Wall construction clarifications

pantalaimon · Post by **pantalaimon** » Thu Jun 30, 2005 12:21 am

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?

knightfly · Post by **knightfly** » Thu Jun 30, 2005 1:36 am

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

jgreen77 · Post by **jgreen77** » Thu Jun 30, 2005 1:52 am

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).

pantalaimon · Post by **pantalaimon** » Thu Jun 30, 2005 3:10 am

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

pantalaimon · Post by **pantalaimon** » Thu Jun 30, 2005 3:14 am

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

excellent stuff.

knightfly · Post by **knightfly** » Thu Jun 30, 2005 11:21 am

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...

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

jgreen77 · Post by **jgreen77** » Fri Jul 01, 2005 12:45 am

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!

cadesignr · Post by **cadesignr** » Sat Jul 02, 2005 12:41 am

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

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

knightfly · Post by **knightfly** » Sat Jul 02, 2005 3:14 am

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...

JR, you understand perfectly. You may now officially insert the battery in your screw gun

Steve

jgreen77 · Post by **jgreen77** » Sat Jul 02, 2005 3:21 am

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!!

jgreen77 · Post by **jgreen77** » Sat Jul 02, 2005 3:31 am

knightfly wrote: JR, you understand perfectly. You may now officially insert the battery in your screw gun Steve

Can I ask why you guys call me "JR" - it is actually JP... (just so you know)

knightfly · Post by **knightfly** » Sat Jul 02, 2005 3:34 am

JP it is; glasses must be even closer to needing an upgrade than I thought, sorry; Steve

rod gervais · Post by **rod gervais** » Sat Jul 02, 2005 3:42 am

knightfly wrote: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...

Steve -

He was refering to (I f am not going crazy) staggered stud walls - single plates with staggered studs.

If i am correct - then there should be a benefit to the use of RC.

Seeing as each stud directly connects to the opposite wall through the top and bottom plates- there should be a benefit if you decouple the drywall from those studs (and effectively from the plates).

With a true seperate wall frame there is no advantage.

Rod

knightfly · Post by **knightfly** » Sat Jul 02, 2005 4:36 am

Thanks Rod, makes sense to me too; I don't recall IR-761 showing such a combination though, but do agree that above m-a-m regions RC might help a staggered stud wall.

At the low end, only mass and air gap seem to matter much, so I'm not sure whether we would ever HEAR a difference if the wall were improved a few dB at 500 hZ ??!? Steve

rod gervais · Post by **rod gervais** » Sat Jul 02, 2005 9:15 am

knightfly wrote: At the low end, only mass and air gap seem to matter much

Steve,

I need to think about that for a bit - I don't know if I agree or not yet.

For it to be the case that would suggest that decoupling the 2 faces of the wall would not matter as much as I thought it did at low frequencies.

I say this because we can handle all the high frequency we want quite easily with simple frame walls, but the low frequencies are a problem.

So with a staggered stud wall we improve low frequencies attenuation in the 30 - 300Hz range - but still have those coupled plates....... would we gain anything from the installation of RC to effectively decouple at least one face from the plates in those ranges?

Me dunno - me have ta think about it............

However - you would probably lose on the face of it in the 10 - 20 Hz range where stiffness rules.

Let me burn it for a while and see what comes out...........

Rod

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Wall construction clarifications

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