Steel Studs---16" or 24" OC

[knightfly]

Mark, check out my last post on your other thread and see if that answers your questions. Oh, I didn't see this til I wrote the other - don't let your drywall guy do it "the regular way" by mudding the cracks - acoustic caulk won't crack like mud can, so your room will stay soundproof much better... Steve

[khallgren]

I think Mark's concern is that if screws are used on the second layer they will create a short through the first.

In my case I've got vinyl siding then 1/2" plywood then two layers of 5/8" drywall. I'm in the process of putting up the first layer of drywall with 1" screws. I'm a bit concerned about the second layer (I believe for the same reason Mark is) penetrating the first layer and entering the exterior plywood.

Would this be bad from a transmission loss perspective (the screws holding drywall layer 2 penetrating into the outside layer of plywood)?

I could switch screw lengths so they only penetrate the first layer of drywall--but is this a good construction practice? I'm gluing the drywall and then screwing it in. A contractor friend told me the only point of using screws in this case is to hold the drywall until the glue dries. You could actually take the screws out once the glue has dried and the bond would be just as tight. The glue is the drywall construction glue sold commonly at the home depot here in the Detroit, MI area--Liquid Nailz is the brand name.

I suppose I could remove the screws and seal with acoustical caulk--but would this step be necessary?

[knightfly]

Sorry, I didn't catch that part of the question.

The things you need to watch for in multi-layer drywall, is that you create NO AIR SPACES between layers, and that you (usually) do NOT completely laminate layers together. Screws between layers will have very little effect on total leaf performance - however, failed adhesive allowing the second layer to fall off or dis-locate can be disastrous. This applies even more to ceilings.

I don't recommend using ANY adhesive between layers, but if you do it should be a thin one, and screws that penetrate BOTH LAYERS and enter whatever support that is the frame, SHOULD be used (quickly) wherever the adhesive is applied so that you "suck in" or flatten the glue joint making the gap between layers as small as possible.

If you're putting drywall directly on studs (no RC) then each layer's screws should penetrate into the frame for holding power. Using adhesive will allow you to spread the fastening schedule (how far apart the screws are) to nearly double. This would have a small effect on total leaf performance (less screws gives SLIGHTLY better STC) but probably less than a dB. Some people recommend using a bead of caulk on the studs UNDER the first layer, then fastening with screws - for wood studs, this should help even out the surface and make for less chance of wall vibration between the stud/drywall joints.

Normally, a multi-layer drywall leaf should NOT be laminated (100% surface-glued), because it has the effect of lowering the resonant frequency of the partition by combining the masses of ALL the layers - this will LESSEN the isolation by not allowing the individual layers of drywall to flex separately. Flexing is what allows the partition to convert part of the sound energy into heat.

Keep in mind that, for a music studio, STC is a poor choice of standard because it doesn't consider frequencies below 125 hZ AT ALL. A much better system is the (rarely used yet) MTC, or Music/Machinery Transmission Class standard which takes the lower frequencies into account. Drywall manufacturers (and others) don't like this new standard very much, because it makes their stuff look worse on paper.

However, to stop bass guitar and drums you need good low frequency Transmission Loss, without losing the mid/upper isolation. Not an easy task, but if TL isn't balanced correctly, then the PERCIEVED isolation is worse, because it gives the (leaked) sound that "telephone" quality that cuts thru (psychoacoustically), simply because it doesn't sound NATURAL.

For lower frequencies, close stud spacing is better because it stiffens the wall and helps keep the longer wavelength bass frequencies from exciting the panel as much. In some cases, STC might be worse with panels mounted directly on studs but MTC would IMPROVE, due to this increased stiffness.

If you're concerned about the minimal degradation caused by long screws, one thing that might help (minimally) would be to use a LARGE (at least 24 oz) ball pein hammer (the BALL end) and dimple (gently, the size of the hammer is just to get the right radius depression) each screw location PRIOR to driving the screw, then drive the screw in the center of the dimple. Once the layer is up, put a dab (or dollop, your choice :=) of acoustic caulk over the screw, and strike it off level with a large flex-knife or putty knife.

If you have the option of building double-framed walls, I would recommend building one frame on 16" centers and the other on 24" centers, and using dissimilar mass on each side - if you make each of the two leaves a DIFFERENT resonant frequency, then you'll get better isolation by minimising the resonance dip (walls pass sound at their RESONANT frequency, so if it passes ONE leaf it would be blocked by the other, not being at the SAME resonance).

I read of one case (don't remember where) where a studio builder built a double frame wall between control and tracking rooms using both leaves constructed exactly the same (double 5/8 drywall on metal studs, panels on the inside of inside frame and the outside of the outside frame, rockwool in between) and had a HORRENDOUS leakage problem, I believe it was centered @ 44 hZ... When they discovered that it was the WALL resonance, they added a third layer to ONE side only, and the problem went away.

I'm still learning as to what works and what doesn't (as far as sound proofing), so don't take this to be a final word, it's just what I've picked up by listening to ONLY credentialed sources (such as the BBC, F. Alton Everest, John, Eric, thank you)

The main thing I want to caution everyone on, is that craftsmen who've NOT worked in acoustics will NOT always do the right thing for your purposes. That "drywall guy" probably knows what will keep your walls LOOKING good, because if he's been doing it very long he knows how to avoid CALLBACKS from buckled walls, etc - BUT, unless he's done studio construction he likely WILL NOT know what to do to keep the walls SOUNDING good.

Things that work mechanically, and are typical methods in NORMAL construction, are sometimes the WORST way to do it for OUR purposes. Things such as putting RC over EXISTING drywall layers, and adding more drywall - VERY BAD. Mudding drywall joints, especially on under layers, NOT GOOD - use acoustic caulk. (gaps sometimes need to be left between things for expansion - just don't expect anything BUT acoustic caulk to KEEP them SEALED sound-wise)

Gotta run for now, I hope some of this helped... Steve