Brick outer leaf / drywall inner leaf.

[Soundman2020]

Hi there "Leland2", and Welcome!

It would be better for you to start your own thread about your build, rather than post questions on somebody else's thread.

I was planning on floating an inner stud wall

How are you planning to float your wall? Floating a wall is just as complex as floating a floor, and you very likely do not need to do either.

Below this frequency the TL drops by half from around 61 db to around 31db

Ummmm... there's something very wrong with the way you are doing your calculations. Below MSM resonance, TL should rise again, as the stiffness of the wall becomes the dominant factor. And I can't see how you could get a change of over 30 dB in the space of just half an octave! Plus, I very much doubt that you would get 61 dB of isolation at 34 Hz. Something doesn't add up here... I suspect that there's something wrong with your calculations, or the way you did them...

This does seem extreme, and it would seem that only the kick drum range would be an issue..

What type of kick drum are you using that has a fundamental round 34 Hz? That's some kick!

This all seems very adequate for my purposes,

If your wall system will have an MSM frequency of 34 Hz, then it won't isolate at all below 48 Hz, and only isolates decently above about 70 Hz. Are you sure that's OK?

How can one ascertain the point where increasing the air gap or 2nd leaf density will be compromised by other considerations?

That depends on what "other considerations" you are talking about! For example, if you are talking about failing to seal the cracks around the edge of the wall properly, that would cost you a large chunk of your isolation, regardless of how big the air gap is.

How does one account for flanking transmission when looking at the design of such a wall?

That's easy! Don't allow any flanking! Assuming that you are building on a nice firm solid concrete slab-on-grade, and that you take care to ensure that no part of the inner leaf touches any part of the outer leaf, then flanking is not an issue.

In my case I also have windows that are 8' x 4'. I suspect those will have a bigger effect on my max TL in the room than the wall structure.

Then you will need to do the math for that too! You will have one sheet of glass in the outer leaf, and another sheet of glass in the inner leaf. Do the math to make sure that the MSM frequency and isolation will be the same for that part of the wall, as it will be for the rest. If not, increase the mass of the glass, or the size of the air gap.

As an aside, I wonder whether floating the inner stud wall is even worthwhile

As I mentioned above: trying to float a wall is just as hard as trying to float a floor. Here's why: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173 The exact same applies to walls.

I could put studding directly onto the brick much more easily - and the cost of floating the wall versus the extra TL is hard to calculate in this case.

I don't understand what you are trying to say there: If you attach your inner-leaf studs directly to the outer-leaf wall, then the entire wall is flanking, and you have a fully coupled 2-leaf system, which isolates like crap! (excuse my French). It's pretty easy to calculate the isolation of that: use mass law alone, as though it were just one leaf. And I don't understand what this has to do with floating your wall: if you attach it to the brick, then why would you want to float it? And if you DON'T attach it to the brick, then floating it is probably not going to do any good, for the reasons outline in that link. Firstly, how would you float it? I assume you mean on some type of rubber pads. If so, how would you make sure that you get the correct deflection for that type of rubber, along the full length of the wall? Can you be sure that the deletion will be constant, and not be either too great or too small at some points, which would negate the floating completely, and leave the wall coupled to the floor again. Etc. It's far better to just have your wall resting directly on the floor, without attempting to float it. As long as your floor slab is suitable, this is the best method for isolating a room. Make sure it is properly decoupled (not touching the existing outer-leaf walls, or the outer leaf ceiling), with enough mass on it and enough air gap, and you will be fine. Floating a wall is not necessary unless you have very extreme isolation requirements: Decoupling is all you need.

The cost can be calculated, but I don't think you'll like the answer: as Greg pointed out, floating a wall is an expensive thing to do, and complex as well. Floating the entire room makes more sense, but is VEEEERRRRYYY expensive, and even harder to do. Take a close look at that thread that I linked you to above: it's about floating a floor, but the exact same concepts and principles apply to floating a wall. I would not recommend that: just decouple your inner-leaf from your outer-leaf, check that you have enough mass and air gap to get the frequency and isolation you need, then you should be fine.


- Stuart -

[Leland2]

I can see I need to be more cogent! I do not intend to hijack - I was mostly interested in the brick/studding combination and the calculations for TL thereof.

When I said float the walls, I actually meant decoupling in the vertical plane - ie they float in front of the extant walls. My apologies. For my situation, I have already established that floating the walls in the horizontal plane (the floor/ceiling) would be expensive and also pointless given limits caused by other conditions in the building. Hence my interest in flanking conditions - if the wall is in contact with the outer floor and ceiling, how much will one benefit from decoupling the walls in the vertical plane only? Obviously, the floor/ceiling construction method and materials would be pertinent to that conversation.

The figures I quoted came from inputing my specific values into the spreadsheet provided - I must say I was surprised myself as it didn't really match my empirical experience. Hence my conclusion that other factors must come into play. Perhaps I'm misinterpreting the figures. (My inputs - leaf 1: 225kg/m2 - leaf 2: 15kg/m2 - and a 4.5" gap)

I'd be interested to know the source of the equations - can anyone provide a reference?

As an aside - as I did not want to hijack the thread:

The sheet material I mentioned is called Versapanel - a form of concrete particle board. It is much more expensive than plasterboard, but has the benefit of being moisture resistant and reusable. There are specific problems using plasterboard in this building - I've already had to remove something like 3000 ft2 of it. And it is very hard to dispose of here - it is classed as a toxic waste and requires specialised recycling which is expensive and difficult to access.

My reference to the windows as a limiting factor for TL is that doors and windows are traditionally a weak point for TL. There is little point in having amazing wall construction if the weak point negates all that effort. Extant 8'x4' wooden framed casement windows are not exactly high efficiency when it comes to TL, even if I put 12mm laminate glass as a secondary glazing in the inner wall of the room.

FYI, this is not a new building - it is an Edwardian age building in the UK not built to anything like modern standards. It has specific problems for which the solutions in its repair and conversion are very different to standard solutions for modern buildings . If I was going for supreme isolation, I suspect the only solution is knocking the whole thing down and starting again, which is not possible! Having said that, I'm sure I can get its isolation to a better level than it is currently!

I will in due course start my own design thread. It wasn't my intention to hijack!

[Soundman2020]

I can see I need to be more cogent!

Right. When discussing technical subjects, it's better to use the correct technical terms, rather than colloquial terms.

When I said float the walls, I actually meant decoupling in the vertical plane - ie they float in front of the extant walls.

Technically, that is "decoupling", not "floating". Floating, as the name implies, is raising something up vertically so it is no longer in contact with the ground below, just like water floats a boat. When you separate things away from each other horizontally, you are not floating: you are decoupling.

I have already established that floating the walls in the horizontal plane (the floor/ceiling) would be expensive and also pointless given limits caused by other conditions in the building.

The limitations of the bilding is not the only reason whey it would be pointless and expensive: here are the other reasons why floating a wall or floor would be expensive and pointless: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173 .

if the wall is in contact with the outer floor and ceiling, how much will one benefit from decoupling the walls in the vertical plane only?

Let's rephrase that in the technical terms, so that we are all on the same page, then I'll answer: If the inner-leaf wall is in contact with the outer-leaf floor and ceiling, then how much benefit do you get from decoupling the inner-leaf wall from the outer-leaf wall". That's the question you are actually asking.

First, the inner-leaf wall should NOT be in contact with the outer-leaf ceiling! It should only ever be in contact with the floor, since you are not going to float your floor, for all of the good, valid, and correct reasons we already mentioned. So your inner-leaf wall will be resting on the concrete slab, just like the outer leaf wall will be, and that is the ONLY point of contact. The flanking limit for a situation like this is somewhere around 60 to 70 dB, depending on a few factors, but assuming you have a fairly massive concrete slab-on-grade, then closer to 70. Of course, it's highly unlikely that you can get 70 dB isolation for the rest of the structure, and therefore the flanking through the slab is not an issue. You'll never get to that limiting level, so it's a moot point.

However, if you connect your inner-leaf wall tops to the outer-leaf ceiling, then you place a much lower limit on total isolation, since your wall is now flanking at BOTH ends (top and bottom), and is now fully restrained in 2 dimensions: it can no longer vibrate freely at the top, restrained only by the floor. There's a lot more factors to take into account here, but your isolation limit is probably around 60 dB now, absolute best case, and probably more like 50.

But if you attach your inner-leaf wall directly to your outer leaf wall, you have basically trashed any chance of having good isolation. The inner-leaf is now fully restrained in all dimensions, and any vibration on that leaf is directly transmitted to the outer-leaf, this placing a very low limit on isolation. This is now a fully-coupled system, so isolation is minimal. Maybe 40 dB limit, best case, and probably not much more than 35 or so in the real world. A typical house wall gets maybe 30 dB, so not much better than that.

In addition, since the air gap is now small, there's the real possibility that the MSM resonance will be fairly high, thus placing a further limit on isolation.

My inputs - leaf 1: 225kg/m2 - leaf 2: 15kg/m2 - and a 4.5" gap

From my calculations, that would get you around 52 dB isolation, theoretically, if you do everything right. HOWEVER! It would not be good for low frequencies. The MSM resonant frequency of that system (asssuming you fill the cavity with suitable insulation) would be around 35 Hz, which means the wall does NOT isolate below 48 Hz, and only isolates reasonably starting at 70 Hz, with good isolation above 104 Hz. So, that's not much good if you plan to have drums, bass, electric guitar, keyboards and things like that.

I would suggest that you increase the air gap, and increase the mass on the inner-leaf. Double your mass, increase the air gap to 6", and you'll be getting more like 60 dB isolation, MSM f0 will be around 21 Hz, isolation starts at 30 Hz, is good at 42 Hz, and great at 64 Hz.

Of course, all of the above assumes you will fully decouple your walls, except from the slab.

My reference to the windows as a limiting factor for TL is that doors and windows are traditionally a weak point for TL.

Sort-of true. But not necessarily. Doors and windows are only weak points because of the seals: if your windows are non-operable (fixed in place, and cannot be opened), sealed very well, then the exact same equations apply to those as to the rest of the wall. So it is merely a matter of choosing the right glass and air gap in order to achieve the isolation that you need. It's not complicated.

There is little point in having amazing wall construction if the weak point negates all that effort.

True, which is why it is so important to seal EVERYTHING perfectly air tight. Including the surface of your brick wall. Brick is porous, unlike glass, so it is entirely possible that unsealed brick could be a bigger detriment to isolation than a properly sealed window. Seal it with any good quality masonry sealant, before you start on the inner leaf. Air-tight seals are critical.

Extant 8'x4' wooden framed casement windows are not exactly high efficiency when it comes to TL,

The are exactly as efficient as their surface density (assuming that the frames are in good shape, non-operable, and properly sealed. If the surface density is too low, then replace the glass with higher density stuff, such that the MSM f0 is hte same as for the rest of the wall.

even if I put 12mm laminate glass as a secondary glazing in the inner wall of the room.

12mm is rather thin for your situation. I haven't done the math but I would guesstimate that you'd need something like 22mm on the outer leaf, and 16mm on the inner leaf, with an air gap of maybe 7" at least, to be able to get close to matching the rest of the wall.

If I was going for supreme isolation,

You didn't mention that yet, and it is the single most important number of your entire design: What is your isolation goal, in decibels? How much isolation do you actually need? Without knowing that, then all of the above is just academic chit-chat, and meaningless to your design. The entire design is based on isolation. That dictates many other aspects of the build, in a sort of "domino effect", way down the line, even in areas you wouldn't really think are related. But the are.

I'm sure I can get its isolation to a better level than it is currently!

Sure, but what IS that level? It's the key to your entire design...



- Stuart -