mass loaded vinyl "curtains"

Hi, I am creating a band practice space in a basement that I rent. The area is 12' x 10' with 6.5' ceilings. I am conerned with soundproofing only one 12' wall as it is shared with the neighbours. And I'm not concerned about the ceiling. I have read through a lot of the advice on this forum, all great stuff, and I know how I would ideally do this. However, I'm renting so I can't remove the inner layer of drywall, or do any other kind of serious construction, and my budget is only about $500. The band is quiet enough that we can sing audibly without a P.A., and the neighbours don't mind hearing a little, so I don't need a perfect isolation solution. So here's my idea, please let me know how effective this might be. I'm thinking of just getting some mass-loaded vinyl and hanging it like a curtain a few inches in front of the wall. Mass-loaded vinyl has an STC rating of 27 on its own, the existing wall probably has about the same (most likely single wood stud, 1/2" drywall, mediocre insulation). So given that this is completely detached from the wall, and with a big enough airspace, would this break 50 STC? I know this is a "3-leaf" system, but there's not much I can do about that. Would it help to add insulation to the inside of my vinyl curtain? Would it help to build a frame for it instead of just letting it hang? Thanks for any suggestions. Peso

Every time I think I've found all the info I need, lo and behold there's more! After looking through more of the forum I see that the claim that MLV has 27 STC is not accurate, the number applies to a wall assembly with MLV in it (though some manufacturers seem to claim otherwise). So the new plan is to build a non-load-bearing metal-stud frame a couple of inches from the existing wall, use two layers of gypsum glued together with green glue, with Roxul AFB insulation between the studs. Of course, the middle leaf is still a problem, but it'll cost me only a few dB right? I have hopes that this system will give me 60 STC. Is that reasonable? Peso

Peso wrote:

Every time I think I've found all the info I need, lo and behold there's more! Of course, the middle leaf is still a problem, but it'll cost me only a few dB right? I have hopes that this system will give me 60 STC. Is that reasonable? Peso

Ha, I know exactly how you feel.... Part of the problem is that there seems to be alot of information out there -- too much information, not all of which appears to be (or could be) entirely correct. This whole third leaf concept is a particular area which might be especially iffy. Please don't get me wrong, I don't deny the existence of value in considering the amount of leaves. I just very strongly believe that it's not just a simple matter of: 'if it's third-leaf it's ALWAYS worse than if it's two-leaf'. I don't see how it can be such a factor as to override everything else in every case. Just consider a few hypothetical examples: 1.) Say you HAVE a double leaf system to start with, but that it's only a single 2X3 stud wall with a single sheet of 1/2" sheet-rock on each side. Now say you add another leaf (because for some reason you can't touch the original wall), 9 feet away with 3 layers of 5/8" sheet-rock on one-face, creating another 9 feet of dead air space. Does anyone really think that the sound isolation is going to WORSEN or remain mostly the same just because it's technically a 3-leaf system? Someone would have to please explain to me how that 9 feet of dead air space is not going to help, or in fact be the overriding factor. 2.) Say you have a double leaf just like above; but inside the wall are uninsulated metal heating ducts that you can't remove or replace or otherwise alter. You want to have a studio on one side of this wall, but realize the ducts are a major path for sound to go where you are trying to block it from going. a.) Should you accept this as the 2-leaf system it is and leave it intact? b.) Should you move one leaf a bit further away to a new second row of plates and studs? c.) Or should you ignore the leaf on the far side and build another leaf out from the second leaf between the studio and the duct-work? In this last choice you would have a technical 3-leaf system, BUT it would be 2-leaves between the major potential sound path (the duct-work) and the studio.... in the first and second choices, you would have technical 2-leaf systems, but only ONE leaf with respect to the duct-work. So, what's the better choice and why? I don't know the answers to these questions I pose... I ask them however to provoke serious thought, not speculation, about what constitutes a third leaf and what doesn't, AND more importantly, to what degree are the effects on sound transmission. PICTURES: 1.) Original wall with vertical vent ducts (in gray) that lead to the 'don't want sound to go here' room; pink is fiberglass insulation; 2-leaf system? 2.) Sheet-rock is torn off of side facing studio; a new isolated leaf is added; the middle is filled with fiberglass; ducts are STILL between leaves; this system is technically still 2-leaf? 3.) Sheet-rock is torn off side facing studio and a new isolated leaf is built FAR from ducts and other leaf; this is obviously better than # 2 above, but by how much? ducts are STILL between leaves; technically 2-leaf? 4.) Sheet-rock is torn off side facing studio; the remaining leaf is 'ignored' and an entire new 2-leaf system is built of isolated plates and studs (or staggered); insulation is optionally removed around ducts, BUT air is allowed to be free-flowing to a space around the studios 2-leaves and with a large volume of air (elsewhere in the building); technically 3-leaf system? 5.) This is the same as #4 except that the new 2-leaf system around studio is not on isolated studs; BUT free-flowing air space between outer (middle) leaf of studio is larger than in #4; technically 3-leaf system? 6.) Same as #3 EXCEPT that the orginal sheet-rock is not removed; there are 2-leaves between studio and ducts; this is technically a 3-leaf system? But is it better than #3?

Peso, Unfortunately, you can't add dB the way you are doing. If you have two 30 STC walls, you don't get 60 dB. You get either 33 or 36 dB TOTAL (depending on what particular "dB" you are adding. dB are not "things" you actually measure but are RELATIVE measurements. When you talk about a 30 (or 57 or 60 or whatever) STC wall, you are talking about the difference in level on one side vs the other not any specific level (unless of course, you know what the absolute level is on one side. Luftweg - You can tear apart any theory if you adjust the numbers far enough. A nine foot air gap will change the effect of three leaves in terms of how much you can hear (or measure) on the "far" side, but the three leaves are still there and WILL be worse than two leaves would be however, such a large air gap will REDUCE the effect on the other side. After all, if the air gap had no effect, the only way to isolate sound would be if there was only one (two leaf) building on the planet because when the sound that does leak out gets to your neighbor's house, whether they are 10 feet or 10 miles away, their house is going to create a third leaf. len

Luftweg wrote:

This whole third leaf concept is a particular area which might be especially iffy.

There is nothing whatsoever "iffy" about any of this - at least not from a scientific point of view - your mind might be another thing altogether.

Please don't get me wrong, I don't deny the existence of value in considering the amount of leaves. I just very strongly believe that it's not just a simple matter of: 'if it's third-leaf it's ALWAYS worse than if it's two-leaf'. I don't see how it can be such a factor as to override everything else in every case.

1st off - what you believe or don't believe has nothing to do with anything - for example - people believe that the earth was flat - and the center of the universe - and yet - for all of their belief - it is neither. And no one is saying that it's always worse - it can well be an improvement - BUT - the question is (always) what are you giving up in return - and (after that) what are your real limiting factors. For example - take an existing stud wall - 5/8" dw each face -0 with fiberglass insulation (standard fluffy stuff) - STC is about 36 - now add a new wall inside of it - 1" clear space - with insulation and one layer of dw on the inside face - STC is about 50.......... So it's an improvement of 14 dB weighted - see - not a loss at all - HOWEVER - step inside and remove that 3rd leaf - and suddenly the iso value goes from STC 50 to STC 57....... with less material - less mass....... now move that one layer of drywall to the interior face - and bingo - STC 60. So you can make a difference of STC 3 just by relocating one sheet of drywall. And from the perspective of the human brain and listening levels - 10dB is a DOUBLING of isolation. I would suggest that this is much greater than you give creedence to. As far as your 9' example goes - this is absurd to even discuss - in fact - no-one on this board would even consider suggesting a 2 leaf system in that condition - in fact - we would always reccomend a 4 leaf system......... the same as we do with airlocks. The correct way to picture this is - whenever you can (within reason) install 2 leaf system - this is critical with small air-gap assemblies - whenever this doesn't make sense - i.e.: closets - air locks - corridors - make either 3 leaf or 4 leaf systems (whichever makes more sense). By that way - this is just one small piece of a whole that creates isolating value - probability is that before you would every reach any real degree of isolation in most buildings - flanking will take over to limit your acheivements. Sincerely, Rod

rod gervais wrote:

There is nothing whatsoever "iffy" about any of this - at least not from a scientific point of view - your mind might be another thing altogether.

I'm wondering if anyone has any links to any site that explains the physics of why creating a third leaf always worsens a wall's isolation capabilities. I mean, by this rationale, if I'm sound and I go from my live room, through the control room wall and into the control room, and through the back wall of my control room to the outside of my house, technically there are three leaves there and by not having the control room wall you're saying that the live room sound should technically be louder outside because there's a control room wall adding a leaf. And if I tare down the control room wall you're saying the sound will be quieter outside. But I know this isn't true because if I take the glass out of the control room wall and let the sound go through to the outside I can hear a big difference outside. Even Einstein conceded that there are certain X factors in physics you simply cannot account for or explain. Aren't there some X factors here you guys aren't acknowledging? Or am I just immensly misunderstanding how sound works? I'm simply asking, not trying to defy anyone.

SonicClang wrote:

rod gervais wrote:

I'm wondering if anyone has any links to any site that explains the physics of why creating a third leaf always worsens a wall's isolation capabilities. I mean, by this rationale, if I'm sound and I go from my live room, through the control room wall and into the control room, and through the back wall of my control room to the outside of my house, technically there are three leaves there and by not having the control room wall you're saying that the live room sound should technically be louder outside because there's a control room wall adding a leaf.

Sonic, I know you can read - because this is not the first time we've communicated. Perhaps you should re-read exactly what I wrote - because this is not what I said at all. In my example I show a 3 leaf system improving on a 2 leaf system - but the improvement is considerably less than would be possible by simply placing the 3rd leaf in the right location. How does that turn into "you're saying that the live room sound should technically be louder outside because there's a control room wall adding a leaf" In the case of a wall that uses 4 layers of drywall - the difference that can be realized by placing the last 2 sheets in the right spot (outside each wall face) instead of the wrong spot (on the inside face of the 2 walls) is a massive 23dB weighted average - that's more than 4 times quieter just because of where you place the material Think about it............

And if I tear down the control room wall you're saying the sound will be quieter outside. But I know this isn't true because if I take the glass out of the control room wall and let the sound go through to the outside I can hear a big difference outside.

You never heard ME say that...........

Even Einstein conceded that there are certain X factors in physics you simply cannot account for or explain. Aren't there some X factors here you guys aren't acknowledging? Or am I just immensly misunderstanding how sound works?

No - if you listen to Einstein you would understand that sometimes things are just the way they are - that's what you're losing track of. Sincerely, Rod

I wasn't neccessarily referring to any statements you made Rod. I was just kind of talking to everyone here who knows more about this than I do. :D I'm seriously not trying to argue with anyone, I'm simply a pupil asking my mentors and/or teachers questions about the theories and science so I can fully understand what you're talking about.

Folks, Let's begin with an understanding that the math is extremely deep - nothing simple here - and I am neither an acoustician nor a physist. I am a specialist in construction - and therefore rely on tests to see what works and what doesn't. When I tell you that there is no magic here - that's because the test data proves what I say to be true - and for me - it doesn't matter why it works the way it does- all that matters is that it does. This is why I continue to tell everyone - use what has been tested - the only way to know (outside of test results) what will work - or what results you might have if you vary from what's been tested - is if you spend the duckies to go yourself and pay to have tests performed on the idea you have for construction. BUT - understand - all of this gets really very deep - and for music especially so (seeing as there are no established standards for assemblies in the frequency ranges we're dealing with here...... All of your assemblies are subject to existing structural pecularities that might exist - flanking - and of course the center frequency of what you might be constructing itself. For example - a wall built with a particular reasonant frequency can affect the amplitude of that frequency by +3dB as it passes through the assembly. Now - seeing as (begining at 3' from the surface) each doubling of distance gives a reduction of 6dB - 2 walls opposite one another in a 3' wide corridor - with the same construction and thus the same reasonant frequency - could conceivably leave you (at that frequency) with exactly the same amplitude in both rooms - so no reduction of sound levels at all........ That is due to the 3dB increase into the hallway - - a 6dB loss for the hall width - bringing it to a net 3dB loss - and the rise of 3dB (due to reasonance) bringing it right back to where you began........ note - this effect works within roughly an octave of the center frequency and happens (to some extent) with harmonics of the frequency.......... So picturing that one condition in your mind - does anyone really believe for a second that there is some easy to understand explanation to all of this that acousticians don't want to share? You want proof of the losses due to 3 leaf systems in comparison to the same exact materials placed correctly in 2 leaf systems ?????????? Read this compilation of test results released by California: http://www.toolbase.org/docs/SubsystemN ... atings.pdf I hope this "puts it to bed" because I am getting tired of saying the same things over and over again........... Enjoy, Rod

I never said I wanted proof, I simply stated that I wanted to understand it. The link you gave is exactly what I was looking for. Go calm down, drink a beer, and let me read. You didn't have to repeat anything you've said a hundred times before, I'm fine with a link. What does a good pupil do every time he thinks something has holes? He asks questions. If you have a problem with that notion, you don't have to reply to my questions.

SonicClang wrote:

I never said I wanted proof, I simply stated that I wanted to understand it. The link you gave is exactly what I was looking for. Go calm down, drink a beer, and let me read. You didn't have to repeat anything you've said a hundred times before, I'm fine with a link. What does a good pupil do every time he thinks something has holes? He asks questions. If you have a problem with that notion, you don't have to reply to my questions.

Sonic, This wasn't directed at you - you don't ask the same questions over and over again........ you learn from answers and then come up with new (usually good) questions following and expounding the theme - nope - I don't have a problem with you at all......... SO (if you don't mind) I will continue to reply to your questions..... Sincerely, Rod

Cool! :) I think I'll stop hijacking this thread now. sorry Peso!!!! :oops:

I don't mean to stir-up any new 3rd-leaf controversy, but since I currently have a mediocre wall in which the studs are physically connected to both leaves, then which option is better: a) add my two layers of gypsum board to the existing inner leaf, giving me a 2-leaf system with one side bulked up with mass, or b) leave a few inches of air space between the existing wall and my two layers of gypsum board, therefore disconnecting the outer leafs but having a 3-leaf system. And here's my second question, a theoretical one about the non-linear addition of dB reductions: Room B is sandwiched between room A and room C, and the walls between the rooms are all rated at 30 STC. So when a 100dB signal is generated in A, a meter in B would read 70 dB. Similarly if a 100 dB signal is generated in B, a meter in C would read 70 dB. Now going back to just a 100dB signal being generated in room A. What does the meter in C read?

Peso wrote:

I don't mean to stir-up any new 3rd-leaf controversy, but since I currently have a mediocre wall in which the studs are physically connected to both leaves, then which option is better: a) add my two layers of gypsum board to the existing inner leaf, giving me a 2-leaf system with one side bulked up with mass, or b) leave a few inches of air space between the existing wall and my two layers of gypsum board, therefore disconnecting the outer leafs but having a 3-leaf system.

Actually I've already been trying to figure that one out - and am having a hell of a time coming up with an answer without being there to see and perform some tests on the property. SO the real "best" answer to that question is this: "Hire a local expeert in the field to come down to your property........ he can perform some tests and determine what's direct - what's flanking - and what your best possible soloution would be. It's also [possible that he might tell you that you cannot get from A to B with what you have.

And here's my second question, a theoretical one about the non-linear addition of dB reductions: Room B is sandwiched between room A and room C, and the walls between the rooms are all rated at 30 STC. So when a 100dB signal is generated in A, a meter in B would read 70 dB. Similarly if a 100 dB signal is generated in B, a meter in C would read 70 dB. Now going back to just a 100dB signal being generated in room A. What does the meter in C read?

That is impossible to predict without having a ton of additional info not included in your question. And that is one of the problems with all of this. Perhaps if you were to provide some detailed drawings - with sections and specifications of all of that materials involved - their exact properties - exactly how they were to be constructed and interconnected - along with exacting dimensonal details in 3 dimensions and the tone you plan on producing, I could provide a rough estimation. Sorry - but rough is as good as it gets........ this because not all construction performs exactly as (or sometimes even close to) predicted - which is why there has to be testing performed - it's the only way to really determine how anything will react. By the way - you'll notice that none of those tested assemblies show a different assembly attached to them for testing purposes - so you could take a wall lab tested for STC 70 - along with a floor tested for the same STC - and a ceiling for (again) the same STC and end up with a room that only has an STC of 50 (or worse). Which is why we get so critical about being so anal about sealing each and every penetration in the assembly- and how critical decoupling is - and so on and so on - because all of this affects the room/building as a whole........ which is why it takes someone who understands acoustics to design how it all goes together. Better for you to ask me how to build a nuclear power generator - the results are more predictable.......... Sincerely, Rod

My rooms are FINALLY getting to the point of actually being sealed up from one another, and I'm definitely finding out how important it is to seal everything up. And it's funny because as soon as I find one weak spot and fix it, I hear another. Thankfully I've been a total anal jerk about caulking everything even when all the people who've helped me complained about it. Caulk is my friend. Most of my weak spots are coming from doors where there are tiny spots the foam weather stripping isn't being compressed. That's easy stuff.

Regarding the math of adding STCs (or TLs) it should be said categorically that you just can't do that ... To add or subtract dB values you have to do a complicated math thing that i don't remember offhand, but as an example say you have two sound sources that are 30 db, when you add them together you get i think about 33 db. and so you see there is something funny going on here. a few minutes of google searching should lead you to the method. has to do with algorithms (fun!)

Reminds me of the Richter magnitude scale that measures intensity of earthquakes:

. . . an earthquake of magnitude 5 has ten times greater displacement than one of magnitude 4 and one of magnitude 8 has 10(8 − 4) or 10,000 times greater horizontal amplitude than one of magnitude 4. (Source)

Decibals are logarithmic. With the addition of each 3 db, the sound is twice as loud. So from 30 to 33, the db is actually doubling. From dictionary.com. logarithmic: The power to which a base, such as 10, must be raised to produce a given number. If nx = a, the logarithm of a, with n as the base, is x; symbolically, logn a = x. For example, 103 = 1,000; therefore, log10 1,000 = 3. The kinds most often used are the common logarithm (base 10), the natural logarithm (base e), and the binary logarithm (base 2).

Perhaps my questions are going too far into the realm of theory for being in the stucio construction section, but I've been a bit confused on the theory side of things, and I think a solid understanding of the theory will help me in my construction decisions. With a little more research I've figured out the following. Decibels: Sound can be measured in power per area, like watts per square foot. Decibels measure the ratio of two units of power (or power/area). The formula is: power difference in dB= 10 * log (Power A/Power B) So dB is only a COMPARISON of two sounds. In acoustics, the referece power, "power B" in the above equation, is the threshold of hearing. So when we say something is x dB we mean relative to the threshold of hearing. In other words, using the above equation, the threshold of hearing=0dB. A sound with twice the power of the threshold of hearing works out to be about 3dB. The reason why the decibel is a useful unit, is because our hearing works logarithmically. Each addition of 10dB sounds twice as loud to our ears. Now in my example with rooms A, B and C, the walls reduce a given signal by 30dB. Using the above equation, what the walls are really doing is reducing the power of the signal 1000 times. So in a perfect situation (walls of infinite length and height), since the second wall also reduces power 1000 times, you do in fact get a 1 millionth of the power in room C, and the dB reduction is 60dB, or 30+30. The reason why we use the decibel as a unit, is because you CAN add it linearly. The reason why this doesn't happen in practice is because the two walls end up being connected through other walls, floors, ceilings etc. The linear addition is useful in that it gives you the upper limit, best-case scenario. I think I've also found the answer to my other question, regarding the 3rd leaf, in this great, great!, document: http://www.toolbase.org/docs/SubsystemN ... atings.pdf On page 43 (pdf page number, not the numbers shown on the pages) the first example is what I have: single wood studs, 5/8" gypsum, no insulation. STC=34 On page 53, the second last example is my first option, adding a layer of gypsum to the one leaf of the existing wall. It would give me STC=36. Now I'd actually be adding 2 layers connected with green glue, so I'd get better than that, but for the sake of this comparison let's go with just one extra layer. On page 80, the first example is my second option. A 3-leaf system, with an airspace between the existing wall and a new leaf with a single layer of gypsum backed with insulation. This would give me STC=52, clearly the better option. Now, if I could take the middle leaf out (p78, 3rd example) it would be even better, STC=56. But that's not an option for me, and it's clear that a decoupled 3-leaf system is far better than a non-decoupled 2-leaf system.

I should point out that Dan Fitzpatrick is entirely correct in saying that two sound sources don't add linearly. That's because you are adding power (not multiplying), so a 30dB source plus another 30dB source results in 33dB. Just for fun, let's see the math. new dB=10*log(sourcepower x 2) =10*log(sourcepower)+10*log2 =(dB of a single source) + (dB of twice the hearing threshold) =30dB+3dB =33dB The difference with walls reducing power is that they reduce the signal power by a factor, ex TL of 30 means dividing the signal power by 1000, as opposed to subtracting power. So the dBs subtract across a wall.

ok, i think you have a typo there then ... it says 30 db + 3 db = 33 db :)

Oh my head...

SonicClang wrote:

Decibals are logarithmic. With the addition of each 3 db, the sound is twice as loud. So from 30 to 33, the db is actually doubling.

Sonic, Not quite there yet......... the 3 dB increase is a doubling of power - not amplitude. It get's pretty confusing actually - Take 1 speaker - send a mono signal through it at 100dB - add a 2nd speaker with exactly the same signal and amplitude - the sum is a total of 103dB.......... which is a doubling of power. Add 2 more speakers - exact same signal and amplitude - sum total is 6dB - 6dB is a doubling of SPL (sound pressure level) (This is A weighting) - Add 4 more speakers - exact same signal and amplitude - you now have 109dB (8 speakers total at this point) and are one dB shy of a doubling of perceived volume by the average human being (C weighting). That's how it all comes together........... Sincerely, Rod

len-morgan wrote:

Luftweg - You can tear apart any theory if you adjust the numbers far enough. A nine foot air gap will change the effect of three leaves in terms of how much you can hear (or measure) on the "far" side, but the three leaves are still there and WILL be worse than two leaves would be however, such a large air gap will REDUCE the effect on the other side. After all, if the air gap had no effect, the only way to isolate sound would be if there was only one (two leaf) building on the planet because when the sound that does leak out gets to your neighbor's house, whether they are 10 feet or 10 miles away, their house is going to create a third leaf. len

Len. Well, you've pretty much summarized most of my points. Other factors -- if sufficently affecting -- can override the taboo third leaf effect. Those things, such as huge air gaps or mass layers, etc. may be so great that the fact the third leaf is there -- or was created -- may be almost moot. BUT, I'm not trying to directly compare PURE 2- and 3-leaf systems. I brought up that, theoretically, third or higher leaves may be impossible to avoid, and the 'counting' may depend upon your reference point(s), and yes, at some stage in almost any project you might encounter them. I wasn't trying to 'cause trouble' for no purpose; there are specific things in my proposed project that are seeming to force me into making compromises -- thus there are multiple ways I could go. There are places where (again, depending upon the reference points) the very same wall as constructed might be considered both a single-leaf and a double-leaf (or a double-leaf and a three-leaf). For example, I have ducts between above joists that need to be considered. Should I ignore the ducts and 'pretend' that the above floor and the below ceiling are a 2-leaf system? But if so, then with respect to the duct the system is still 1-leaf (which would provide inferior stopping into the duct?) On the other hand, should I consider the ducts (that lead straight to the space I'm trying to stop sound from going!) and construct a double-leaf below them? If so, then with respect to the duct the system is 2-leaf, but to the above room (with its floor) the system is 3-leaf. So, that's why I've been wracking my brain considering the consequences of following either choice, because NEITHER of them would be purely ("hands down") single-, double-, or triple-leaf systems. AND, as far as the physics comparisions that have been made here: Anyone who has studied physics knows that alot of concepts, laws, etc., relate only to references points and are not absolute. For instance, you can drive down the highway at 65 mile/hr, but you can't say that your absolute speed is 65 mile/hr -- you can only say that it is 65 mile/hr in reference to some 'fixed' point behind you. The Earth (and the whole universe) isn't fixed but itself is moving too, so that's why you can't say your speed is absolute. A triple-leaf (or double-, or single-, etc.) is only so in reference to 2 points. Some places along any wall may change their leaf number if somehow the reference points change. What I (and everyone else) should be looking for is the best compromise, and it is just that because I don't just have 2 reference points, but rather 3-dimensional spaces with many points in certain places that might allow (technically) 1-, 2-, or 3-leaf systems to exist simultaneuously. AGAIN: I don't dispute the theory of the 'third-leaf'; I would use entirely 2-leaves everywhere IF that were possible.... but it isn't, and so I need to make a compromise -- what the best compromise is should be the real question I suppose.

rod gervais wrote:

There is nothing whatsoever "iffy" about any of this - at least not from a scientific point of view - your mind might be another thing altogether... 1st off - what you believe or don't believe has nothing to do with anything - for example - people believe that the earth was flat - and the center of the universe - and yet - for all of their belief - it is neither.

Belief was maybe a poor choice, but a comparision to 'Flat-Earthers' is not exactly fair. I never said (or meant to say )I didn't believe in the 3rd leaf concept. What I had issue with is that some people seem to believe that something with 3 leaves is always inferior to something DIFFERENT that's 2-leaf. The point is that the differences besides the number of leaves have value. It wasn't all academic, and I'm not trying to 'cause trouble'. I have certain areas in my project that CAN'T be either ONLY 2- or ONLY 3-leaf with respect to every set of points between the inside and outside of my room.

rod gervais wrote:

And no one is saying that it's always worse - it can well be an improvement - BUT - the question is (always) what are you giving up in return - and (after that) what are your real limiting factors.

That's almost exactly what I've been trying to mean. And I'm forced to compromise in my project (with respect to different flanking pathways) and need to know which compromise is best. There's almost no way (certainly not cheaply) that I could have an entirely pure 2-leaf system (not one with the same degree of recommended best solid isolation of leaves and degree of mass in the leaves).

rod gervais wrote:

For example - take an existing stud wall - 5/8" dw each face -0 with fiberglass insulation (standard fluffy stuff) - STC is about 36 - now add a new wall inside of it - 1" clear space - with insulation and one layer of dw on the inside face - STC is about 50.......... So it's an improvement of 14 dB weighted - see - not a loss at all - HOWEVER - step inside and remove that 3rd leaf - and suddenly the iso value goes from STC 50 to STC 57....... with less material - less mass....... now move that one layer of drywall to the interior face - and bingo - STC 60. So you can make a difference of STC 3 just by relocating one sheet of drywall.

Actually, isn't that the basis of those diagrams I think everyone here has seen as 'demonstration' of the 3rd leaf effect? I have no problem with that and those; it's actually almost easy as pie to understand. What would be more further illustrative would be to do similar treatment with adding isolated leaves OUTSIDE of the 2-leaves (although I do realize that would still be similarly 3rd leaf and would be worse than if the middle leaf was removed), and to have those extra leaves be considered with free-flowing airspaces versus dead airspaces. I don't think anyone here is going to add a wall BETWEEN 2 leaves (especially rigidly attached to the same studs and reducing the dead airsapces), but rather there may be times when an external 3rd leaf -- perhaps only in a few places along the whole wall, and with free-flowing air to OTHER spaces (besides the areas that need to have sound blocked from them).

rod gervais wrote:

And from the perspective of the human brain and listening levels - 10dB is a DOUBLING of isolation. I would suggest that this is much greater than you give creedence to.

Coming originally from engineering (though I'm a PharmD now), I'm aware of the concepts of logarithms, exponents, and geometric changes with respect to linear changes. Yes, if I remember correctly 10 decibels is an increase or decrease by a factor of 2, of the perceived sound volume (1 Bell is the doubling or halving?). Again, I don't think I ever argued anything about what value this has (or even ever talked about db).

rod gervais wrote:

As far as your 9' example goes - this is absurd to even discuss - in fact - no-one on this board would even consider suggesting a 2 leaf system in that condition - in fact - we would always reccomend a 4 leaf system......... the same as we do with airlocks.

I realize it's way over the top.... it was only meant to 'drive' home the point that sometimes the 3rd leaf effect needs to be disregarded. PLEASE, tell me more about the airlocks, what defines them and how you treat them.

rod gervais wrote:

The correct way to picture this is - whenever you can (within reason) install 2 leaf system - this is critical with small air-gap assemblies - whenever this doesn't make sense - i.e.: closets - air locks - corridors - make either 3 leaf or 4 leaf systems (whichever makes more sense). By that way - this is just one small piece of a whole that creates isolating value - probability is that before you would every reach any real degree of isolation in most buildings - flanking will take over to limit your acheivements.

Now you're talking. THIS is more the kinda stuff I wanted to hear... And yeah, it's the flanking I'm concerned about.... For instance, if I can eliminate solid contact flanking, but it involves creating a third leaf in some areas, isn't that a reasonable compromise -- one that will provide a better sound isolation result. It's NOT the concepts of the 2-leaf system, or 3rd leaf I don't understand.... it's rather the adjustments to this 'rule' that need to be made from time to time to treat 'real' world situations.... thanx, K