RC channel O.K. over shear wall ?

[jbgoode]

My friend and I are attempting to construct a relatively soundproof room in which to record drums late at nite.

We are putting RC channel on the walls/ceiling and then 2 layers of gyp bd. Along the way the city had us add shear wall on the inside of the wood studs ( 3/8" OSB ) to which we now plan to apply the RC channel. My friend read in some forum or another that putting RC channel on top of a continuous surface rather ( like our shearwall ) rather than directly on the wood studs, would actually worsen our STC rating. I rather disagree, thinking that breaking the transmission path still would be an advantage, but I can't get this idea out of his head now.

Can anybody chime in on the merits or lack of in applying the RC channel over shear wall type construction ??

Thanks - JB

[RICK FITZPATRICK]

If your building a room, is this a room within a room, or are you simply sheetrocking an existing space. Also are these walls part of a double wall system, and last, why did the city make you add OSB to a wall to make it a shear wall? To answer your question, your friend is correct. No doubt the shear wall is part of the structural or maybe a load bearing wall, or what ever. If it were me, depending on how this shear wall is in relationship to the other walls, I would decouple or remove the connection of the other walls to the shear wall, and build another wall in front of it. But then again, if your not building double walls anywhere else, it still might be advantageous to simply sheetrock over the ply, but this depends on many other factors. Let us know a few more details and I'm sure someone can give you the correct advice, but at this point, RC should NOT be mounted to the OSB. My .02
fitZ

[knightfly]

Your friend is right - putting RC over existing wallboard, assuming there is also wallboard (or other paneling) on the OTHER side of the frame, gives you a triple leaf wall (mass-air-mass-air-mass), which may slightly improve isolation at higher frequencies but will WORSEN it at low frequencies by quite a bit.

What you need for isolation is two masses separated by one air/insulation space - the heavier the mass, the better - the wider the air gap the better - hermetic sealing is necessary, or lows get through quite a bit easier.

think of air as a spring - its function in a sound wall is to decouple one side from the other by being soft enough NOT to let vibrations from ONE of the sides to cause the OTHER side to vibrate. IF you take that spring and "cut it in two", you've just made two shorter, stiffer springs - now, the coupling between layers has improved (causing a WORSE isolation factor)

There is a lot more on this here -

http://www.johnlsayers.com/phpBB2/viewforum.php?f=2

Done right, RC can improve a wall or ceiling by anywhere from 6 to 10 dB using the same exact materials and dimensions - done wrong, it mainly just costs time and money... Steve

[RICK FITZPATRICK]

Hey Steve, if you fasten RC to a shiething, and then sheetrock so as to create a 1/2" airgap, IF there is NO shiething on the other side of the wall, then this is a 2 leaf system, correct?
fitZ

[Innovations]

What kind of a shear wall are they asking for? Usually under the UBC lateral stability can be achieved with let-in diagonal braces, which will definitly not count as a leaf. if they are saying that it has to be plywood try to see if putting a thick layer of plywood on the far surface would satisfy them.

The fact of the matter is that for RC to work you need a large and cushioned air mass behind it to the next surface.. Just the half-inch of the RC itself, particularly if empty, will not do it.

[knightfly]

Just ran this thru the Insul calculator - two sheets 1/2" gypsum, separated by 1/2" air gap with light insulation and RC - STC 39, M-A-M resonance at 254 hZ, TL @ 50 hZ is 20 dB - basically just a couple dB worse than a standard house wall made with 2x4's, no RC, light insulation, and 1/2" rock on each side - very little point in this but interesting that it's almost as good as a wall that's 3" thicker... Steve

[RICK FITZPATRICK]

two sheets 1/2" gypsum, separated by 1/2" air gap with light insulation and RC - STC 39, M-A-M resonance at 254 hZ, TL @ 50 hZ is 20 dB - basically just a couple dB worse than a standard house wall made with 2x4's, no RC, light insulation, and 1/2" rock on each side - very little point in this but interesting that it's almost as good as a wall that's 3" thicker... Steve

Interesting. Just wondering thats all, you know me. 10 years ago I was in the broadcast booth in the local sports arena in Sac, just as they were mounting RC on the CONCRETE walls. Hmmmm.
fitZ

[knightfly]

Yeah, just one of many reasons a lot of construction people swear that RC does no good, along with nailing through the sheet rock/rc/studs, etc... Steve

[Innovations]

Originally Posted by RICK FITZPATRICK

Interesting. Just wondering thats all, you know me. 10 years ago I was in the broadcast booth in the local sports arena in Sac, just as they were mounting RC on the CONCRETE walls. Hmmmm.
fitZ

Well, they may have actually been doing it for a different reason. RC ws not originally designed just for decoupling but as a metal furring strip for installing gypsum board over uneven surfaces

[Innovations]

Originally Posted by knightfly

Just ran this thru the Insul calculator - two sheets 1/2" gypsum, separated by 1/2" air gap with light insulation and RC - STC 39, M-A-M resonance at 254 hZ, TL @ 50 hZ is 20 dB - basically just a couple dB worse than a standard house wall made with 2x4's, no RC, light insulation, and 1/2" rock on each side - very little point in this but interesting that it's almost as good as a wall that's 3" thicker... Steve

What is this insul calculator you speak of?

[RICK FITZPATRICK]

Well, they may have actually been doing it for a different reason. RC ws not originally designed just for decoupling but as a metal furring strip for installing gypsum board over uneven surfaces

Yea, I know, just thinking outloud Actually, the contractor told me they were soundproofing the booth though. It was the first time I saw RC.
fitZ

[knightfly]

Inno; 'bout halfway down this page -

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=2122

BTW, the full version costs over $1000. If you go into material properties and change the 690 to 2100, then change panel thickness to total concrete thickness, it does a pretty good job of modeling concrete.

The double-ended arrow at the top menu is for panel size - it defaults to a huge panel, so you need to change this each time you open the prog.

One of the two layers per leaf will only let you use one panel, but the other will let you use several - so if you wanted to model a wall with thinner wallboard between two thicker ones, the best you can do is use two thicker layers and one thinner.

All in all, not bad for a freebie -

Have fun... Steve