Multipurpose room with John’s inside-out walls

Hi everybody, I’ve been reading and searching the forum for the past months and finally I’m ready to submit my Sketchup project :D. I really must say I’m so glad to have finally found such a rich source of pro info on such a debated matter. Well… this is a brief introduction: Designing stage of a single room within a room located at the bottom floor of an apartment block in Rome with the following design features: - John’s inside-out wall layout. - Load-bearing steel frame walls. - Steel frame not suspended ceiling but resting on the inner leaf wall structure (no sloped ceiling, will add cloud later). - Non-floating floor. - No soffit mounted speakers but freestanding (at least for now). - 24" (61 cm) rockwool superchunks in corners (except corner near door) and 6° splayed slot resonators on both sides of listening position. - Rod’s superdoor design for the door. I’ve made some measurements of the room with a friend’s pro sound level meter, and I’ve also bought a stethoscope as suggested in an old post by Knightfly and made some amazing discoveries! Such a simple and useful tool indeed :shock:… Attached is a work in progress Sketchup file of the room if anyone could be so kind of having a go at (frames are wood color but are meant to be steel, and cloth finish, new door and some minor details are deliberately omitted in the design at the moment). And here are the details: Location: Rome, Italy Project: designing stage of a multipurpose CR for composing/mixing TV and Cinema stuff and soundtracks, also some acoustic instruments and vocals recording. Goal: I’m moving from a much smaller room and I’m seeking for a bigger and dead silent (if possible) environment, and with natural daylight at last!! Therefore spaciousness is very important (when is it not? :)) and I’m trying to avoid any space-wasting overkill. Room dimensions: lenght 552 cm, width 413 cm, height 288 cm (18,1 x 13,5 x 9,4 feet) Room specs & surroundings: The room is in a partly-below-ground-level apartment in the center of Rome just facing the riverside street. It is situated at the bottom of a sturdy building built in the 30’s. No humidity issues. Loudness: I work 90% of the time between 60 and 70 Db (occasionally higher at mixing stage) but my loudness is luckily not the main issue since the adjacent areas are part of my apartment (except the street side of course). My main concern is structure-borne traffic low freq. rumble coming from the street (right side of room), and some occasional footsteps and door slamming noise coming from adjacent main entrance of the building (front of room). Sound level meter testing revealed a peak volume of 50db of street traffic rumble. Main peak at 63Hz, lower peaks at 31,5Hz and about 80Hz due to the room’s length of 540 cm (17,7 feet), confirmed by John’s Fundamental Frequency Calculation page on the SAE site. Existing structure details: - walls: tuff bricks* and mortar with plaster finish. stethoscope test: street noise clearly heard (especially the window wall towards the street). Adjacent areas are bathroom (rear), street (right), building’s main entrance staircase (front), hall (left). *: tuff is a porous volcanic stone, very popular as a building material here in Rome a while ago. - ceiling: hollow clay bricks and mortar with plaster finish. stethoscope test: some street noise heard (little less than the walls). The above 1st floor is mine too (two bedrooms) but if I do not suspend the ceiling I guess I should be able to avoid flanking to and from the existing ceiling. - floor: marble surface, unfortunately not sure of what’s layered underneath, probably more mortar and hollow clay bricks. Empty basement below. stethoscope test: very little street noise heard. This is a pic of the empty basement that’s beneath the room: Room’s existing features: - Two big double paned (with thick laminate acoustic glass) windows facing each other (one external and one internal) on the right wall facing the street. I know I know… I think that makes it a 4 leave system but unfortunately I still wasn’t aware of that at the time :( (window on the right is surely a room symmetry issue so I guess I’ll hang a heavy draping in front of it while I mix). - One hollow wooden door on the left wall (I’m planning to remove it and go for Rod’s superdoor design). - One old style cast-iron radiator (not removable) just beside the door on the left wall. Budget: 5000 € (7000 USD) for construction, can raise a little if necessary. No extra working gear needed. ------------------------------------------------------------------------------------ And here are a few questions, if anyone can kindly spare some time. I’ve really tried to research as much as possible before posting them, please tell if some of these are clearly answered in some other post that I’ve missed, in that case my bad… Q1- The walls. First of all, do I really need to go for the room-within-a-room approach to stop this amount of traffic rumble from getting trough or would that be overkill? My guess would be that I should but some expert advice would be so welcome… My goal is as close as possible to a dead silent CR so whatever I’ll need to do I’ll go for it. As I said since space is an issue I’m thinking of using John’s inside-out wall layout. I’m wondering if one of these two versions could be enough to keep that amount of low freq traffic rumble from getting inside while saving a little more space… (pics do not include cloth finish, slot resonators etc.) The first is a thinner version I’m considering with just 50mm (2”) airspace and one single gypsum layer. The second is a thinner still version with 2 layers of gypsum and no external rockwool layer. I’ve tought of this one ‘cause I gather that since my specific problem is mainly lows I should concentrate on mass rather than on highs absorption (oh dear is that very wrong :???): ------------------------------------------------------------------------------------ Q2- Inner/outer leaf flanking issue. As you can see in the 2 pics below there is a major problem, as a matter of fact… please don’t laugh!! :oops:…. Unfortunately I cannot get to have a complete “room within a room” all around the perimeter on account of the 2 heavy double paned PVC acoustic windows on the right. I’ll explain myself… I installed the 2 windows a year ago just before finding this precious forum… dòh! The internal one is mounted on a timber structure that stands in a cavity of the existing right wall. You can see this earlier post for a clearer view if you wish: viewtopic.php?f=2&t=12319&p=92648&hilit=pelmet#p92648 Since the internal window (the one resting on the timber structure) weighs up to 220 Kg (480 lbs! :shock:) I had to secure it to the existing wall, and for safety reasons I also bolted the window’s PVC frame top to a heavy steel beam that was sunk in the brick wall just beneath the ceiling, thus making it completely part of the outer leaf. I did not even consider the idea of installing it on the internal stud wall leaf, if I did so I should in any case have secured the top of the window to the ceiling somehow, thus coupling the whole internal structure with the building… Even if I got a third lighter window on the internal studwall I guess that would make it a triple leaf system, unless it is already a 4 leaf since the two windows are double-paned… “So why don’t you just seal the old ******* window off?!?” you might say, well… being able to work while looking at the sunset is the whole point of this project, I think you can understand what I mean 8)… So in any case I still need to seal the gap (50mm (2”) wide) between the new internal studwall and the outer existing wall where the window begins and ends… I’ve made lots of research on this outer-leaf/inner-leaf flanking issue and was quite relieved when finally I’ve read Rod Gervais (talking about minor flanking issues on his superdoor layout) saying this: “…tests have proven that a through jamb (in my case the connection between the stud wall and the window frame? editor’s note) does not effectively lessen the total isolation value of a wall assembly to any great degree. So don’t worry about any minuscule amount of isolation you may lose. Just build the frame straight through the cavity.” So when the internal walls reach the borders of the window I was thinking of making a connection using some decoupling material, maybe a “jamb” made of neoprene and/or rubber batts and lotsa caulk or… any other suggestion? ------------------------------------------------------------------------------------ Q3- Bolting the walls’ bottom plate to the floor. Do I absolutely need to bolt the steel wall’s bottom plate to the marble floor? Or can I make the walls & ceiling a free standing structure? I guess I could temporarily brace the top plates of the inner stud walls to the outer existing walls one at a time while building and raising the frames until all 4 are connected and thus supporting eachother and subsequently taking the braces off. I saw Lilith Envy doing something similar (using wood frames though) and using just one bolt every 3 meters... I wouldn’t mind doing the same. That said… can I completely avoid bolting the walls trough the floor thus ruining the old marble surface? I mean even if code allowed this do you think it’s ok to avoid bolting? If not no prob, I’ll put some bolts like Lilith has done, better safe than sorry :) ------------------------------------------------------------------------------------ Q4- The non floating floor. Since the stethoscope test revealed a very faint street noise heard trough the marble floor do you think I could really avoid a proper floating floor? I’ve read Sharward’s famous post regarding floating floors and I’d rather avoid building one unless you think it’s mandatory for my situation. I had the chance to get a lot of this 20mm (0,7”) thick ecorubber (1x1meter mats of a high density (700 kg/m3) vulcanized elastomer) for free from a dismantled studio and I’d like to use it someway. Could a 20mm (0,7”) thick layer of this ecorubber resting on the marble surface with wooden tiles on top (as cosmetic finish) be of any advantage? Of course only if I can avoid bolting the wall’s bottom plate to the marble floor ‘cause I gather that that would nullify any decoupling. I guess this ecorubber is too rigid to work as a spring like standard neoprene pucks, but could it be of any advantage even for a minimum decoupling? ------------------------------------------------------------------------------------ Q5- Steel frame gauge. There seems to be a lot of different info over the net regarding the translation of steel frame "gauge" (USG?...) into mm, I’ve looked around a lot and I’m still not sure… Here in Italy they’re not very familiar with “gauge” as a measurement unit and, you know, I wouldn’t like to go guessing on such an important parameter :) Could anyone confirm that the 20 gauge load-bearing steel you frequently talk about is 0,037 inches (0,95mm) in thickness? Dear Aaronw, you used 20 gauge steel framing in your studio, do you recall that being 0,037 inches thick? Thanks… Maybe you all refer to this (USG)? http://mdmetric.com/tech/steelthk.htm ------------------------------------------------------------------------------------ Q6- Acoustic treatment considerations. Rear: I think I shouldn’t diffuse in the rear, I’ve read John recommends to diffuse in the back only if rear wall distance from listening position is greater than 3 meters which is barely my case so do you think the internal rockwool layer of the rear stud wall should be enough as a highs absorber? For rear bass-absorbing instead I’d rather avoid (unless mandatory for my situation) proper and wide acoustic hangers for heavy rear bass treatment due to limited size of room and especially for the position of the door in the rear left corner. Do you think that single superchunk on the rear right corner is too miserable for the job? Front: I guess the inside-out wall should be enough or should I beef it up a little more? ------------------------------------------------------------------------------------ Q7- Can anyone kindly point out some good steel frame ceiling span calculator? There’s lots for wood frames but really couldn’t find one for a steel ceiling… ------------------------------------------------------------------------------------ Thanks for reading this far guys, hope the info was clear enough and that the post met the forum posting requirements… Please do let me know :) Pat

Hi Pat, and welcome! Wow! Great starting post! It's refreshing when someone does actually read the stickies, and do everything that is asked, and them some! Here's my answers (for what they are worth) to some of your questions:

First of all, do I really need to go for the room-within-a-room approach to stop this amount of traffic rumble from getting trough

Absolutely. Traffic rumble is all low frequency, so you need to design your wall with that in mind. Meaning lots of mass, large air gap (more on that later), and lots of insulation (more on that later).

The first is a thinner version I’m considering with just 50mm (2”) airspace and one single gypsum layer.

Not good. 2" is probably not enough to get your MSM frequency way down where it needs to be. The normal recommendation here is at least 4", and even more for low frequency issues. The reason is that the resonant frequency of your MSM wall depends on a several factors, with two of the biggies being mass and the distance between the leaves. More mass means lower frequency. Larger gap means lower frequency. Ideally, you need to get that resonance down to half of the lowest frequency that you need to isolate. In your case, that means a big air gap and lots of mass.

The second is a thinner still version with 2 layers of gypsum and no external rockwool layer.

Even worse! Losing the insulation in the cavity will cost you at least 4dB of isolation, probably more like 10 dB, or maybe even more. The entire cavity should be filled. The more you fill it, the better results you get. Insulation accomplishes several things at once, but mostly it damps resonance inside the wall (flutter echo between the leaves, etc.), and drives down the resonant frequency of the MSM system. You do want both of those!

I gather that since my specific problem is mainly lows I should concentrate on mass rather than on highs absorption (oh dear is that very wrong :???):

It's not really wrong, it's just that you have to think of this not as a bunch if individual pieces each doing its own thing, but rather as a system working together, which each pieces has an effect on the whole. In fact, an MSM wall is a bandpass filter, which does a terrible job of isolation at its resonant frequency, but a much better job at high and lower frequencies. Anything you do to the wall will change that frequency, driving it either up (bad!) or down (good!). taking away mass drives it up. Reducing the air gap drives it up. Taking out the insulation drives it up. Adding another leaf will drive it up. It's a tuned system, and you need to tune it as low as possible by choosing the correct mass, gap and fill.

Even if I got a third lighter window on the internal studwall I guess that would make it a triple leaf system, unless it is already a 4 leaf since the two windows are double-paned…

Ouch! Yup, it is already a 4-leaf system, and since the glass isn't very thick, the resonant frequency is probably way up there, where it should not be. Adding a 5th leaf would drive it even higher. Did you ever put your stethoscope on the glass, to see how much noise is getting through? My guess is that you'll be surprised...

So when the internal walls reach the borders of the window I was thinking of making a connection using some decoupling material, maybe a “jamb” made of neoprene and/or rubber batts and lotsa caulk or… any other suggestion?

I'm not so sure that would work, because Rod is not talking about 4-leaf double-pane windows made of thin glass and mounted in a barely sealed PVC frame, but rather he is talking about his extremely massive single-leaf super door mounted in an equally massive triple-sealed wooden frame! That is two rather different animals, I'd say... I suspect (but I might be wrong) that if you mounted it as you suggest, you'd end up with sound still coming through that window, either through the 4 or 5 leaf system directly, or through flanking, or through the lousy seals, or through the PVC frame. If it were my room, and I needed really good low frequency isolation, I think I'd just bite the bullet, bang my head on the wall, yell and scream in frustration for a few minutes, then figure out (at the very least) a way to de-couple that inner window from the outer leaf and make it part of the inner leaf, or even better, I'd take it out completely, along with the other window, and replace them both with proper thick laminate glass windows, correctly separated and installed... But that's just me. One thing that dis strike me is the weight of that window. 220 kg is pretty massive! Do you have the specs of the glass and construction of that window? I'm wondering why it is so heavy...

Do I absolutely need to bolt the steel wall’s bottom plate to the marble floor?

Isn't Rome in an earthquake zone? Where I live (Chile) it would be insane to NOT bolt a wall to the floor. Maybe in places that never, ever get even tiny tremors you could get away with it, but not here. Even if code allowed it, I still would not do it, since I plan to spend a lot of time inside that room, and it will be my head that it all falls down on if things aren't firmly attached where they should be. Not sure about Rome, but I certainly would not risk it here. I kind of like my head the way it is right now. I don't think it would work any better with dents and holes in it...

Rear: I think I shouldn’t diffuse in the rear, I’ve read John recommends to diffuse in the back only if rear wall distance from listening position is greater than 3 meters which is barely my case so do you think the internal rockwool layer of the rear stud wall should be enough as a highs absorber?

Sounds good to me!

Do you think that single superchunk on the rear right corner is too miserable for the job?

Yep! It's a small room. It will need lots of bass trapping. a single superchunk in one corner is nowhere near enough. I'd go for full superchunks in at least 4 of the 12 corners in the room, then test it and see how it is doing, to see if you need to go for more corners too. For the front of the room, you might want to reconsider flush mounting (soffit mounting) your speakers. To my way of thinking, it simplifies the entire front end, and has many benefits. Come to think of it, I don't think I've ever seen any of John's own designs WITHOUT soffit mounted speakers. As far as I recall, they all have them. That's a good enough reason on its own! Anyway, that's my $0.02 worth. Hope it is useful for you! (Of course, others here might well disagree with me here, in which case I'll give you a discount and drop that to only $0.01.... :) ) - Stuart -

Dear Stuart, thanks a lot for the effort you've put in answering my questions, I really appreciate it :) :) And your explanation of low freq isolation in reference to the importance of both air gap and mass was really enlightening to me :). So one more question on this matter, if I may... Considering that in my room the lows of the traffic rumble have a PEAK volume (my bad, I initially wrote "average" in the intro) of "only" 50db could that allow me to have just a small (2") air gap between existing wall and inner leaf? Or should I have a 4" air gap even if it was, say, 30db? I mean, can the thickness of the whole inner leaf assembly be proportional to the amount of rumble volume it has to stop?

Soundman2020 wrote:

One thing that did strike me is the weight of that window. 220 kg is pretty massive! Do you have the specs of the glass and construction of that window? I'm wondering why it is so heavy...

You're right, I should have been more detailed regarding the nature of the windows :oops: ... Actually they are thick laminate double paned (or quadruple since every pane is made of two glasss layers coupled toghether by a thin soundproof film) with argon gas in the air cavity: The internal one (the 220 Kg one) one has the following glass layering: 8mm+8mm (glass), 12mm (air), 4mm+4mm (glass). The PVC frame has 5 separate internal air chambers so to improve thermo-acoustic isolation. That's why it's so heavy... it's pratically bullet-proof :lol: :lol: :lol: I wish I saw this forum before wasting all that money :roll: In fact stethoscope test on glass is completely silent, while on the hollow PVC frame some noise can be heard. External window is the same only glass is a little narrower.

Soundman2020 wrote:

you might want to reconsider flush mounting (soffit mounting) your speakers

I know, you're quite right, but I'd really prefer to do it later on. Also I'll be changing my old pair of Genelec 1031 in future so I'd like to design the soffit directly on the new monitors.

Soundman2020 wrote:

Quote: Do you think that single superchunk on the rear right corner is too miserable for the job? Yep! It's a small room. It will need lots of bass trapping. a single superchunk in one corner is nowhere near enough.

Right, actually I was planning to put one superchink in all the 4 main vertical corners, unfortunately the room's door is right next to the lower left corner so the rear will have to be asymmetric someway... Any idea of where to find a span calculator for a steel framed ceiling? thanks again Stuart :) Pat

I mean, can the thickness of the whole inner leaf assembly be proportional to the amount of rumble volume it has to stop?

It's not just a matter of "how much", but rather also of "what frequency". If you make the gap thin, your MSM resonance will probably be way too high to be useful. Don't forget that the wall is transparent to sound at the MSM resonant frequency. In fact, it is MORE than transparent: it actually helps sound get through. Above and below that frequency, the isolation improves slowly. At 1.4 times the resonant frequency, you only just start getting isolation. At 2 times the resonant frequency, you start getting reasonable isolation. You can do the math yourself, and figure out the theoretical resonant frequency of various alternatives that you come up with, to see what works best. It isn't that hard for just two leaves, but for three leaves the math gets nasty....

You're right, I should have been more detailed regarding the nature of the windows :oops: ... Actually they are thick laminate double paned (or quadruple since every pane is made of two glasss layers coupled toghether by a thin soundproof film) with argon gas in the air cavity:

Ahhhh!!! Now THAT would explain it! So it isn't just plain old "thermopanel" double glazing, which is what I was assuming. That makes sense! So it's thick laminate on both sides, with a thin air gap. That should help quite a bit, actually. It would be really good if you could figure out how to separate that from the outer leaf and make it part of your inner leaf. Is the window operable? By that, I mean can you open it, or are the panels fixed and sealed in place? From the photo it looks like it might be operable.

Any idea of where to find a span calculator for a steel framed ceiling?

Nope! But maybe Brien can help out with that one? He'll probably know.