Mitch Lee - Composer Studio Build

... Aren't my studs making it one big leaf already? (no matter the material between the GB layers) ? On the other hand...

Well, sure, to a certain extent the studs are coupling the two sides of the wall, yeah. No argument there. That's why a single-stud wall is such a lousy isolator. But the way I see it, there is still an awful lot of GB surface area in such a wall that isn't directly coupled by the studs (around 90% for 24"OC, by my calculations), which is only separated by "spring" (air+insulation), so I reckon that wall will still be doing a lot of "three-leaf" bad stuff. I'm basing that assumption on the following diagram, which I think comes from the SAE web site (Not sure: I lost track of where I found it): After all, a pair of single-stud walls cannot be acting entirely as two simple one-leaf walls if the STC jumps a full 10 dB when you take OFF one layer of GB, then another 7 dB by taking off another. Something resonant is going on inside those cavities that causes a drop in transmission loss with the GB present, and whatever it is disappears when you take away the GB in between, which is REDUCING mass. To me, that looks like the three-leaf effect at work. According to mas law, taking off those two panels should have reduced the STC by about 6 dB (half the mass), but instead it increased by 17. So there's a 23 dB discrepancy between mass law prediction and measured real life! That has to be three-leaf and four-leaf effect, if I'm not wrong. And that's using the STC rating, which doesn't even take into account low frequencies (it skips two and a half octaves!). If you considered everything down to 20 Hz, I reckon you'd see a much larger difference, since that's where the biggest losses for three-leaf are. The way I see it, that bathroom wall is going to cause the "STC-40" or "STC-50" situation in the above diagram, but if he takes of the drywall on the bathroom wall and beefs up the inside, he can theoretically get the STC 63 situation. That's a pretty large jump! Either 13 dB or 23 dB. Well worthwhile, either way, if you ask me. Or am I wrong in my reasoning? - Stuart -

I know the picture and tests, I'm more looking into "what is considered a leaf". When are we talking one or more leafs? Ah, forget it. I know how it works and that's what's important. Nevermind what we name it... right? It's just a thing that's been on my mind lately. :cop: (you know me, crazy about semantics)

Ah, forget it. I know how it works and that's what's important. Nevermind what we name it... right? It's just a thing that's been on my mind lately.

Yeah, I DO know what you mean! I still can't figure out when a third leaf stops being a third leaf. I mean, if you put it one centimeter away, guaranteed it is a third leaf, but if you put it 30 meters away, guaranteed it is NOT a third leaf. So where do you draw the line? I'm still trying to get that figured out. I guess you could say that it is the distance at which the maximum reduction in TL due to that leaf is less than "X" decibels, compared to the wall without that leaf. But what should "X" be? 3 dB? 6 dB? 10 dB? 1 dB? And what is the equation for calculating it? AAAARGHHHH!!!!!! Answers! I want answers!!!!! Why can't acoustics be intuitive and easy????? Why can't it be like nuclear physics, or brain surgery, or something simple like that??? :) - Stuart -

yeah... typical matter of "the more I know, the less I think I know!"

Hey Guys. I still have a couple questions regarding the ceiling. So should i view the existing ceiling as one leaf. So I then am just framing the 2nd leaf below it? Just like the walls?

I still have a couple questions regarding the ceiling. So should i view the existing ceiling as one leaf.

That depends on what it is! Is it just one leaf, or is it two? Or maybe even three? I checked back on the thread, but I couldn't find where you mentioned what the existing ceiling is made of, nor what is above it. - Stuart -

I will have to ask the landlord exactly. But from what i can tell.....its for sure a drywall (one sheet) ceiling. I know I know that doesnt tell you much. But for now let's just pretend its a "normal" sheeted ceiling. that even possible?

OK, so you have at least one sheet of drywall up there, and I'm guessing there is another floor above you, right? Another room upstairs? Or is it directly into the roof space? Either way, you already have at least one leaf up there, and almost certainly there are actually two leaves. So once again, if you want good isolation with whatever is above you, then you need to take of that drywall above you, and either use RC (or RSIC clips and hat channel) and a put back up a couple of layers of thick drywall, with insulation above it (fiberglass, mineral wool). Or better still, take off the drywall and leave it off, build a new joist frame a couple of inches below the one that is already there but resting ONLY on your inner leaf walls, put your two layers of drywall on that, once again with thick insulation in the space above it. Of course, if isolation is not important to you, then you can just leave it like it is. If you build another ceiling below that one WITHOUT taking off the existing drywall, then you will have a three-leaf system again, and your isolation at the low end of the spectrum will not be good. - Stuart -

there is no one above us. Its the top floor. Just the roof. Unfortunately I wouldnt be able to take off that drywall. So Ill have to go with the 2nd best option. I was just going to frame in another ceiling that would sit on my inner wall. and built similar to my vertical walls. It would be great to not have to build a ceiling at all. But from what I can gather that would make the new walls pretty much useless. is that right? thanks again. Mitch

Hey Mitch.

I was just going to frame in another ceiling that would sit on my inner wall.and built similar to my vertical walls. It would be great to not have to build a ceiling at all. But from what I can gather that would make the new walls pretty much useless. is that right?

Well, it won't make it useless, it just means that you will have a three-leaf system, which implies that the low-frequency isolation is not as good as it would be with a two-leaf system. What you can do to compensate (to a certain extent) is to add additional mass to your inner leaf, which helps. But at HIGH frequencies, the three-leaf system is actually a better isolator than a two-leaf. So it depends on what your problems are: Are you having (or do you expect to have) problems with high frequencies, or with low frequencies? In other words, are you going to record bass guitars and kick drums, or are you going to record female vocals and violins? Also, what kind of exterior noise problems are there? Is there structural noise in the building itself, such as elevators, pumps, HVAC motors, etc. that creates low frequency noise? Impact noise? Other kinds of noise? All those factors are important considerations in deciding if it is better to build a third-leaf ceiling with extra mass, or just go with the existing ceiling. My guess is that building the third leaf is probably the better bet, but understanding that it might have some poor isolation properties at the low end. But I'm not convinced that you'll even have a bad three-leaf problem: Maybe, maybe not. How is the roof above constructed? For example, if it has a large "attic" space and a very high peak, then the third leaf effect will be minimal. But if the roof is flat and parallel to your ceiling, , just a few inches higher, then that will maximize the effect. It would be great if you could post an accurate diagram or photo of exactly how that roof is built. - Stuart -

I chatted with the landlord and he said that it is single drywall then Joist....Some insulation. Then anywhere from 18 inches to 3 feet of space to the roof. Option 1 Question 1: If I was to just throw up (with green glue) another sheet of drywall ontop of the single sheet that exists now...would I gain any benefit? Option 2 Question 2: If I was to add another sheet of drywall (to the existing roof) and then frame in another ceiling that rested upon my inside walls and then another 2 layers of drywall on the inside of this new ceiling......would this be even a better option?

I chatted with the landlord and he said that it is single drywall then Joist....Some insulation. Then anywhere from 18 inches to 3 feet of space to the roof.

That sounds hopeful! With that much air space, I think that the third leaf effect would be minimal.

Option 1 Question 1: If I was to just throw up (with green glue) another sheet of drywall on top of the single sheet that exists now...would I gain any benefit?

Yep! It would gain you something. Hard to say how much, but theoretically if you double the mass then you get 6 dB extra isolation. GG will gain you some more still, especially at the low end.

Option 2 Question 2: If I was to add another sheet of drywall (to the existing roof) and then frame in another ceiling that rested upon my inside walls and then another 2 layers of drywall on the inside of this new ceiling......would this be even a better option?

Yep! You'd be doubling the mass again, so you'd gain another 6dB, minimum, and probably much MUCH more, since you'd have a true MSM ceiling then. So you might gain 20 to 30 dB, if you build it correctly and carefully. Once again, theoretically. Your mileage may very. etc. You would still have the three-leaf issue, but once again I don't think it will be a big deal, due to large air gap up in the roof. - Stuart -

3 new questions: 1. A question about the "super door" that Rod talks about on page 94. How is the sheet lead and plywood attached to the door? Super duper duper glue? :) 2. My air space between my 2 walls should be what? I have right now only 1.5" space. should i go more? i could...its just makes my room smaller. thoughts? 3. One thing we havent chatted about is my brick wall on the one side. I was going to just have my walls come to the brick wall. but then i started thinking. what issues might I have if i just leave it? I attatched a pic of a drawing with a 1" gap then a new wall. whats your thoughts? Mitch

help me o wise one.