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PostPosted: Tue Aug 18, 2015 7:10 am 
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Location: Fair Oaks, CA (Sacramento)
I'm a music teacher converting my garage into the recording studio for my live broadcasts / private lessons / recordings. The plans are on the attached PDF (Walls are 8 feet high)
Attachment:
Conversion Plans.pdf
While I'm recording I'm usually plugged in, so I don't care about protecting my neighbors from my sound, but I don't want to be finally nailing a recording on take #43 and have a Harley roll by outside my studio and wreck the whole thing. I'm just a music teacher with 5 kids 5 and under, so I don't have money to throw away, but this is how I make my living so I need to spend whatever is necessary to construct it well.

I have the 2x4 walls up but I'm second-guessing the interior layering advice I got now that I've read several posts here. Before I go spending a ton on materials that won't work, I was hoping to get some advice and for someone to tell me my plans are massive overkill and a ridiculous waste of money.

I'm on a 4" concrete slab that was poured on top of the existing garage concrete floor.

Here's what is built so far:
Stucco > 2x4 > 3/4" gap > 2x4

Now - here's what I was thinking of adding:
R19 insulation > Genie (or some other brand) Clips > RC Channel > Padded Tape > Quiet Rock > Acoustic glue around all Quiet Rock seams > Green Glue > 5/8 Drywall

My questions:
*What of this advice I have received is terrible and what would you do if it were your garage conversion?
*I also have to put in a single window, about 5' x 4'. What's the best way to go about this?
*If I need RC Channel, what size/type?
*Do I need to put putty pads around all the outlets?
*What is the difference between the "Sound Board" everyone is talking about and the Quiet Rock product they sell at Lowe's?
*I plan to put foam wedge squares on the inside to control sound during recordings. This doesn't do anything to help keep noise out, right?

Thanks so much for your time!


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PostPosted: Wed Aug 19, 2015 1:47 am 
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Hi there " JaminColler", and Welcome! :)

Quote:
The plans are on the attached PDF
How much of that already exists, and which parts are the new construction that you will be adding? Maybe you could color-code it: for example, make all of the existing walls red, and all of the new walls green, or something like that.

Quote:
Walls are 8 feet high
Is that measured from the top of the new slab to the bottom of the ceiling joists? Or was that from the top of the original slab to the joists? Also, tell us something about the roof and how that is built. Sound is three-dimensional - it doesn't just go out sideways through your walls: it also goes up through your roof, so the roof is a major part of your isolation. It's not just Harleys in the street that you need to be worried about, but also helicopters and other aircraft flying over, not to mention wind, rain, hail and thunder. And about a million other things...

Quote:
I have the 2x4 walls up
You mean the external garage walls? Or did you already build some of the framing for the inner-leaf? If so, that's a bit confusing because your PDF plan doesn't show any inner-leaf... Or rather, it doesn't show any usable, correctly designed inner-leaf...

It would really help to have photos of where you are right now!

Quote:
I'm on a 4" concrete slab that was poured on top of the existing garage concrete floor.
I'm just wondering... but... WHY? If you already had a good concrete slab down there, then why was another one poured on top? Was there something wrong with the original slab? Was it cracked, pitted, broken, or some other problem like that?

Also, how was that new slab poured? Is it floated on proper slab isolators, or just poured directly on top of the original slab? If the latter, how did you bond the new concrete to to the old? Did you put any steel mesh in the new concrete?

Lots of important details are missing here...

Quote:
Stucco > 2x4 > 3/4" gap > 2x4
What is the stucco attached to? It can't just be floating in mid air! :) It must have some type of base under it. Hopefully it isn't just wire mesh. And there is no insulation in that cavity at all? No other drywall, OSB, plywood, MDF, etc? Once again, photos would help to understand this better.

Quote:
R19 insulation > Genie (or some other brand) Clips > RC Channel > Padded Tape > Quiet Rock > Acoustic glue around all Quiet Rock seams > Green Glue > 5/8 Drywall
Quite a few comments here: "R19" is a thermal rating, not an acoustic rating. What you need for MSM insulation is either mineral wool insulation that has a density of about 50 kg/m3, or fiberglass insulation that has a density of about 30 kg/m3. "R19" doesn't say anything at all about the acoustic properties of the insulation. Indeed, it is possible to have thermal insulation with an R19 value that has zero acoustic properties at all! For example, Styrofoam would fit that: no use at all in acoustics, but not bad for thermal purposes....

Next, if your inner-leaf frame is properly decoupled from your outer leaf (does not touch it anywhere), then you do NOT need Genie clips or Resilient Channel. And regardless of whether or not the frame is decoupled, you NEVER need both clips and RC! The purpose of Genie clips (or RSIC clips, or whatever) plus hat channel is to decouple the drywall from the studs from the studs. The purpose of RC (Resilient Channel) is to decouple the drywall from the studs. So you do not need both! If your drywall is decoupled, then it is decoupled. Doing so twice is not only a waste of time and money, it could possibly defeat the entire plan, since you would have two resilient decoupling systems, each tuned to it's own frequency, thus creating something similar to a 3-leaf system: You might well damage your isolation like that. And if your frame is also decoupled, then you are decoupling three times, potentially creating the equivalent of a 4-leaf system!!!

If your framing is done correctly, as a fully-decoupled 2-leaf MSM "room in room" system, then you do not need clips (which are expensive and slow to install), nor do you need hat channel, nor do you need RC (which is expensive). And with any/all of those, you do not need padded tape: that's not what it is meant for, and it won't do what you are expecting.

"Quiet Rock": Forget it. Expensive, for no significant benefit. Isolation is achieved with mass. Sound waves cannot read the price tag on your mass, and they don't care anyway. They simply respond to the amount of mass they find in front of them. So get the cheapest mass that will do the job. Plain old fire-rated 16mm (5/8") drywall is usually the stuff that fits the bill. But do look around: sometimes things like OSB, plywood, MDF, or fiber-cement board can be found less expensive than drywall ... pound for pound, of course! If you find fiber-cement board, then it only needs to be about half as thick to get the same mass as drywall, because fiber-cement is about twice as dense. If you find plywood at a good price, then it needs to be about 50% thicker, since it is about one third less dense. Etc. So check the density of whatever materials are available in your area, and do the math to see which one is least expensive. But my bet is that it turn out to be 5/8" fire-rated drywall.

"Acoustic glue": Not sure what you mean by that. Did you have any specific product in mind? Or did you mean acoustic caulk? Caulk isn't glue.

"Green Glue"_ It is great stuff, and does work as advertised, but do you need it? What is your goal for isolation, in decibels?

Quote:
*What of this advice I have received is terrible and what would you do if it were your garage conversion?
First, before doing anything, I'd take a real close look all around the existing garage structure, and make sure that I have only one sing leaf everywhere, in all directions around me, and that said leaf is very well sealed: no cracks, gaps, holes, penetrations, etc. Then I'd makes sure that the entire outer-leaf has enough mass on it, and that it is consistent all around. In other words, that I don't have one wall that is concrete blocks but another that is just siding on studs, or a a roof that is just shingles on battens over the trusses. If I found anything like that, I'd figure out a way of adding mass to the offending parts, to get it all up to the correct amount. The measurement here is "surface density", which just means "how much does each square foot of the wall weigh?" If it isn't enough, then I'd add more until it is, once again making sure to seal the new mass very well, so the entire outer leaf becomes air-tight. I'd calculate the correct surface density by using the correct equations, or by using empirical rules.

Then I would spend several weeks designing the inner-leaf in great detail, and double-checking to make sure that it does not touch the outer leaf at any point. Not even a single nail. I'd also design my HVAC system and my electrical system at the same time, to ensure that they both do not damage my isolation shell, and provide the correct amount of isolation.

Then I'd build my inner-leaf framing, put insulation in the cavities, put my drywall (or other sheathing) on, seal it, maybe paint it.

Then I'd measure the acoustic response of the room using the simple (and free!) REW software, analyze the results, and design and build suitable treatment for each of the problematic locations and frequency ranges in the room.

Quote:
*I also have to put in a single window, about 5' x 4'. What's the best way to go about this?
You'll need two panes of glass for that: once goes in the outer leaf, the other goes in the inner leaf. The type of glass you need is "laminated glass", and hopefully you can find the good stuff that has a proper acoustic interlayer in the middle, but if not the ordinary laminated glass with an ordinary interlayer is fine. The thickness of the glass is determined by the surface density of the walls: it needs to be the same, or maybe a bit higher. Since glass is about three times as dense as drywall, it only needs to be about one third the thickness. So for example, if you figure out that you need two layers of 5/8" drywall to get the density you need for the amount of isolation you need at the frequencies you need, then your glass would need to be about 3/8".

Obviously, you cannot have operable windows (ones that open) since that would destroy your isolation completely, so your frames can just be very simple wooden frames, hefty enough to handle the weight of the glass, and well sealed to the wall framing. The glass needs to be sealed into the frame as well, of course, and I like to do that with rubber all around the edges of the glass, on all sides, held in place with wood trim that is thick enough to keep the glass held firmly in position. There are other methods too, such as using setting blocks, glaziers tape and caulk.

Quote:
*If I need RC Channel, what size/type?
You don't need it if your framing is done right, but if you did then you should really only go with the original RC-1... if you can find it! (sometimes called "RC-Deluxe" these days)! There are other things out there that call themselves "Resilient channel", but in numerous tests and studies, RC-1 always comes out ahead as compared to others: Curiously, it wasn't even designed with acoustics in mind originally! It was designed to prevent drywall from cracking. But it turned out to be the best darned design there is for resilient channels. However, you do not need it, if you build your framing correctly.

Quote:
Do I need to put putty pads around all the outlets?
Not if you do your electrical system correctly!! Because if you do your electrical correctly, there will not be any holes in the drywall, anywhere in your room, so you won't need to waste time and money on trying to seal them up. You would only need putty pads on the outlets, switches and light fittings if you decide to not do your electrical system the best way. The "best way" is with surface-mount systems, such as these:

http://www.calcentron.com/Pages/fram-tr ... aceway.htm
http://www.export.legrand.com/EN/dlp-wa ... ng_95.html

Quote:
*What is the difference between the "Sound Board" everyone is talking about and the Quiet Rock product they sell at Lowe's?
Same animal, different name. And you don't need either of those, because sound waves cannot read price tags... :) Just use plain old ordinary 5/8" fire-rated drywall.

Quote:
*I plan to put foam wedge squares on the inside to control sound during recordings. This doesn't do anything to help keep noise out, right?
Correct. It does nothing at all to keep sound out, or in. That is treatment, not isolation. And the typical foam wedges sold cheap on the internet won't do much to improve the interior acoustics, either! It would be much better to design proper treatment as part of the room construction itself, then add the additional treatment that will be needed, after you test the room. Some types of treatment absolutely will be needed, without any doubt, so that can be designed in as part of the basic construction, but fully predicting the response is not all that accurate, since small difference in construction materials, dimensions, techniques, etc. can have large differences in response. So it's always better to test the room when it is finished, see what problems it has, then design specific treatment to deal with those issues, and put that treatment in the most effective places. For example, if it turns out that you have a flutter echo problem associated with the two side walls, the obviously the treatment for that problem has to go on one (or both) of the side walls: putting it on the ceiling or the front wall wouldn't be very useful! Etc.

So I'd suggest that before you do anything else, please post some more details about how the garage is actually built, where you are right now with additional construction, and also some photos of how things look right now.

- Stuart -

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PostPosted: Wed Aug 19, 2015 2:38 am 
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Location: Fair Oaks, CA (Sacramento)
Great stuff! Thanks for the quick and detailed response!

*What existed was the garage box. All interior work is additional. The interior walls are up, including a double wall with a 3/4" gap inside the "Music Studio" portion - the only part I care about soundproofing. The original stucco is just on wire :( The new stucco (where the garage door used to be and the window I took out on the right) is on 3/8 plywood.

*This picture
Attachment:
2015-08-17 12.12.52.jpg
is from the left side of the plans facing the intersection of the bathroom and the studio.

*This picture
Attachment:
2015-08-17 12.13.10.jpg
is from inside the studio facing the bathroom. The plywood represents the window and the second leaf between the studio and the utility room is not up yet. The rest of the double walls are.

*8ft from the top of the new concrete to the bottom of the joists.

*The original concrete was a garage floor, so it sloped by about 4 1/2" toward the street. I put down plastic and wire for the new pour. Did not bond it to the original.

*Thanks for the insulation advice. I was planning on R19 just because it would be thick enough to fill the void of 2, two-by-four walls with a 3/4" gap. I'll check out my other options, but I am supposed to fill that entire void with insulation, right?

*My inner leaf does not directly touch the outer leaf. However, my inspector will not pass it unless it's tied into the same joists at the top. How much does this matter?

*You're right - I meant acoustic sealant, not glue. I was planning on the Green Glue brand of sealant.

*How do I put in the window with out connecting the sill to both leaves? Won't the insulation and/or dust & dirt above slip down on and between the glass panes?

So with the new input I'm thinking to eliminate the clips and RC channel and just go 5/8 sheet rock Green Glue-d to another layer of 5/8 sheet rock. I've been told to stagger the sheet rock seams so the second layer seams don't line up with the first layer, and to use acoustic sealant around only the second layer. Also I should not let the sheet rock on the walls directly touch each other, the ceiling, or the ground, but have a small bead of sealant at all those points. Does that sound right?

Thanks again! Better and quicker response than I could have hoped for


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PostPosted: Thu Aug 20, 2015 5:05 am 
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Quote:
The interior walls are up,
Did you seal the outer-leaf walls air-tight before you did that? It's MUCH easier to do that before framing the inner-leaf.

Quote:
*This picture...
Great! Makes it a lot easier to see what is going on.

Quote:
*The original concrete was a garage floor, so it sloped by about 4 1/2" toward the street. I put down plastic and wire for the new pour. Did not bond it to the original.
Did that pass code in California?? I'm surprised. I though they were very strict on seismic regulations, and bonding or anchoring all new concrete to old concrete. Maybe I'm wrong about that...

Quote:
but I am supposed to fill that entire void with insulation, right?
Correct. The closer to 100% full you make it, the better your isolation is. You need at least 4", but filling it completely is best. However, I have heard about possible fire code violations when completely filling the cavity: check with your inspector before you do that, to make sure it is OK. Did you already pass your framing inspection?


Quote:
*My inner leaf does not directly touch the outer leaf
Yes it does:


The top plate of this inner-leaf wall is directly touching the outer-leaf ceiling joist:
Attachment:
JaminColler-Flanking-1.jpg



There s a major flanking path from the inner-leaf to the outer-leaf right here. That alone totally destroys your isolation:
Attachment:
JaminColler-Flanking-2.jpg



The top plate of this inner-leaf wall is directly touching the outer-leaf ceiling joist:
Attachment:
JaminColler-Flanking-3.jpg


Quote:
However, my inspector will not pass it unless it's tied into the same joists at the top. How much does this matter?
Why, ask him which regulation prevents that. But wait until you have your inner-leaf ceiling framing in place first: the way it is right now, I can understand why he would not allow it. There is nothing to prevent the tops of your walls "flopping around" right now, but once he sees your inner-leaf ceiling joists in place, it will be clear where the support comes from. Did you explain to him that the ceiling framing will do that job? Maybe he didn't understand that there's going to be a ceiling there.

Quote:
*You're right - I meant acoustic sealant, not glue. I was planning on the Green Glue brand of sealant.
:thu:

Quote:
*How do I put in the window with out connecting the sill to both leaves? Won't the insulation and/or dust & dirt above slip down on and between the glass panes?
Two sills, one in each leaf. Two frames, one in each leaf. Two panes, one in each leaf. You will need to put silica gel (or some other desiccant) in the gap between the panes, to take up the moisture trapped between, and most people cover the gap with a piece of OC-703 wrapped in black cloth, pressed into the gap. I prefer to put a couple of dabs of glue on one side, to make sure it stays in place. There's an equation for calculating how much desiccant you need, but I don't recall it off hand.

Quote:
So with the new input I'm thinking to eliminate the clips and RC channel and just go 5/8 sheet rock
Right! But do make sure that you fix your framing problems first!

Quote:
5/8 sheet rock Green Glue-d to another layer of 5/8 sheet rock.
careful there... Despite the name, Green Glue is not glue, and you cannot use it to glue to panels of drywall together. Each panel still needs to be nailed in place, as per code. Use half of the nailing schedule on the first layer, and a full schedule on the second layer, but offset from the first layer so you don't get nails hitting nails. And the nails for the second layer have to be 5/8" longer than the ones on the first layer, of course! Rather important.... You can't glue the second layer of drywall to the first, and even if you could, Green Glue is not adhesive. It is a viscous-elastic polymer whose only function n life is to act as a constrained layer damping compound between the two layers, with all the acoustic benefits that that provides. But it isn't glue: it never hardens.

Quote:
I've been told to stagger the sheet rock seams so the second layer seams don't line up with the first layer,
Correct. Some people change the orientation of the sheets for the second layer. Eg, put them vertically on the first layer, and horizontally on the next layer. Or just stagger them. Both methods work.

Quote:
and to use acoustic sealant around only the second layer.
Nope. Do it on every layer. If not, how else will you get a good edge seal? Speaking of which, I noticed that there's no trace of caulk under your new sole plates: Are you SURE you caulked those properly when you put them down? Three beads of caulk along the full length? If so, it should have squeezed out from underneath, at least in some places, as you anchored the sole plates to the slab. But I don't see that.... That's a problem. You absolutely need air-tight seals on all your framing, and all your drywall.

Speaking of "anchoring your sole plates", I don't see that on the photos either: Have you done that yet?

Quote:
Also I should not let the sheet rock on the walls directly touch each other, the ceiling, or the ground, but have a small bead of sealant at all those points. Does that sound right?
Correct. For each layer, rest the drywall on shims before you nail it, then after it is nailed pull the shims out, put the backer rod in (push it in deep), then caulk.

- Stuart -


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PostPosted: Thu Aug 20, 2015 7:24 am 
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Location: Fair Oaks, CA (Sacramento)
Thanks again!

*Sealing the outer leaf - I assume you mean more than just making sure the air vents are covered in stucco. I'm realizing I'm really ignorant on the sealing of framing. Haven't read about that before.

*The concrete passed because I put in a stem wall first that was tied into the original footings.

*I now know the "R" factor in insulation doesn't mean anything for the sound quality, but I can't get insulation guys to talk about any other rating. The wool stuff I've seen is crazy expensive! Do you think an 8" 'R30' high density batting would do reasonably well in reducing noise or is that even an ineligible question?

*Talk to me about an inner-leaf ceiling. When I read your response, it seemed obvious, but I hadn't put any thought into it. How can I achieve an inner-leaf ceiling at this point?

*The window thing makes sense, but now I'm worried about the door jamb. What do you usually do there?

*Point taken on the Green (non)Glue.

*I'm totally screwed on the floor plates. I didn't caulk those and haven't read anything about that. Makes sense, though. Anything I can do at this point to help salvage that?

*I was told that I could put RC channel and more sheet rock on after initial sheet rock layers if I still had issues, but it seems like that would create a 3rd leaf, which seems to be frowned upon...?

So I guess I'm kind of salvaging my work at this point, making the best of a situation that required more prep than I knew I needed. I really appreciate the input!


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PostPosted: Thu Aug 20, 2015 5:01 pm 
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Quote:
*Sealing the outer leaf - I assume you mean more than just making sure the air vents are covered in stucco.
Look for gaps, cracks, holes, etc. Plug them with acoustic caulk. If it even looks like a crack, seal it. And even if you are 100% certain that it is not a crack, seal it anyway, just in case... So inspect that outer leaf as well as you can, from both sides, and do your best to get it sealed. If there is a flaky area (poor quality stucco, damaged, etc.), then you can stabilize that. Check at your building supplies store, and ask what products they have for that. Alternatively, dig out the bad section and patch it with new Stucco, but do use some type of bonding agent mixed into the stucco tp make sure the new stuff "sticks" to the old stuff.

Quote:
I'm realizing I'm really ignorant on the sealing of framing. Haven't read about that before.
Every place that your framing butts up against something else (floor, other framing in another wall, doorway, etc.) and there is a possibility that there could be a gap where air could leak through, seal that with acoustic caulk. The basic thing to keep in mind here is this: If air can get through, then so can sound.

Here's a scary graph for you. It shows how much REDUCTION in isolation you get from having a tiny air gap in a wall. It's not very clear (I couldn't find the original) but if you look closely, you can make out the text.

Attachment:
effect-of-gaps-on-TL-GOOD!!!.jpg


To interpret it for you: The X axis shows how much isolation you would have got if there were no gaps, and the Y axis shows how much you actually get, for each of the curves shown. For example, look at the second curve down, which is for "0.02% open". In other words, if the gap represents 1/5000th of the surface area of the wall. So say you had a wall 10 feet long by 8 feet high, for a surface area of 10x8= 80 square feet. An "open area" of 0.02% means that there is 0.016 square feet of "gap", which is a bit less than 2.5 square inches. That would be the same as a tiny crack, just 1/64th of an inch high, under half of your wall. If you have that situation, you'd think that such a miniscule crack would not do anything to your isolation. But the graph says otherwise: It says that if your wall was designed for 50 dB of isolation, that tiny little insignificant crack will bring your isolation down to about 37 db. So you would lose a whopping 13 dB of isolation!!!! All from a tiny little crack. To put that in perspective, an average house wall gives you about 30 dB of isolation, and a well isolated studio can indeed give you about 50 dB of isolation. But a few cracks in the seals would bring that down to not much better than a normal wall....

That's scary!

Did I mention that sealing cracks and gaps is important? :)

Quote:
*The concrete passed because I put in a stem wall first that was tied into the original footings.
Ahhh! Ok, that makes sense.

Quote:
*I now know the "R" factor in insulation doesn't mean anything for the sound quality, but I can't get insulation guys to talk about any other rating.
Then you are probably talking to the wrong people, or you are in the wrong store! :) Seriously, note the brand and product codes of all the available insulation that you see at the store, then go on-line to the manufacturers web sites and find the complete specs yourself. Some place in there it should tell you what the density is, which will be measured on either kilograms per cubic meter (kg/m3) or pounds per cubic foot (lb/ft3, or more commonly PCF). If you go with mineral wool, you need a density of about 50 kg/m3, which is roughly 3 PCF. If you go with fiberglass insulation, then you need a density of about 30 kg/m3, which is roughly 2 PCF.

And if you want the best stuff there is, just ask them for "Owens Corning OC-703". A bit more expensive, but its really good stuff. It is used a lot in studios, because it is very, very effective, and also easy to work with: you can cut it with a bread knife, and the itchy-scratchy fiber problem is much less severe.

Quote:
*Talk to me about an inner-leaf ceiling. When I read your response, it seemed obvious, but I hadn't put any thought into it. How can I achieve an inner-leaf ceiling at this point?
Assuming that your new framing does NOT touch the existing ceiling joists, then you can use metal joist hangers, like this:

Attachment:
metal-joist-hanger-01.jpg


Attachment:
metal-joist-hanger-05.jpg


Attachment:
metal-joist-hanger-04-SHR.jpg


You get the idea: You nail part of it to the top plate and studs, then slip the joist in place and nail that too.

But check with your inspector to make sure that would be allowed, and which type he would recommend. There should not be a problem with doing it that way.

Worst case, you'd have to go to "plan B": Cut your studs down a bit, put new double top plates on them, then rest the joists on top of those in the usual manner.

Lots of ways to skin a cat!

Quote:
*The window thing makes sense, but now I'm worried about the door jamb. What do you usually do there?
You need full perimeter seals on your doors, meaning a seal that runs all the way around the edge: top, bottom (threshold) and both sides. You need two complete and independent seals on each door. The very best ones are made by a company called "Zero International" (Google them, then check out the acoustic door seals in their website catalog, especially the threshold seals). There are other options too, but they have the best stuff (but not cheap...)

Then you follow the same process here: to cover up the gap between the two door frames, wrap a piece of OC-703 in black fabric, and press-fit it into the gap. Or put a few dabs of glue on it, on only one of the frames, and glue it like that.

Quote:
*I'm totally screwed on the floor plates. I didn't caulk those and haven't read anything about that. Makes sense, though. Anything I can do at this point to help salvage that?
All is not lost! Yes you can: clean along both edges of each sole plate where it meets the floor, vary carefully, to get all the dust, dirt, grime, sawdust, etc out, so you have perfectly clean concrete and perfectly clean wood, and caulk along both edges of each plate. Make sure to force the caulk as deeply as you can into the crack, and use a finishing tool to force it in even deeper and clean up the edge. ( https://www.google.cl/search?q=bathroom ... 39&bih=459 )

It's not as perfect as running the beads under the sole plates as you lay them, but if you are careful it will work just fine. And also take extra care when you do under each layer of drywall: Backer rod, carefully pushed into the gap to the right depth, then caulk carefully.

Quote:
*I was told that I could put RC channel and more sheet rock on after initial sheet rock layers if I still had issues,
:shock: :!: :ahh: :!: :roll: :cen: .... Ummmm.... well, whoever told you that is one person you can safely ignore for advice on acoustic isolation! He doesn't have much of a clue.... Here's what would happen if you did that:

Attachment:
RC-over-base-layer-graph.jpg

The graph shows exactly the same situation, but with the base layer being plywood, rather than drywall. So the details aren't accurate, but the concept is.

The blue line shows how much isolation you would get if you did it properly, without the base layer (plywood, in this case) and with the RC mounted on the studs correctly, then a layer of drywall on the RC. You would get an STC rating of 61 like that, which is really good. But if you were to ADD the layer of plywood in there, between the RC and the studs (which is what your friend is telling you to do), then you would get the red line result: your isolation would drop to STC-42, with most of that loss being in the low frequency end of the spectrum. The green line shows what the isolation would be if you didn't have the RC and just nailed your drywall directly to the studs: STC-40, which is pretty much the same as the "sandwich" case. In fact, the low frequency isolation would actually be slightly BETTER for that final case (no RC)!!!

So even though the "sandwich" case has a lot more mass on the wall (because of the layer of plywood in there), the isolation is way worse! Not intuitive at all, and not what most people would expect. The reason is very simple: as you guess, that creates a 3-leaf system, but even worse, the air gap for the third leaf is very, very small, meaning that the resonant frequency is very high. Since isolation only happens above the resonant frequency, you get much worse overall isolation.

Resonance is a powerful effect; it can destroy your isolation when it happens in the wrong places, or it can reinforce your isolation if you design the wall so that you are using to to your advantage.

But anyway, the guy who gave you that advice basically doesn't understand acoustic isolation at all, least of all the effects of resonance, so don't take any advice he gives you...

Quote:
but it seems like that would create a 3rd leaf, which seems to be frowned upon...?
Yup! Frowned upon for very good reason. Sometimes you have no choice, and you have to have a 3-leaf system, and in that case there are work-arounds that you can do to compensate for the problems. But it should be avoided, where possible.

Quote:
So I guess I'm kind of salvaging my work at this point, making the best of a situation that required more prep than I knew I needed.
Fortunately, you are not too far advanced! There are still relatively painless ways of fixing what is there and getting you back on track. Some folks who come to the forum are much further advanced than you, and have a much sadder situation, where they basically have to rip out everything they did, then re-do it the right way.

If you have a few minutes, take the time to read through the thread below. It's one of my favorites, as it shows how a really bad situation can occur, and it also shows how that can be fixed when the builder is determined to get it right, and does whatever is needed: It has a sad start, but a happy ending, and I'm sure it will make you feel better after you reed what poor old Beeros had to go through: Pull up a chair, grab some pop-corn, and prepare for a fun ride...

viewtopic.php?f=2&t=17363

As you can see, you are in a much better situation than he was when he first arrived, so there is most certainly hope for you!

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I really appreciate the input!
:thu: That's what this place is here for!


- Stuart -


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PostPosted: Sun Aug 23, 2015 12:19 am 
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Location: Fair Oaks, CA (Sacramento)
Wow. Fantastic! Just a couple last questions.

Quote:
Look for gaps, cracks, holes, etc. Plug them with acoustic caulk

This is the acoustic sealant we've talked about, right? Just want to be sure there's not another "caulk" product out there I should look for.

Quote:
Assuming that your new framing does NOT touch the existing ceiling joists, then you can use metal joist hangers

I'm definitely going to try this. Talking with my framer in the next couple days about that. If I can't get a second leaf in the ceiling, do it make sense then to use RC channel on only the ceiling to create a 2nd leaf up there but not use RC channel on the walls? Or put perhaps a better way, what's my second best alternative if my inner leaf does touch the existing joists?

Thanks again! Sharing this site everywhere I can. Wish I had found it sooner. My musician friends definitely need to come here!


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PostPosted: Sun Aug 23, 2015 7:00 am 
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what's my second best alternative if my inner leaf does touch the existing joists?
Disconnect it so that it no longer touches! That's actually the first, second, third, fourth and fifth best alternative... all other alternatives below that aren't even worth thinking about....

In other words, that's the single most important, imperative, absolute "must do" item on your list. If the inner leaf is directly connected to the outer leaf, then you might just as well chop down the inner leaf and use it for firewood, because it isn't going to be a lot of use for isolation...

The entire concept of "room in a room" construction is exactly that: you have an outer room, and you have an inner room which is absolutely, totally, fully and completely disconnected from the outer room. There can be NO mechanical connections at all between the two leaves. Not even a single nail or screw. That's why we talk about the two leaves being "decoupled", acoustically. Which leads to the matter of "flanking paths": a flanking path is just any type of mechanical connection between the inner leaf and outer leaf, that transmits vibration between them. Anything at all that is reasonably solid is a "flanking path", even if it is very small. A nail can transmit a huge amount of energy across the gap, but if you have your joists on one leaf directed contacting your joists on the other leaf... well, it's hard do think of a better way of transmitting energy between them!

That's why I highlighted those three glaring issues in your current framing. You have multiple flanking paths all over the place. Until you fix those, it makes no difference at all what you do with the rest of the building, or the materials you use. The flanking paths provide a direct "short circuit" between the leaves, allowing sound to bypass everyghing else you do.

That's the reason for my comment: "Assuming that your new framing does NOT touch the existing ...." In other words, if it DOES touch, then don't bother doing that, because nothing you do will make much difference. If your inner leaf is connected to your outer leaf, then the maximum isolation you will ever get is limited by how much energy passes along that link. In real-world terms, that would be somewhere in the region of 30-somthing decibels, which is only slightly better than a typical house wall.

Quote:
This is the acoustic sealant we've talked about, right? Just want to be sure there's not another "caulk" product out there I should look for.
Acoustic caulk is the best, for sure, but if you are on a tight budget there are less expensive alternatives. Savings can be significant, because you are going to need many cases of caulk. If your budget is good, then just go with Green Glue acoustic sealant (not to be confused with Green Glue compound.... not the same stuff at all...

Quote:
Talking with my framer in the next couple days about that. If I can't get a second leaf in the ceiling, do it make sense then to use RC channel on only the ceiling to create a 2nd leaf up there but not use RC channel on the walls?
You could do that, yes. Once again, assuming that there are no mechanical connections between leaves. However, if you did that then you are not anchoring the top plates of your walls to anything, so you'd be back to the problem with the inspector: he's not going to pass your framing if the tops of the walls are not attached to something. Either they have to be attached to each other though the ceiling framing, or they have to be attached to the outer leaf ( :shock: :roll: ), or they have to be restrained with suitable sway braces (expensive...).

Quote:
Thanks again! Sharing this site everywhere I can. Wish I had found it sooner. My musician friends definitely need to come here!
:thu: Yes they do! Way too many musicians understand music but don't understand sound, and even less acoustics. I've always wondered how much better their gigs would be if they DID understand sound and acoustics, and used the acoustics of the venues where they play to their advantage, instead of allowing the acoustics to make them sound bad, as is often the case.... :) When I run live sound for bands, I try to explain some of the basics to them in a few minutes, so they understand why I do the things that I do. Those who listen and work with me end up sounding spectacular. Those who don't, usually sound just as mediocre as they always do.... 8)


- Stuart -

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PostPosted: Sun Aug 23, 2015 2:45 pm 
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Location: Fair Oaks, CA (Sacramento)
Decoupled! Replaced the top 2x4s of the top plate with 1x4s so none of the walls touch the existing ceiling joists. Had to use 2x6s to span the gap on the inner ceiling, but with the aid of a strong back it should be able to take the double rock. The ceiling is a little lower than I would have preferred now, but hopefully well worth it.

A couple more questions from the day:

*I'm spending all this energy sealing up the outer wall, but my framer thinks the inspector is going to require vents where the roof meets the wall since it's now considered conditioned space, and I already have a turbine roof vent up there. How do other California studios get past that?

*I've read that I might want to use 1/2 sheet rock for a layer and 5/8 for a different layer because of the similar resonate frequency of the two layers of the same thickness. Thoughts?

*Regarding the windows and doors: Since I've just got a 3/4" gap between the inner and outer leaf, can I just run the sheet rock up to about a 1/4" gap and then use acoustic sealant between instead of the cut OC-703?


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PostPosted: Fri Sep 25, 2015 6:15 am 
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Location: Fair Oaks, CA (Sacramento)
I'm about to get my framing inspection and then I'm hanging the sheet rock. Can you tell me your thoughts on these questions:

*I'm spending all this energy sealing up the outer wall, but I have a turbine roof vent up there and the "garage" has a firewall where it meets the house, so there's no other roof ventilation once I sheet rock the top. How would you deal with that?

*Regarding the windows: Since I've just got a 3/4" gap between the inner and outer leaf, can I just run the sheet rock up to about a 1/4" gap and then use acoustic sealant between instead of the cut OC-703?

Thanks!
-Jamin


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