or won't it just matter ? Thanks Alex

Yes, a second air gap (3 leafs of mass with two airspaces) will make low frequency (bass) TL worse. If you can avoid a second air gap, then by all means do what it takes to avoid it.

ah! thanks, I was about to make a mistake ! what are my possibilities then ? Mortar and paint ? Thanks Alex

After re-reading your first post, I realize that I'm a little confused with your description of your existing wall and the wall you propose to build. When you say cinderblock and concrete do you mean hollow blocks filled in with concrete or...?. A photo or a drawing would help a lot if possible. If your cinderblocks are hollow, then you already have a mass air mass system. Any new wall built inside of that will create a 3rd leaf or more depending on how it's constructed.

yes there are hollow blocks and the concrete is put on the side of them (mortar). Yes somehow this is already a mass air mass, but don't I need to do more in term of insulation ? What I intend to do is this:

Ignore the outside and inside not relevant in that case since that wall is in between two room.

Given that your blocks are hollow, your best bet (if possible) is to fill in the cavities first. Then build your second leaf out from that. If you can't fill the cavities, then you are better off just adding layers directly to the block wall. It also depends on how much isolation you need too.

oh that's bad, filling the cavities is impossible. I have two walls like this one of them is 20cm and has 3 air space in it. The other wall is only 10 cm and separate the room from the outside, I have already played the drums behind this wall, I must do something for it unless my neighbors are going to kill me. Now you say I can't built another wall, I am very confused regarding what to do now !

I think you are in trouble; there is already more "leaves" in your wall system than you need, and adding even more air gaps will only make things worse. About the only possibility now is to add heavy mass to the existing inner and outer surfaces WITHOUT allowing another air gap; this will improve your walls somewhat, it's too difficult to calculate for a math idiot like me :cry: Just for clarification, here are a few walls and which would be considered a separate leaf - sorry, haven't calculated actual TL on these and may never get time to do... Steve

alright I get it, I can only add to the mass, I will then add a 2cm thick layer of mortar on each side of the wall. Thanks. Alex PS: I don't know about anywhere else but here in south of France we use hollow blocks all the time to build houses, some have 3 separations (like mine) of air inside the block, I guess it is good for thermic isolation but when it comes to sonic one, the best is to demolish everything :S. Thanks Alex

A last thing, I have a wall that is 30cm tick and made of stone and concrete (only mass), theoratically I should build another wall creating a mass-air-mass system. But knowing that this wall is burried underground, is it necessary ? Will it make a significant difference if I don't build another wall ?

I doubt it; several feet of earth is hard to beat for sound isolation... Steve