Wall construction clarifications

I have a few questions based on stuff I've seen here and other places, and from discussions with other engineers, and with some architects and contractors: 1) When constructing walls with staggered studs, do the studs for both sides of the wall share common headers/footers? If so, isn't that an undesired mechanical connection between the walls? 2) I've read in a few places, that sound does not like to travel through different mediums, because of the different resonant properties of materials. I've also read that one side of a wall should be of a different thickness than the other. If that's true, when building a room where I'm more concerned with preventing sound from going out, which side of the wall - the inside or the outside - should be thicker (let's say 2 layers of 5/8"/16mm drywall on one side and 3 layers on the other) ? That's it for now. Thanks in advance for any help on this. -Greg

the walls would transmit less sound if you used different head and sill plates. not positive on the second question, i dont think it matters which side you would put more layers, if it does i would think you'd put more on the noise side

subatom, Thanks for the reply. Separate head and sill plates makes sense. We'll see if anyone else has info on the wall layers deal. -Greg

You're right that the staggered stud wall isn't as good as completely separate frames - you can improve that a few dB by using Resilient Channel under the inner wallboard layers, but it's better if you have the space to use completely separate frames. Light steel studs for those frames will give better isolation (for higher frequencies) than wood studs, because the steel studs flex more. However, for lower frequencies (the ones musicians care about but STC doesn't consider) more rigid is better. As to directionality, there is none. If the sound is attenuated (at a particular frequency) by 60 dB one direction, it will be the same the other direction. However, when you consider acoustics from WITHIN the room, there are choices to be made - virtually ALL rooms can benefit from bass trapping, and some of this can be gotten by using more flexible INNER wall than outer. One way of getting both good sound ISOLATION and good sound would be to use heavier, stiffer construction for the outer leaf and lighter, more flexible construction for the inner leaf. You should NEVER have more than one air space and two centers of mass (leaves) unless you have several feet of space for walls and lots more money than sense. The most efficient walls use this principle. The main things that affect wall performance are mass of leaves, width of air space, type and amount of insulation, stiffness of frame/wall panels, dampening of panels, different inner and outer leaf resonances, and airtightness of construction. Forcing the sound to travel through different types of material also helps, due to refraction loss at layer boundaries. Since the speed of sound in gypsum is about 22,000 feet per second and in wood it's about 12,000 feet per second, putting a layer of particle board or MDF between two layers of gypsum is a good way to further attenuate the sound. Both those materials also have good mass, which doesn't hurt either. For a good music wall, you could do an outer leaf of two 5/8" wallboard separated by a layer of 3/4" (or even 1/2") MDF or particle board, all mounted solid and caulked with acoustic sealant, onto a 2x4 wood frame - putting kraft-faced fiberglas insulation in the stud cavities in the normal fashion will allow the fiberglas to help damp the panel somewhat. Nothing should be put on the other side of that frame. Then at least 2" of 3 pound per cubic foot mineral wool (SAFB is one term) between frames, then a steel stud frame, also insulated with normal spun fiberglas touching the inner wall panels, which should be two layers of 5/8" sheet rock/wallboard - if you mount all inner layers on Resilient Channel that's screwed to the studs, then caulk with acoustic sealant thoroughly, you should have a very good wall for music. What you DON'T want in a wall is for both leaves to have the same resonant frequency - if that happens, that frequency will go right through with little attenuation. That's the point of the dissimilar wall construction in a nutshell. When bedding sill plates for walls, you want just enough absorbent material to fill gaps but not enough to actually create a "spring" mount - this should be accomplished by the Resilient Channel, and only on one leaf of the wall. Hope that helped... Steve

Steve, Thanks, that helps a lot. I'll run over it more...what's the best way to affix the sill plate to the (concrete) floor - glue, those .22-fired fasteners, screws? -Greg

If the concrete is smooth and level, you could just run beads of acoustic sealant near each edge and Ramset (.22 fasteners) the channel to the concrete. If the concrete is rough and uneven, doing that will distort the channel. In that case, I'd use a couple layers of "Sill Seal" (pink foam stuff, designed for interface between sill plates and foundations) and only use fasteners where the high points of the concrete meet the channel. Let the acoustic sealant and Sill Seal fill in the gaps. Just be sure to caulk with more acoustic sealant as each layer is added. If you have to "tip up" an entire wall due to previous construction, you need to improvise - in those cases you can sometimes run the bead of caulk on the bottom plate with the wall horizontal (but positioned where it's going to be) then, tip up the wall and fasten in place... Steve

Thanks, Steve, Do the same rules apply for timber studs? That sounds like the method for the steel studs. I'm more concerned with lessening the low end and, since the overal STC ratings seem to be close, I'm probably gonna go with wood, since they're easier to work with (when you have to cut them - and I will) as well. I also have a question about ceiling height and acoustics/room modes; would you rather I post it in the Acoustics section? -Greg

Yeah, sort of - if you use timber studs over concrete, you need to use Pressure Treated ones for sill plates same as normal house construction, and the Sill Seal will act as both a seal and a vapor barrier. The acoustic sealant should also be used between layers if the floor is uneven, just to make sure there is a hermetic seal. Fastening to the concrete I'd use lag shields, pre-drill holes in the plates large enough for some lee-way, then fill the hole in the plate with acoustic sealant, put a big washer on the lag bolt, and snug it down. This should all be done in one session, so the sealant is fresh when it contacts the surfaces. Steel studs are only 20-25 gauge, and are easily cut with tin snips or aircraft snips BTW... And yeah, if you post acoustics questions in acoustics, they'll be easier to find later. I can point you to a couple of good calculators if you're on PC and have Excel, one simple one I wrote years ago if you have Lotus, and zip/nada if you're on Mac, sorry... Steve

Thanks a lot. I'll post my other question ni the acoustics forum. -Greg

PLEASE READ MY ANNOUNCEMENT AT TOP OF THE FORUM...

Thanks for the heads up. That post about nailing (screwing) vs. glueing (laminating) makes a huge difference, since I expected to be screwing (not nailing) each piece to its predecessor. So I'll just screw the first piece to the studs and glue each one after that. -Greg

Gluing Drywall to Drywall I mounted my first layer of drywall to the studs (with some MLVB in between) with regular drywall screws in a horizontal layout. I plan on mudding and taping the seams & screw heads in the regular fashion on this first drywall layer. Finishing drywall for painting can be a pain, and I need some more mass on my wall - so I am going to install a second layer of drywall in a vertical fashion over the first (this makes it easier to finish, because all of my seams will have the recessed channels). My Question: Would anyone advise or not advise a method in which I use few or no screws to attach my 2nd drywall layer and can you recommend a process to keep the sheets as close together as possible? I have read the other posts in regards to putting holes in drywall & understand that gluing is OK, I really just want my taping & sanding the seams to be as easy as possible. I am an amateur tape & mudder - and can't afford professional help.

Both of you need to hold up a bit; it's sounding like you've been looking at posts that were superceded with the help of people more experienced at this part of construction than me; I'm out of time for the moment, but please do NOT build til we clear this up. Thanks... Steve

This appears to be a resurrected thread but is still relevant to us 'noobs' A few more wall construction clarifications from me... 1) When using a staggered stud wall (say, 2x4's on a 2x6 plate) How can you fit 4inch rockwool batts between your studs when the opposite stud is eating into the space between from the other side? You could compress the batts against that stud but wouldn't this couple the two wall leaves? ie: Inside wall>rockwool>outer stud>outer wall. 2) When attaching inner ceiling joists onto top plates of a staggered stud wall, surely that top plate will transfer all energy from ceiling to outer wall. Is it possible to have a common baseplate, but separate top-plates for where space is limited. How would this best be constructed? 3) Best method for joining the baseplates of walls that are at right angles to each other? 45° mitre with glue n screw? metal brackets? end lap joint? 4) Joining the baseplates of walls that are at right angles (as above) on a 4 sided wall would mean that the baseplates wouldn't need fixing to the floor, as the 4 walls would stand up like a 'box' Is this correct? 5) Fixing wallboards (drywall, mdf etc.) to the studs. Do you attach the boards to the top and soleplates directly, or stop short of those and caulk. Would it make sense to fix the studs slightly overhanging the top/soleplates by a few mm, and applying caulk to those top and bottoms before attaching the wallboard? cheers Deluks

knightfly wrote:

Both of you need to hold up a bit; it's sounding like you've been looking at posts that were superceded with the help of people more experienced at this part of construction than me; I'm out of time for the moment, but please do NOT build til we clear this up. Thanks... Steve

I have days & days before I do this - take your time and thanks again.

Okay here is a slightly more technical clarification that i'm trying to ...... em...... clarify. when i think of sound moving through the air gap in a wall, i think of a spring oscillating longditudinally executing SHM as long as the frequency is constant. so say that the frequency of the spring is imposed to be 400Hz. so the spring is attached to both walls and repeats itself 400 times a second. my question is, the density of the air will be inversely proportional to the speed at which the wave is travelling as speed travels faster in denser walls. speed is proportional to frequency and therefore so is the density of air. modulus of elasticity of a spring is defined to be the tension in the spring when it is stretched twice it's natural length therefore how much this air/walls bend will be proportional to the modulus of elasticity. when it comes down to it i suppose i am wondering what the analogous quantities are in Acoustics to the conventional Mass-Spring-Mass?

What I'm wondering is if you are INTENTIONALLY ignoring my requests to update your profile with a location, or just too absorbed in other things to realize I'd even asked? MSM is really no different in walls, the air gap is the "S". The shorter the air gap, the stiffer the spring and the higher the MSM resonance... Steve On glue/screw, etc, can you guys post a link to the exact page you're trying to follow so I can either edit or eradicate it? Thanks... Steve

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=502 This is the thread I am referring to, I will double check my profile to include location. It was "locked" so I could not post under it. I am new here, so I am still learning where all the appropriate threads are. Pleas bear with me. Also, I am I no way equipped with the physics knowledge of sound waves to get into a discussion like "pantalaimon" person before me - I appreciate the layman's terms & explanations (which you guys do a good job of, don't get me wrong).

i have a terrible attention span. sorry steve. i've even added a link to my music too.

on the subject of glueing drywall to drywall.... check my recent Durabond thread here. excellent stuff.

JR, you are fine - I'm pretty sure you already had location info in your profile. And as much as possible, I prefer the "KISS" method too - any terms/concepts you need clarification on just ask, and I'll re-phrase, etc... pantalaimon, no prob; it's pretty easy to get so involved in this stuff you forget what you started out to do; been having that problem for the last 25 years... :cry: Update - I would pay more attention to the LAST post in that thread than the rest - if you've not already downloaded IR-761, you NEED to. The Canadian group has accomplished an AMAZING feat in the tests that led to that document; over 350 different walls, built and tested in the same facility and the same tests, for REAL performance figures. Another update - gluing two panels together (full face gluing) will make them act as one, with ONE coincidence frequency and different bend/flex characteristics than either panel separately. In almost all cases, this is NOT good for sound attenuation - better that the sound has to get through 2 or more layers (each side) of DIFFERING materials, the harder the sound has to work to pass, the less survives to the other side. In fact, Rod and I have discussed a theory he has, that even IDENTICAL layers of 5/8" drywall acts slightly different for successive layers in a leaf, due to differences in stiffness/bending caused by adding layers - no proof as yet, but feasible. USG's Laminating Screw/glue method scares me enough NOT to recommend it; it's just too "iffy" to get the right glue mix and timing NOT to soften the layers with the adhesive enough to weaken the makeup of the gypsum, and end up WORSENING performance. This is the main reason (aside from panels acting as one instead of independently) that I don't recommend this method. Just using glue (of any kind) isn't an option for multi-layer, unless you laminate them OFF the wall and use fasteners all the way through. Not really practical. Flanking through fasteners is only an issue with SINGLE framed, NON-resilient mounted partitions (like "normal" stud walls with wallboard both sides of ONE frame. Once you resiliently mount (or separately frame) walls, the flanking goes away - what little gets thru the first frame will be absorbed in the wall by proper insulation, and offsetting/sealing of all wiring boxes, etc. Things that matter for best isolation - Air gap - the distance from inner wallboard to OTHER side inner wallboard, NOT the distance between framing members - a double 2x4 stud wall with the frames separated by 1" has an air gap of 8", not 1" - The wider this gap, the lower the resonance of the structure. Ideally at least 1.5 OCTAVES below the lowest frequency you want to stop; 8-10 hZ is a good target, and harder than hell to achieve. Mass - the more the merrier, but weakest link theory applies; it's useless to build foot-thick concrete walls if you have a single 5/8 drywall ceiling, or 3/4" OSB floor. Only two leaves of mass - adding more will improve mid-range frequency TL, but HURT the Lows - these are the hardest to stop, so walls should be optimised for low freq. TL and the rest will take care of itself. Even using a paper backed insulation inside a wall can hurt the bottom end, unless that paper is AGAINST one side's first wallboard layer. Hope this helps clear a few things up, gotta run for now... Steve

So what you are saying, is that if I have 1/2" drywall on my walls right now, you would advise either 5/8" drywall (different thickness) screwed directly to the existing layer or possibly sheets of MDF (different material / different thickness). I did download that 350 page document, but I have yet to print it and read that EPIC!

if you mount all inner layers on Resilient Channel that's screwed to the studs, then caulk with acoustic sealant thoroughly, you should have a very good wall for music.

Hey Steve :shock: I was under the impression it was redundant to use RC on staggered or double walls. Whats the deal here? Same here:

When bedding sill plates for walls, you want just enough absorbent material to fill gaps but not enough to actually create a "spring" mount - this should be accomplished by the Resilient Channel, and only on one leaf of the wall.

fitZ

Fitz, you're correct; either RC OR separate frames, not both. If you saw otherwise, either I mis-spoke or you mis-read. Probably both, damned "old fart" syndrome... :cry: JR, you understand perfectly. You may now officially insert the battery in your screw gun :wink: Steve

Thanks for your help Steve. I am going to the Home Depot to price the 5/8" TH drywall & 1/4" MDF... - no glue & just enough screws to keep it up!!