Control Room Construction

Hey guys, I'm living at the foothills of Perth, Western Australia, and I'm currently making plans for a control room. I'm going for a 2-shell design, based on F. Alton Everest's inner-shell, outer-shell construction. The outer shell is already up, made from regular 20cm, two-core concrete blocks and measuring 16.2m long, 10.8m wide and 6.5 m high. This may seem very large for a control room, but it is ideal for a 2-shell design, as it gives greater freedom and space when constructing the inner shell. I'm currently in the process of deciding upon materials for the construction of the inner shell. Do you guys have any suggestions? I've tried searching through the forum, and Google, but can't seem to find any information on materials used for a 2-shell design. I have a solid knowledge in acoustic theory, however construction materials I am indecisive about. One option may be just a stud wall, as I want to angle the walls to create an RFZ. Bricks probably wouldn't be much good. So, here's a few pointers for suggestions: 1. What wall materials would be best to use for the inner shell? Likewise, any suggestions for the floor/ceiling would be much appreciated. 2. Using the materials you have suggested, are there any construction methods that you might of help? 3. Using the gap between inner & outer shell as a bass trap, would fiberglass work best in between the walls? Some extra info: - Budget is not an issue. - The outer shell is intended to treat resonance modes & low frequencies, and the inner shell is intended to treat reflection related issues (early reflections, ITDG, etc) & mid/high frequencies. Here is a picture of the intended design of the inner shell: I also have a few floor plans + several 3D illustrations. I don't want to upload too much, so if you'd like to have a look, let me know what it is you'd like to see. Any other suggestions would be sweet! Cheers, Andy

Hi Andy, and welcome. There is an announcement at the top of the forum about what to do to assure getting as many responses as possible. The announcement leads to this post. Actually, several people, who are experts on this forum, will not reply if you do not do what is written in the post. Many others who are very helpful, will not reply out of respect for the moderators' wishes. - Stuart -

Hi Stuart, Thanks for letting me know, I'm not much of a forum-going-guy, so I appreciate you giving me the heads up :) I'll edit the post asap. Cheers! Andy

Hi Andy, Here are some comments on your updated post. Please don't take them the wrong way: they are intended to be constructive criticism.

I'm currently in the process of deciding upon materials for the construction of the inner shell. Do you guys have any suggestions?

Most people here build the inner leaf with timber or metal stud framing, and put several layers of 5/8 sheetrock on ONE side of the studs (either the inside, or the outside, but NEVER both). You need to leave a gap of several inches between the outer leaf and the inner leaf. Both leaves must be sealed air-tight. You need to fill the gap with mineral wool or fiberglass.

Bricks probably wouldn't be much good.

Why not? Bricks have much higher mass and density than sheetrock: Much better isolation. the trick is to build the wall with no flanking paths, which is not easy, and to seal it properly, but brick is a pretty good material for acoustic isolation.

Likewise, any suggestions for the floor/ceiling would be much appreciated.

How is your existing floor built? Your ceiling should be built in the same way as the walls: two-leaf MSM. Think of it as a wall lying down. Timber or steel framing, two or more layers of sheetrock on ONE side (assuming that you already have a one-leaf roof above), and green glue in between the layers of sheetrock, if low frequency loud noise is an issue that you need to isolate.

Using the gap between inner & outer shell as a bass trap, would fiberglass work best in between the walls?

Well, you certainly do want fiberglass or mineral wool in the air gap between your leaves, but the purpose of that insulation is NOT bass trapping! Bass trapping is something entirely different. The purpose of the insulation is to damp the resonance of the cavity. It is part of the spring in your MSM system. It isn't meant to be part of room acoustics, and is only part of the room isolation. Two different things. Bass trapping is part of the room treatment, not the room isolation, and goes inside the room (in the corners), or is done with hanger in / above the rear area, but not inside the walls. The two-leaf wall is for isolating your room, not treating it. Treatment goes on the inside, and is a separate concept.

- The outer shell is intended to treat resonance modes & low frequencies, and the inner shell is intended to treat reflection related issues (early reflections, ITDG, etc) & mid/high frequencies

Sorry, but it just does not work like that. You seem to have misread Peter Newell, and misunderstood his NER concept. You are confusing isolation with treatment. The two leaves or your wall are part of an MSM system (also known as a MAM system). MSM is "mass-spring-mass". MAM is "mass-air-mass". Same thing. You have a very heavy sealed outer wall (mass), a damped air-gap (spring), and a very heavy sealed inner wall (mass). All three parts work together. It is an acoustic system where the parts work jointly to deal with the entire spectrum. You cannot say that the outer wall deals with one frequency band or the inner wall with another. According to Newell, the outer (asymmetrical) shall contains the bass energy, but does not treat it, and like wise the inner (symmetrical) shell contains the mids and highs, but does not treat them. According to MSM theory, the two leaves work together as a damped resonant system. If you poke a hole in the outer shell, it won't just damage your "resonance modes & low frequencies" treatment: it will wreck the entire isolation system, and all frequencies will be affected. Ditto if you poke a hole in the inner leaf: it will not only damage "reflection related issues (early reflections, ITDG, etc) & mid/high frequencies". It will most certainly wreck your entire isolation. (Yet, counter-intuitively, it might not affect your room acoustics at all! Can you explain why that is?) I think you need to spend a few days reading over this forum to understand the two-leaf concept, isolation, treatment, MSM, flanking, and a bunch of other principles.

Here is a picture of the intended design of the inner shell:

Again, you seem to be confusing the concepts of isolation and treatment. They are two totally separate and distinct things. Materials that you use to isolate are terrible at treatment. Materials that you use for treatment are terrible isolators. Concrete is a fantastic isolator, but pretty lousy for treatment. Mineral wool is fantastic for treatment, but a pretty bad isolator. These are two absolutely different concepts. First you build a two-leaf system to isolate your room, then you build treatment inside the room. Yes, you can use your inner leaf as part of the treatment, and yes an RFZ design is a good idea, and yes the inner leaf can be part of the RFZ design. But don't confuse the concepts. I think you need to buy Rod's book and also the Master Handbook of Acoustics, and work your way through both until you thoroughly grasp the concepts. - Stuart -

I also have a few floor plans + several 3D illustrations. I don't want to upload too much, so if you'd like to have a look, let me know what it is you'd like to see.

A detailed, accurate SketchUp model would be excellent.

Hi Stuart, Thanks for taking the time to reply to my post. It's strange that you mention The Master Handbook of Acoustics... as this is one of the books from which I learnt this theory of a 2-shelled control room. (a list of my book references are below) Here is the excerpt from the book: I understand what you are saying - and I also understand the differences between acoustic treatment and isolation. However, from the reading I have done, I think you can have a 2-shelled design that can both combat isolation and acoustic treatment. One must consider wave vs. ray acoustics, and the differences therein. Waves and rays will react to a similar acoustic situation in almost completely opposite ways. That's why the two approaches are separate. Even physicists apply the wave approach when dealing with frequencies that have long wavelengths. Thus it is safe to assume that the two shells of this design will work to combat both approaches. Therefore, it is also safe to assume that if the inner shell of our structure consists (for example) of 3-ply wood sheets, 'waves' (frequencies below 400 Hz) will not in any way be effected by the 3-ply sheets, because - as F. Alton Everest says - the acoustical size difference between the ply and the wavelength of the frequency is huge! I hope this explains some of my theories... and hopefully provides something to work with in our discussion. Just to clear things up... what do you mean exactly by two-leaf? You use the term both for the isolation of the room, and for the construction of the wall materials. I am currently writing an acoustic analysis of the room for my final assignment at SAE (School of Audio Engineering). I know SAE's reputation isn't great, and I now know why... but let's not discuss that. I have attached the analysis if you want to have a quick browse through... perhaps just skip straight to the 'Treatment' Section. I haven't at all finished it... it is still a working progress, so please feel free to comment on any of my theories (most are directly derived from the books below) (Just as a side note... the analysis is used for educational purposes ONLY! And is not deemed to be an official document whatsoever. Please keep this in mind whilst reading, and please do not attempt to copy or distribute the document.) Here it is: http://freepdfhosting.com/f33f7a60c7.pdf Sorry to have to link the PDF, but it's too big to upload here. Also, so you know, the PDF. is only on the server for 30 days. [Alternative download location: http://digistar.cl/sayers-forum/f33f7a60c7.pdf ] For interests sake, here is a list of books and texts that I have read (some thoroughly, some not so thoroughly): The Master Handbook of Acoustics, by F. Alton Everest Recording Studio Design, by Philip Newell Modern Recording Techniques (Sixth Edition), by David Miles Huber Architectural Acoustics, by Marshall Long Acoustics, by Heinrich Kuttruff Low Frequency Sound in Rooms, by Graeme Huon Sound Insulation, by Carl Hopkins Recording Spaces, by Philip Newell Building Acoustics by Tor Erik Vigran Sound Studio Construction (on a Budget), by F. Alton Everest Determining Optimum Room Dimensions for Critical Listening Environments: A New Methodology, by Trevor J. Cox and Peter D’Antonio Thanks again for your time Stuart, Andy

Also, could I ask, where does MAM / MSM come from? Is that one of Gervais'? I'm interested in doing some more research on it, but can't find much about it in any of my books. Would you construct the inner and outer shell walls using MAM, and then apply the same theory to the two walls as a whole + the air gap between the two walls? Cheers, Andy

Just to clear things up... what do you mean exactly by two-leaf? You use the term both for the isolation of the room, and for the construction of the wall materials.

I'm speechless. You have read all those books on acoustics, and never heard of a two-leaf system, or MSM? Wow. Whatever. Happy New Year.

andihopkins wrote:

Also, could I ask, where does MAM / MSM come from? Is that one of Gervais'? I'm interested in doing some more research on it, but can't find much about it in any of my books.

The concept has been around for decades, and is widely used in acoustic isolation. I'm surprised none of your books mention it! It's not exactly a new concept. Rod is just one of many proponents of the MSM concept, and has built many, many studios, of all sizes, using it. As have many, many other people. Here is a link to one of the original papers on the subject: http://www.box.net/shared/jcaoavdc8g

Would you construct the inner and outer shell walls using MAM, and then apply the same theory to the two walls as a whole + the air gap between the two walls?

No, if I understand you correctly, that would make a four-leaf system, which will give you much worse low frequency transmission loss that for two leaf, all other factors being equal. MAM (or more correctly "MSM") is your entire wall. It is everything and the only thing that separates the inside of your studio from the outside world. You do not want any other leaves on the outside of that, or on the inside of that. All you want is two-leaf MAM. All that you want between you and the outside world is two massive leaves, with a sealed, damped air-gap in between. Period. In other words, you want a "room within a room": There is an "outside" room, which is one leaf of the system, and it entirely contains another room, the "inner" room, which is the other leaf. Theoretically, the two rooms do not touch anywhere, and there is an air-gap all around. The air gap between the two rooms is sealed and filled with acoustically absorptive isolation material. The air acts as a spring, the absorption acts as a damper. The mass acts as mass, on both sides. It is a damped resonant system. It is basic physics. Think of it as a mass hanging from a spring that in turn hangs from another mass, with a damper (think of a car shock-absorber) on the spring. Causing the lower mass to vibrate does not cause the upper mass to vibrate, since the spring is elastic and does not transmit vibration easily. The damper absorbs the energy in the system, and converts it to heat. Vibration does not pass from mass "A" to mass "B" because the spring is elastic and the damper absorbs the energy. That is MSM in a nutshell. Ideally, the two rooms would be entirely decoupled, to prevent flanking, but of course in practice that is impossible. Normally the floor is common to the two rooms, to a certain extent, (unless you float your floor, which is an entire other kettle of fish) but the other five sides of the rooms must remain fully decoupled.

Here is the excerpt from the book:

You are taking MHoA out of context. In that section, Everest is talking about pure mass-law walls, not MSM walls. To do what he describes, you need many times the mass of a simple MSM wall. Mass law governs for part of the spectrum, of course, but you can minimize that and get MUCH better transmission loss using MSM for MUCH less total mass. And much less total cost. If you really want to spend all the money to build 12-inch thick solid high density reinforced concrete walls, floor and ceiling, then you can get decent isolation. But if you don't want to waste your money, you can just build an MSM wall and get decent isolation for much less expense. More links: viewtopic.php?f=3&t=11487&p=83071&hilit=wyle#p83071 http://www.greengluecompany.com/underst ... leLeaf.php http://forum.studiotips.com/viewtopic.php?t=52 http://www.jeacoustics.com/library/pdf/0557.pdf http://www.isover.lv/files/pdf/awg_paper_CH_1.pdf http://www.bobgolds.com/WallCharts/Quad ... eafSTC.htm http://www.soundproofing101.com/soundproofing_2.htm http://irc.nrc-cnrc.gc.ca/pubs/cp/noi1_e.html http://www.ets-lindgren.com/pdf/acoustic_manual.pdf http://squ1.com/archive/index.php?sound ... ssion.html http://christianproaudio.com/studio_construction.htm Etc. Just a few that I found with a couple of quick searches. In your case, you already have a single-leaf wall, and I'm hoping that you filled the cavities in your concrete blocks with cement, concrete, sand or some such. If not, then you can live with it, but it would have been better to do that, for both the extra mass and also to eliminate those are cavities, which might resonate. So all that you need to do is to seal that outer leaf (if you have not done so already), build the inner leaf, which you can do with simple 2x4 framing and a couple of layers of sheetrock, with Green Glue in between. Or more layers if you need better isolation. Fill the air gap (your "spring") with fiberglass or mineral wool (your "damper). Bingo! You have a MSM wall. Do the same for the ceiling. Make sure that no part of the outer leaf touches the inner leaf at any point, or you have a flanking path. - Stuart -

Cheers for the info Stuart. Just did a quick search through, and I could only find MSM / MAM in the Glossary of one book, Carl Hopkins' Sound Insulation. But he only includes it in a few equations. I get the idea of MSM now, thanks for the explanation. However, I do have one query: If the 'first' mass of my MSM (the inner wall) is less dense and less deep than the 'second' mass (the outer wall) then won't the rooms resonances be determined by the outer, more dense wall? Although isolation is important, I think it should come second to room dimensions/ratio and construction materials. What are your thoughts?

Hi andihopkins, Happy New Year! What a lot of people in your side of the world use is metal or wood framing and sheet rock for the interior leaf/wall for thier build. I don't know where Mr. Everest used the acronym MSM or mam either :) But he does allude to it on several occasions. He is still the "goto" author in my book. The above scanned page you present, page 442 Chp21, is an example of a two leaf /mass air mass or mass spring mass, room-in-a-room or inner shell - outer shell, pick your phrase. The standard cost effective build is typically wood or metal framing and sheetrock. So when we get outside of that it may only be cost effective in my "neck of the woods". Don't let all the discussion take away from the focus, we are talking the same thing, just using different words to say it. Good luck, Brien

andihopkins wrote:

If the 'first' mass of my MSM (the inner wall) is less dense and less deep than the 'second' mass (the outer wall) then won't the rooms resonances be determined by the outer, more dense wall?

It is a system. All three affect the resonance frequency. Have a look at the attachment. It is from NRC IR 586.

Although isolation is important, I think it should come second to room dimensions/ratio and construction materials.

It is a total system. Do it all right. Within your budget of course. Andre

andihopkins wrote:

Just did a quick search through, and I could only find MSM / MAM in the Glossary of one book, Carl Hopkins' Sound Insulation. But he only includes it in a few equations.

How many equations do you want it used in? :D I was doing some other research and came across "Double Panel Design Techniques" starting on page 338 in Long. Maybe that will help you. Andre

andihopkins wrote:

(Just as a side note... the analysis is used for educational purposes ONLY! And is not deemed to be an official document whatsoever. Please keep this in mind whilst reading, and please do not attempt to copy or distribute the document.) Here it is: http://freepdfhosting.com/f33f7a60c7.pdf

Andy, I wonder, seeing as you don't mention either reading (or owning) my book - how it is that some of my art-work ended up in your paper? Some of what I see in there are details that I personally drew in CAD and then exported for my book. An example of this would be figures 1.1, 1.2, 1.3 in your paper (which are 2.6, 2.7, 2.8 in my book) - 2.8 is pretty telling of what I speak - even the numbering / lettering of the intersecting points of radiation are identical. The chances of someone using exactly the same intersecting points (in exactly the same room locations) as well as the letters/numbers, along with the exact scale of room to layout ratios strictly by random chance is pretty much not going to happen. These are not the only ones that exist - rather just an easy example of what I am seeing. , Could I inquire as to your sources? Sincerely, Rod Gervais

andihopkins wrote:

Also, could I ask, where does MAM / MSM come from? Is that one of Gervais'?

Andy, To begin - I do not invent construction terminology..........

I'm interested in doing some more research on it, but can't find much about it in any of my books.

Not surprising - no one book can cover everything about the field....... Extensive studies on this have been done over the years.......... obtain publications for tests performed at the National Research Council of Canada, try this link to check out wall assemblies. They have it all - probably some of the most extensive testing I have ever sen http://irc.nrc-cnrc.gc.ca/dbtw-wpd/exec ... =QBE_QUERY

Would you construct the inner and outer shell walls using MAM, and then apply the same theory to the two walls as a whole + the air gap between the two walls?

No - the 2 walls create the whole MAM system - so you could have (as I often use for studio design) 12" cement block - sand filled (with structural cores filled with structural grout and rebar) - and then 1" clear plus a 2x6 wall (perhaps 2x8 depending on height) with a layer of 3/4"m plywood and multiple layers of 5/8" Type X drywall. Then the 1 mass layer is the entire masonry structure = 11 5/8" with an Air space of between 6 1/2 to 8 1/2" depending on wall framing members, whether the wall is parallel to the exterior wall - and then somewhere around 2" of mass on the inner most wall face. I hope this clarified some of this for you - but really study the studies - one of those on over 350 different combinations of wood and metal stud walls - both balanced and unbalanced (different amonut of layers on each wall side) and develope a feel for how the numbers run - what the trends are........ you'll then begin to easily see where you can go with this if you want to be creative....... Rod

I went to Perth SAE with an Andy about 10years ago. I hope you not repeating! :oops:

Hi Rod, Thanks for taking the time to reply, appreciate it. From what I know now, the MSM system is exactly the same as F. Alton Everest's '2-shell' system, and Philip Newell's 'Internal Wall/Isolation Wall' system. I have studied this quite extensively, however I'd never heard of MSM before... thus the confusion/question. I am hoping that you could clear up one area of confusion though, Rod. Both Everest and Newell talk quite a bit about utilizing they're wall systems (or MSM systems) for bass trapping. Additionally, Everest talks specifically about the outer shell, or 2nd Mass, defining the rooms resonances. With knowledge of the variation in behavior throughout varying wavelengths, both Newell's and Everest's claims made perfect sense to me. However, as you may have picked up, some on this forum seem to disagree... What are your thoughts on the subject? Also Rod, your signature at the bottom: 'Ignore the man behind the curtain...' Is that relating to what I think it's relating to?? Cheers mate, Andy

Hey lilith_envy, Haha, nah mate, this is my first run through. Who were your lecturers/class mates back then? Was Dave around yet? Andy

I think so, Paul was my teacher who is now the head of something in Melb or Sydney? Is the studio still off the terrace near the park? I was the 1st group through the "new" building before that it was near an Irish pub! thats pretty much all I remember after school drinks 4 days a week! :blah:

yeah still near the park, they've upgraded quite a few of the studios (gear wise, not acoustics wise, that's all still very glum.) I wouldn't be surprised if previously SAE Perth WAS an Irish Pub!

Could I inquire as to your sources?

No worries Rod. One of the students in my class at SAE has your book, I had a bit of a flick through (not what I would call I formal read) as we are currently in the last month of our course, and working on our acoustic analysis', found those diagrams, and made a quick photocopy for my analysis. I do believe copying 10% of a book is allowed right? Anywho, I will take them out if you so wish. It's not getting published or anything, it's just for an assignment. Cheers, Andy

andihopkins wrote:

Hi Rod, From what I know now, the MSM system is exactly the same as F. Alton Everest's '2-shell' system, and Philip Newell's 'Internal Wall/Isolation Wall' system. I have studied this quite extensively, however I'd never heard of MSM before... thus the confusion/question.

I did not coin the phrase - and do not know who did - however it relates quite simply to 2 layers wraping an inner air layer that creates a spring. [/quote] I am hoping that you could clear up one area of confusion though, Rod. Both Everest and Newell talk quite a bit about utilizing they're wall systems (or MSM systems) for bass trapping. Additionally, Everest talks specifically about the outer shell, or 2nd Mass, defining the rooms resonances. [/quote] I am quite familiar with Everest...... so why don't you begin there - After I read what you wrote - I opened up my copy of MHOA (4th edition) and went searching for what you found - without any luck finding it - so obviously I am missing something - and need you to point me in the right direction - please quote me chapter and verse so that I might re-read and understand.

With knowledge of the variation in behavior throughout varying wavelengths, both Newell's and Everest's claims made perfect sense to me.

They make perfect sense to me as well - all from the point of view of how to design a room dimensionally based on the finished dimensions inside of the room to create a modal atmosphere as close as you can to flat. However, none of that has anything to do with using the wall design as a part of the bass trap system......... but just using good room ratios to smooth out the modal response. That works regardless of the degree of isolation in the wall assembly - however in reverse - you could never increase isolation once your design was finished- because doing so would change the modal point themselves. The first thing you need to understand is that there is no correlation between the TL value of a wall and the absorption value of a wall, and as such it makes it very difficult to use it in a manner in which it has never been measured. All room mode calculators and the math they are based on - are calculated using inside (finished) room dimensions - and the assumption that the inner most wall face is infinitely rigid - with no Transmission Loss through the wall - and ignore that face that an outer face to the wall even exists. Now - I find your concept that the outer wall might control (or at least influence) modal activity within the room interesting - as I make note of that idea in my book - BUT - this is NOT a widely held concept - has NEVER been suggested by Everest, so I do not know where you get the idea from. These has been some study done on understanding the absorptive properties of wall assemblies- with some interesting findings to date (refer to tests performed for the Green Glue company at their website) BUT they are not conclisive - and are certainly not anything you could use o arrive at a predictive state for room design. Right now it is just an interesting study I plan to follow as it moves along.

However, as you may have picked up, some on this forum seem to disagree... What are your thoughts on the subject?

As you can see from my post above - I am one of those who believes you got a little twisted up somewhere along the way and are picturing something incorrectly - but (again) point out to me exactly what you refer to in Everest and I would be happy to re-read the section and see if perhaps it was not I who was not understanding. For the record Andy, it is not enough just to quote a source - you really have to quote it chapter and verse if you wish to make a point and give others the chance to view that which you read and respond intelligently. Now - speaking of responding - I wonder why you did not respond to the question I posed in my first post to you? Sincerely, Rod Also Rod, your signature at the bottom: 'Ignore the man behind the curtain...' Is that relating to what I think it's relating to?? Cheers mate, Andy[/quote]

Hi Rod, Only just saw your first post about 15 mins ago, there should be a response above your latest post. 2 minutes while I reply to your latest post :) Cheers

andihopkins wrote:

Could I inquire as to your sources?

No worries Rod. One of the students in my class at SAE has your book, I had a bit of a flick through (not what I would call I formal read) as we are currently in the last month of our course, and working on our acoustic analysis', found those diagrams, and made a quick photocopy for my analysis. I do believe copying 10% of a book is allowed right? Anywho, I will take them out if you so wish. It's not getting published or anything, it's just for an assignment. Cheers, Andy

Andy, I have no issue about the use - however I do have an issue about (and not of mine in particular) the copying of ANYTHING from a source without naming the source. There are plenty of references I make -or data that I provide - here in this world - that are not my original thoughts - or details that are not my original artwork - and it is my responsibility to name the source for 2 reasons..... the first is that doing so meets the intent and requirements of copyright laws........ the 2nd is so that no one in the world can ever get the idea that the thought or artwork is mine (even by mistake). It is not enough for me to just not claim ownership - I have to DISCLAIM ownership in order to be honest and honerable. Think about it...... Rod

Ok, here are the page/chapter details in MHoA: Page 442 - Chapter 21 Just above the title, 'Outer Shell of the Control Room' he says...

These frequency zones determine construction of the control room—a massive shell to contain and distribute the low-frequency modal energy, and an inner shell for reflection control.

He then goes on to talk about the two main outer shell shapes, rectangular and trapezoidal, etc. If we forget isolation for a second, and assume that we have treated the MSM with materials specific to that of acoustic treatment (and not isolation)... would it be correct to assume that the inner shell/outer shell system would work similar to that of a membrane absorber? As the 1st mass is thinner, and in most cases, less dense, would the 1st mass really have any effect on frequencies bellow 100 Hz or so? It has been pointed out to me that the purpose of the MSM system (when relating to isolation) is not absorption, but reflection. What I don't understand is this... If the purpose is solely reflection, why not just glue in with Philip Newell's stance? You can check out some of Newell's ideas one Page 125 of his book, 'Recording Studio Design - 2nd Edition'. Newell designates the outer wall chiefly for isolation, but he goes on to explain that the inner wall and spring, is used chiefly for acoustic treatment, and low frequency absorption. I'll see if I can find some more info in Newell's book on his wall system. Andy