Effectively decoupled or not?

[xSpace]

"Now given that you've elucidated that RC is indeed a mechanical connection and not a 100% decoupler (am I right in saying that??) that might not be totally accurate (?). Which if so, answers my question, "effectively decoupled or not"."

The difference is the air depth of the two walls and that the RC is part of a wall. It isn't RC specifically, it is the way you are mixing and matching the two.

The fully disconnected wall-air-wall is going to produce better. An RC wall has a reduced ability due to this connection and the reduced air depth.

Will it work? Yes it will work. Is it the weakest link in your wall? I don't know, not having seen the rest of it, but it is the front runner.

But if it is >all< you can do, then it is what it is.

[Soundman2020]

I will build a double wall all round if it is the final recommendation of experience on this here excellent forum. It's just gonna get tight in there....

Not if you build an "inside-out" wall! I just noticed your previous comment on that:

After a bit of thought, I think I know what you mean. That the existing wall should be my interior wall, and I should build the new wall as my exterior... I thought of that - it does gain space, yes. But first, that'll really make a mess of my beautiful (and operational) piano showroom. But more importantly, that wall is already locked to the south and west walls, so existing flanking paths negate the room within a room effect. No?

It looks like I confused you with the issue of "inside out walls", so I just prepared a SketchUp file showing what the term means, and why it would be useful for you. Look at the images below, and it should be clear.

So, what you gain by doing that is that the space between the studs is now exposed to the inside of the room, and you can use that space for your acoustic treatment, such as filling it with rock wool and covering it with acoustic cloth, or building broadband slot resonators on the front of it, or packing it even deeper with rock wool for bass traps, etc. If you build the wall normally, then you STILL have to add the exact same acoustic treatment anyway, except that it now starts about 13 cm further into the room. So in effect you GAIN about 13 cm on each wall, which is roughly 5 inches. If you do that on walls on opposite sides of the room, effectively your room is now ten inches BIGGER than it would have been. You have an extra ten inches of room width that you would NOT have had if you build the wall normally (drywall on the "room" side of the studs). So a room that would have been, for example, 7'4 is wide is now 8'2 wide. Magic!

What you LOSE, or course, is 5 inches of depth in your air cavity, which is the "spring" part of your MSM equation. So you lose some isolation. But you would lose even MORE isolation if you were to just put RC on the outer studs, put the drywall on that, and forget about the second stud frame.


Now, getting back to your drawing: You did not show any drywall on the "outside" studs, but I'm sure there must be some, right? It's just that you forgot to put it in the diagram?

It would be good if you could take the time to build a proper SketchUp model of your room(s) and surrounding areas, so that folks here don't wast time assuming things that just aren't so. It would also make it easier for the experts here to suggest changes to your design that could help you.

You still need two doors in your wall, by the way, regardless of how you build it: if you only plan to have one door in your wall, then it seems rather pointless to bother building an MSM wall in the first place, since your door will be your weakest link. Unless, of course, you plan to build one of Rod's "Superdoors"! That will work.

If this is so, would you agree then that my approach would at least be 50% better than single wall construction with RC,

Putting that in perspective, if it really is 50% better in terms of total power reduction, then that means it would be just 3 dB better.... hardly worth worrying about! It would have to be maybe 10 dB better before I'd think it was worthwhile, and 10 dB is an awful lot more than 50%, if we are talking power!

(Yeah, OK, so that's nit picking, I guess, but the numbers don't lie!)

- Stuart -

[Soundman2020]

Using your own diagram, I just modified it a bit to show what you need to do, to maximize isolation WITHOUT changing the position of the drywall. For both walls, the drywall stays in the exact same location that you show it in your diagram, but I took the liberty of changing the structure, making it into an inside-out wall, to show how you would build that to get the best possible isolation without changing the room size.

While I was doing that, I noticed that you have a line on the lower wall, right behind the RC, labeled "ND Drywall". Is that REALLY a sheet of drywall there? If so, then not only is it pointless to put RC right on top of an existing layer like that (it won't work), but you also have a three-leaf wall, which means that your isolation in the low end of the spectrum would be even worse than for even a standard single-stud wall WITHOUT RC.

It would really help if you would build an accurate and detailed SketchUp model of your rooms, to show exactly what you have.

- Stuart -

[rhythmrhymer]

Awesome. Thanks xSpace and Soundman, I'm (slowly) getting the hang of this...

To clarify one or two things before I nod off, no, there is drywall there now, but we're gonna rip the drywall off that exterior wall first thing tomorrow. That was definitely the idea before installing RC - which we likely won't do; I'm gonna have to process this new info in the morning with fresh caffeine.

My Sketchup skills aren't up to par with you cats. It reads "no drywall", not "nd drywall". I can't seem to tell Sketchup how to make text bigger and that's been bugging me. (maybe I oughtta bite the bullet and buy the pro version)...

And I did sort of half misunderstand the inside out thing for sure, but I've got it now. Lemme review this business in the morning, and thanks for the great advice so far.

GT

[Soundman2020]

there is drywall there now, but we're gonna rip the drywall off that exterior wall first thing tomorrow.

I hope you see this post before you start ripping it off! If you take it off CAREFULLY, then you can save money by re-using it to beef up the inside of your outer leaf. Use some kind of drywall saw, and saw down the edges of the studs, so that you end up with pieces of drywall that are the exact right size to fit in BETWEEN your studs....

That way, you won't have to buy a whole bunch of new drywall, or pay someone to cart away stacks of drywall that could have been re-used, instead of thrown away.

- Stuart -

[xSpace]

Be mindful of electrical wire, plumbing and things that can be behind the wall sheathing. Best to treat it like it is there, rather then it may not be there.

Cut shallow, not deep.

[rhythmrhymer]

Uh oh ... why didn't I think of that? Wait - I did!!

Observe! Nice clean cuts, that'll fit like a glove. Hell of a lot less mess, and I'll be counting a couple of brownbacks in savings.

[rhythmrhymer]

Heck, even the wires are intact...

I've decided also to bite the space bullet and build it right. We're going to run with a double wall construct all round. That back door's bugging me though... bit of a chore but oh well.

I'll do my best to get some better drawings posted soon. Meantime, I'm not in possession of any Green Glue just yet, but I've got lots of acoustical sealant. Is it necessary to secure the two layers with anything more than caulking and a few cleats? (I'll use nails driven in sideways).

GT

[Soundman2020]

I've decided also to bite the space bullet and build it right. We're going to run with a double wall construct all round.

Smart move! I think you won't be sorry about the results! (You might be sorry about the blood, sweat, tears and money, but not about the final outcome.)

That back door's bugging me though... bit of a chore but oh well.

It shouldn't be too bad: Two nice thick solid-core doors, back to back, swinging in opposite directions on hefty hinges, with really good seals all the way around each door, and no handles piercing the wood.... Piece of cake!

Meantime, I'm not in possession of any Green Glue just yet, but I've got lots of acoustical sealant. Is it necessary to secure the two layers with anything more than caulking and a few cleats? (I'll use nails driven in sideways).

I don't think you need Green Glue for "beefing up" the outer leaf. Cleats and generous caulking around the edges should be plenty good enough. I'd save the GG for the inner leaf, where it will be more effective. I think I'd go with something a bit more than just nails for your cleats. Maybe some wooden strips or something. You don't want anything coming loose and spoiling your isolation a couple of years down the line. Or even worse, rattling and vibrating inside your wall...

Don't forget that you need your studs for the inner leaf on 24" centers, not 16". It will drive down the resonant frequency a bit, which is where you want it: as low as possible. IIRC the GG tests were done on 24" centers. The outer leaf already is what it is, but 24" OC for the inner will help.

Glad to see that you rescued your old drywall, too! You'll have more money to spend on GG now!


- Stuart -

[rhythmrhymer]

All righty... right after dropping a couple of queries here, I'm going to pick up the Sketchup ball and get at least closer to delivering some real plans, which I promise are coming... One of my questions - actually they're all kind of bound together like layers of drywall stuck together with GG... is ...

Q1? We took full care to cut the inner layer of drywall (existing wall, to become outer leaf) flush with the studs so that the chunks would simply slip into place to reinforce the outer leaf. Wow. The friggin' studs are so warped and twisted, very few of them fit! What a load of work. Here's the scary part. Once we've fought to get the best fit, some of the gaps are too wide for caulking. It's fairly simple (and much faster) to mud these wider spaces and caulk the rest - but. I've seen mud crack where there's no tape. I'll provide a photo to illustrate, but is this a big no-no? It certainly seems too easy to be true.

Q2? [ a very 'grey area' question ] For new walls (the inner leaf) - wood studs vs. steel studs. Rod prefers to work with wood (I second that) but concedes that light guage (Canadian spelling) studs deliver slightly better results... In Canada, our wood studs are stored outside and subject to rather severe weather changes. I've seen a perfect wood stud transform itself into a banana literally overnight after bringing it indoors. Add to that being rather badly allergic to dust makes me favour (Cdn. spelling again) steel studs - cleanup is also such a breeze. However, the ceilings got to fly, so... does anyone know where there's a thread where there are some stats posted as to what or how to calc the load bearing ability of a given wall? In laymen's terms??? I don't have an engineer in my back pocket, and am a bit afraid of their invoice book for all of 5 minutes of "work". I do promise to get some fairly complete Sketchup drawings up, but I'm awful slow at that. Now, Stu mentioned going for 24" centers to lower the walls fundamental (excellent advice) - but doesn't that also lower it's ability to carry a load? This particular room only needs to float just over 11' of gypsum / GG / gypsum. I had hoped and thought to run with steel studs, and 6" wooden ceiling joists.

I'm gonna post this, and go grab my camera for some photos, and jump on the curveball of Sketchup learning.

[rhythmrhymer]

This photo isn't the widest gap, it's just the clearest photo. (cell phone for a camera). Also when some chunks of drywall don't fit, we pound them into place [with care] , but sometimes they break in places anyway. Some of the corners are smashed, so I've filled a few with mud to see how they dry overnight. I realize gravity might claim a significant portion of it. It's just I've never seen any recommendations for mudding over caulking, but for me it seems a hell of a lot easier. Also, I think my caulking gun may have bit the drywall dust and I need a new one and may be affecting my thinking. I'm getting about 1/8" linear per squeeze... any recs on a good one?

[rhythmrhymer]

Bad gun, or just old and tired?

[rhythmrhymer]

Hope I'm not going overboard, but here's another photo might help. As mentioned, I plan to add another layer on the inside of this outer leaf.

signed
wanna play drums all day without disturbing nobody

[Ro]

no you have to use caulk since it's flexible and therefor can expand or move a bit, leaving no gaps.

[xSpace]

Bad gun, or just old and tired?

I never have much luck with this type of gun, it seems to produce less output per hand pressure applied.

A simple dripless type is my gun of choice.