Soffit Wall Construction

I understand the wall construction part, but I'm not sure about the drywall layers :? Again, i've attached a drawing :idea: to show what i think it should be. Please ease me of my pain! Thanks Luke

OK, Luke, here goes - First of all, here are some definitions, then some rules of wall construction as I see them, with reasons if I'm lucky enough to remember them - afterward, I'll tell you what I think you should do where, and why - The most effectuve use of materials can be had using a mass-spring-mass type of wall. The mass is whatever layers of paneling are attached to either side of the frame. A leaf is all the layers of mass on ONE side of the frame, whether it's one layer or four. An Air space is any non-solid, including insulation. The wider the studs in the frame, the deeper the air space. Air is considered a "spring", because it is relatively stiff and will transmit sound from one leaf to the other. 1. Never put RC on both leaves of a wall - it allows too much movement on the part of one of the leaves, decreasing TL through the wall. 2. Two different thicknesses of wallboard beats two of the same size for mid/upper frequencies, two thicker panels does better at low frequencies. 3. Low frequency attenuation is improved by NOT having RC, by increasing mass, and increasing stiffness. 4. Fastening a second layer of wallboard to a first layer, when the first is mounted on RC or STEEL studs, should be done with screws through the first layer and into the RC or stud ONLY - if these screws touch WOOD studs the isolation of the wall will be reduced by as much as 9-10 dB. The STEEL studs are flexible enough not to cause a loss in performance. 5. Fastening a second layer of wallboard to a first layer, when the first is mounted on WOOD studs, should only be done using 4" wide stripes of slightly thinned joint compound over the stud area, then fastening with Type G laminatings screws approximately 2-3" away from the studs and parallel to them, on approximately 12" centers. This allows the two layers to act as separate panels, which maintains each panel's coincidence dip separate from the other. It also keeps any fasteners from penetrating BOTH layers of wallboard AND entering the WOOD studs. NOT doing this will lose about 9-10 dB of isolation due to flanking through the fasteners and into the studs. 6. Insulation should be placed against inside wall panels to increase damping of the panel - This helps TL at low mid/high bass regions, as well as decreasing the effect of the Coincidence Dip which occurs in drywall at between 2 kHz and 4 kHz. Optimum density for in-wall, rigid insulation is around 2.5 to 3 PCF. 7. RC should never extend far enough for its end to contact the intersecting wall - cutting it about 1" short works well. Wallboards should also not directly touch the intersecting wall, but should have a gap of approximately 1/8" between them. This gap must be caulked with enough acoustic sealant to ensure complete seal. I'll probably add to the rules above as I realize why I'm suggesting things, but here's what I would do on your walls - For the inner walls of the control room, all the way around, I would go with RC on studs followed by 5/8 followed by 1/2" wallboard. This should be on 24" OC studs, with the RC placed on 16" centers. Place the RC with open side DOWN for the very bottom course, with all other courses opening UP. Where RC needs to be spliced, do it over a stud. Overlap the two pieces by about 2", and fasten in two holes through BOTH pieces. This does NOT include soffits, which I would make as rigid as possible and float away from the walls with 1" rigid fiberglas, while mounting them as firmly as possible to the floor. I would build the shelves and the soffit fronts from 1-1/4" solid plywood or waferwood, back them with at least 2x4 studs. The closer you can get them to feel like part of the foundation, the better. You will want to isolate the speakers from the soffit shelf with no more than 1/4" neoprene or maybe 1/2" Celotex. Doing the isolation right at the speaker will lessen the effect of the speakers "rocking" under impulses such as kick drum, thereby introducing transitory time alignment errors which can sometimes be heard as phasing. For the rest of the walls, I'll refer to your floor plan here http://johnlsayers.com/phpBB2/viewtopic.php?t=504 and assume that the drawing is oriented like a map, with North at the top. For the entire tracking room, outlined in red, I would make its inner walls two layers of 5/8" wallboard, with a layer of 1/2" Celotex sandwiched in between - Follow rule #5, above, for both additional layers, making sure that no fastener touches a stud after the first layer is on, and that the top layer of wallboard is fastened with long enough Type G screws to tie into the first layer of wallboard instead of the Celotex. I'm assuming you will be tracking live drums and bass in that room, so you need good low end isolation as well as the rest of the spectrum. The celotex layer will break up the coincidence dips of the two layers of 5/8 wallboard, and the wallboard will provide enough mass (along with the other leaf on the control room side) for good isolation for setting mics while listening to the speakers. For this entire wall, I would use closer stud spacing of 16" centers to make it stiffer and improve bass loss further - Stay with 24" centers where possible for the control room inner wall. For the East wall of the lobby, I would use a layer of 5/8 and a layer of 1/2" wallboard directly on studs, with a bead of acoustic sealant on each stud face prior to fastening. Non-coincident seams, 1/8" gap all around, second layer using LAMINATING Type G screws and joint compound as per rule #5. The North wall of the lobby I would use RC, 5/8, 1/2" wallboard per rule #4. Reason is the tracking room has no RC, so this wall SHOULD have it. This path is close to outside access, so should have double layer of dissimilar wallboard for best isolation. If you can widen the gap (and splay it) between the two doors (one being the one you don't show) between tracking room and lobby, even better for sound control. The Inner South wall of the Vocal booth should NOT have RC, as the control room DOES. The other three walls of the vocal booth SHOULD have RC, because their opposites DON'T have it. An alternative would be to use the non-door part of the south wall of the vocal booth for slat resonators, the east wall 1/2 and 5/8 drywall, and the north and west walls two 1/2" wallboard on RC, following rule #4, then LAMINATE 1/2" Celotex over the wallboard using long Type G laminating screws and 100% coverage, thinned joint compound. This would change the coincidence dip of one of the sheets of 1/2" wallboard, as well as get a start on some of the absorption you will need in the vocal booth. Studio glasses, if you can afford it, should be LAMINATED glass, at approximately 1/3 the thickness of the TOTAL wallboard thickness for each side of the wall. If one side has 1/2 and 5/8 wallboard such as the control room side, then the total thickness of wallboard is approximately 28.5 mm, so the glass for that side should be about 9 mm. For the tracking room side with its 2x 5/8 wallboard + 1/2" Celotex, the formula changes. It's based on MASS, not thickness, so the Celotex won't add as much as it seems. Probably about half, so we would have an effective thickness (if it were all wallboard) of about 1.5" of wallboard, so the glass should be about 1/2", or 12 mm. Following these recommendations will get you as close to the performance of the wall as glass can get. Using laminated glass will improve low freq TL by 3-4 dB - also, the lamination damps the glass panel so that standing waves won't cause as much resonance in the glass. For this reason, I would NOT splay the glasses because doing so will lose 2" of air space for every 10" of window height. That lost air space will cause more bass bleed than the high frequency problem of flutter. Just don't make the window any bigger than you need to, and follow the isolation recommendations on the SAE site for mounting, venting the relief, isolating the two wall frames, etc. I'm about out of steam for now, if I've forgotten anything we can go some more later. I'm attaching a re-do of your last drawing just for kicks... Steve

what a post... :!: :!: BRAVO...very helpful and straight fwd

8) 8) 8)

:mrgreen: :shock: :mrgreen: Total Respect Steve! This is amazing information! Ok, let me get my feet back on the ground.... I have but a FEW questions: Considering that I am not too concerned about lobby or outside noise; should I still go heavy on the lobby walls? I also was thinking about putting only ONE door in there (on the Live Room side) and just a frame on the lobby side. It's very tight in there (lobby). I don't want it to be a labyrinth to bring equipment in the Live Room. Do you see a solution? About NOT splaying the window. Not sure I understand. Do you mean to keep the window panes "parallel"? Can't find a supplier of Celotex locally. Any alternatives? Cheers!

"Considering that I am not too concerned about lobby or outside noise; should I still go heavy on the lobby walls?" *Probably not necessary then. You should stay heavy on the lobby's East wall, to avoid bleed from the tracking room thru the lobby. for the door frame(s) at the tracking room, I'd do the reveal (space between frames) with heavy (3-4") OC 705 or Knauf. I'd absorb as much of the lobby walls/ceiling as you can. That way, if you need more bass trapping in the live room you could crack the door an inch or three and use the lobby as a low bass trap. "I also was thinking about putting only ONE door in there (on the Live Room side) and just a frame on the lobby side. It's very tight in there (lobby). I don't want it to be a labyrinth to bring equipment in the Live Room. Do you see a solution?" *see above. "About NOT splaying the window. Not sure I understand. Do you mean to keep the window panes "parallel"? " *That's exactly what I mean. John will probably dis-agree strongly with this, but from 3-4 different sources I've seen, maintaining a wide air gap is at least as important as minimising flutter. If you could widen the gap between your walls at the window area, you could do both, but I wouldn't set up less than a minimum air gap in the window I described above that was less than 8 inches. If you figure that 1 inch in 10 will get you a 6 degree splay, that means that the width of a 2x4 (3.5") would be just right for a 36" glass. If you maintain a minimum 8" distance between glasses at the narrowest point, your wall (including wallboard and channel) would be a total of about 18" thick. (8" space, two 3.5" frames, 2.5" total wallboard, + at least 1/2" channel = 18" - more if you were to use the RSIC-1 clips and "hat" channel instead of RC. "Can't find a supplier of Celotex locally. Any alternatives?" Check for Homosote, Acoustic panel, Sound board, Johns Manville - the stuff you're looking for is compressed paper and will crumble if you try very hard just with your fingers. This link, posted by JRS Godfrey, caused me to talk on the phone to Auralex' Product Specialist for 35 minutes http://www.pac-intl.com/rc_update_12-02.html More on that later... Steve

Hi Steve, All I could find as a replacement for Celotex is a product called "Enermax". Is this any good? The R value is 4.7 and density is 14.5. I am waiting for the NRC. http://www.emcobp.com/en/02/p_02_01_01.aspx?p_prid=22&p_backpage=p_02_01_pr.aspx&p_backid=28 Thanks

Luke, I'm not familiar with Enermax stuff and their web info is pretty sparse - however, it does claim to be a wood product and weighs about HALF what an equivalent size sheet of plywood does, so it should be similar in characteristics. I would think that you could use it the same way, between two layers of wallboard - the main uses of this particular layer are decoupling and "change of pace", plus disrupting the "coincidence dip" of the wall, so it should work fine. Just be sure not to screw it directly to studs if it's a second layer on wood studs - in that case, all but first layers should be strip-glued and fastened with Type G laminating screws parallel to, close to, but NOT INTO the studs. If it's going on either RC or steel studs, screws are fine for successive layers, but make sure to stagger seams and seal them in ANY case.. Steve

Alright Steve, I will try it. It's all I can find in these parts anyhow. I posted more specs in the materials section here: http://johnlsayers.com/phpBB2/viewtopic.php?t=593 Thanks! :)

knightfly wrote:

OK, Luke, here goes - First of all, here are some definitions, then some rules of wall construction as I see them, with reasons if I'm lucky enough to remember them - afterward, I'll tell you what I think you should do where, and why - The most effectuve use of materials can be had using a mass-spring-mass type of wall. The mass is whatever layers of paneling are attached to either side of the frame. A leaf is all the layers of mass on ONE side of the frame, whether it's one layer or four. Steve

Steve, just want to give KUDOS for a very informative post. VERY HELPFUL indeed!

knightfly wrote:

5. Fastening a second layer of wallboard to a first layer, when the first is mounted on WOOD studs, should only be done using 4" wide stripes of slightly thinned joint compound over the stud area, then fastening with Type G laminatings screws approximately 2-3" away from the studs and parallel to them, on approximately 12" centers. This allows the two layers to act as separate panels, which maintains each panel's coincidence dip separate from the other. It also keeps any fasteners from penetrating BOTH layers of wallboard AND entering the WOOD studs. NOT doing this will lose about 9-10 dB of isolation due to flanking through the fasteners and into the studs. Steve

This is the first I've heard of this. I'm going to have to research "laminating screws".

Steve, just want to give KUDOS for a very informative post. VERY HELPFUL indeed!

How did I manage to miss this one, in my careful perusal of the forum???? It is excellent. It should be required reading for all newcomers. In fact, I think it should be a sticky! - Stuart -

Soundman2020 wrote:

It is excellent. It should be required reading for all newcomers. In fact, I think it should be a sticky! - Stuart -

I second the motion!

knightfly wrote:

5. Fastening a second layer of wallboard to a first layer, when the first is mounted on WOOD studs, should only be done using 4" wide stripes of slightly thinned joint compound over the stud area, then fastening with Type G laminatings screws approximately 2-3" away from the studs and parallel to them, on approximately 12" centers. This allows the two layers to act as separate panels, which maintains each panel's coincidence dip separate from the other. It also keeps any fasteners from penetrating BOTH layers of wallboard AND entering the WOOD studs. NOT doing this will lose about 9-10 dB of isolation due to flanking through the fasteners and into the studs.

There's no mention of adhesive for the layers of drywall - will the joint compound act as glue?

There's no mention of adhesive for the layers of drywall - will the joint compound act as glue?

The individual layers of drywall should not be glued together: Only nailed or screwed. Gluing them would reduce the isolation properties. You want them to remain as separate layers, that act independently, not as one single layer. The only "glue" you would ever want to use between layers is Green Glue, which isn't really glue at all. It's a highly effective acoustic damping polymer that is specially designed to go between layers of drywall, but it is not an adhesive. - Stuart -

Are the Type G laminating screws strong enough to hold the sheets together? They're listed as being a temporary solution - if so, what would be a permanent solution? I guess I'm confused as to how you can permanently attach one sheet to another without drilling into a stud or gluing. It seems like over time it wouldn't hold up. What have you experienced?