Hi Andrew,
You mentioned trying to figure out your third-leaf loss: You probably already have it, but the full equations for 2-leaf and 3-leaf resonance are:
2-leaf-3-leaf-double-triple-leaf-equations.jpg
There's one caveat to all of this: the equations assume that the cavities are sealed air-tight. If that is not the case, then there will still be some effect, but it will be reduced in intensity (depending on how "unsealed" it is). Extreme example: if you hold a small piece of drywall just one foot square, a few inches away from a good 2-leaf wall, then you might be able to detect some resonant vibration in that piece, but it will have pretty much zero effect at all on the wall. On the other hand, if you built an entire leaf in the same place as your "small piece", covering the complete surface of the 2-leaf wall, at the same distance as your "small piece" was, and you left a one square foot hole in that surface, then you'd get a lot more effect on the overall isolation, nearly as much as having a fully sealed leaf...
Yeah, it gets complicated!
How much isolation are you shooting for in your place?
- Stuart –
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Thanks Stuart
It’s an awfully long time since I've had to do any serious maths so its been a bit of a journey reading academic papers.
Thanks for the triple leaf formula - I actually discovered one earlier this afternoon when I found a post from Andre in the green room referencing a paper by Yunikur which outlined the equation - but yours is clearer.
Isolation: I'm expecting inside measurements up to 115dB. My nearest neighbour’s house is 25-30m from my building and I’d like to be at 40dB by the time I hit his back wall. If I take a conservative approach and only use 16m of the gap between the studio and his house and using 4db per doubling of distance I'd need ~60dB at 1m. All planning has been around 70dB TL with the expectation that something will cause an issue somewhere along the way and leave me closer to 60dB.
I'm chasing this triple leaf issue to ground really just to be cautious - this pic should contextualise..
Soundman PM240821.png
Glenn and I have been bouncing around alternatives for my outer wall but for a host of reasons I'm going to stick to the "metal cladding outside" option with my outer isolation leaf on the inside of the same frame as pictured. Intuitively, I'm comfortable that the very low mass of the metal cladding together with the open airflow behind it will cause me no great issues from a triple leaf standpoint but I wanted to try and put some numbers around it even if the formulas I use would produce a worse case than I'd have in practice.
If there was any meaningful impact I was considering whether I should add more mass to the outer leaf vs the inner - I'd seen somewhere else you'd recommended that in similar circumstances.
Andrew
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"If there was any meaningful impact I was considering whether I should add more mass to the outer leaf vs the inner - I'd seen somewhere else you'd recommended that in similar circumstances."It's in the Wyle report at some point. I don't have the reference on hand, but basically their conclusion is that the optimum performance for a 3-leaf system is when the mass on the middle leaf is the same as the combined mass of the other two leaves, and both air-gaps are equal: In other words: d1=d2, and m2=m1+m3, where m1 = m3. I also seem to recall that this is the arrangement required for the equation to be valid, but I'm not 100% certain about that. It makes sense, though.
70 dB isolation is a tall order: it’s about as much as you can hope for in a typical project studio / home studio. so if you are in any doubt as to what mass to use, I would suggest increasing it as much as you can. And I would also suggest using Green Glue between at least two layers on each of the walls. That stuff is pricey, but very much worth it, if you need high isolation. Also, your seals will be critical, and your HVAC system is going to need some careful design... especially the silencer boxes and duct runs. It can be done, though! Not impossible.
- Stuart –
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Hi Stuart
As you can see, I do have some wriggle room on the 70dB
- I really need 60dB – 59dB actually,
- I only used 16m of the 20-25m to my neighbour’s house for decay; and
- The room that has the 115dB max is a further 6m from my neighbour’s house than where I’m measuring from
From having viewed so many projects on this and johnsayers forum I’m acutely aware that studio building is a series of compromises and I'm fully expecting something to come along that forces me to have to give some up TL.
I did see that “doubling the middle leaf mass” observation in Wyles - my problem is that any studies I’ve found assume
- Sealed air gaps; and
- Leaves of reasonably similar mass
In my case I’m working to an MSM of
inner leaf of 37.5kg/m2 – 200mm air (sealed) – outer leaf 37.5kg/m2 – 90mm air (unsealed) – 4kg/m2 “leaf”. There is not much research that covers one unsealed air gap and a 3rd leaf that is ~10% surface mass of the others!
If I use the analogy in your original reply, my case is like building a barrier of 1/8th" wide drywall (if that existed) 3 ½” outside the fully isolated structure. ….and my hypothetical “drywall” is corrugated.
I suppose that in the context of my TL wriggle room if I knew that the worst case was a loss of 6dB in TL I’d just accept the impact and move on.
Andrew
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Hi Andrew,
I wouldn't be too worried about your third "leaf", to be honest. There might be some loss from that, but I can't see it being large. What I would suggest is that you find some way to damp it, so that any vibrations that are induced in it will not be a big issue. Maybe some insulation pressed up against it, or perhaps some type of spray-foam product that doesn't harden and remains soft and rubbery when cured. There's actually a type of "acoustic damping paint" that has some useful effects there:
https://www.atsacoustics.com/quietcoat.html I used that once on a place in North Carolina, and it seems to work, although not as wonderfully as the marketing hype claims! (Surprise!) If you could spray the inner face of your metal siding with that, and put some mineral wool (not fiberglass) in the gap, you should be fine. I don't see this robbing you of a lot of isolation.
For many years I've been working on a spreadsheet that predicts isolation for various 2-leaf and 3-leaf assemblies. It isn0t accurate yet, but it is decent for ball-park estimates. With your 37.5 kg/m2 mass on each leaf, plus your 20 cm air gap, and good seals everywhere, it says you should get 70.5 dB, assuming everything is perfect. It also estimates a "real world" value, based on typical imperfections and workmanship issues, and it says 63 dB for your case. So with a bit of care, you should certainly be able to get over 60 dB. It also estimates f to be around 20 Hz, considering just a 2-leaf system (ignoring your outer siding), and for that 3-leaf system, it estimates F- = 21 Hz and F+ = 28 Hz. As I said, it isn't accurate at all, but it does give you a reasonable indication. It also says you should be getting 41 dB, just from the mass alone, without considering the MSM effect.
So, overall, I'd say you are fine! Good quality materials and attention to detail in the build should get you where you want to go.
- Stuart –
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Thanks Stuart
On the dampening, the most common approach here would be what I have under the roof - something I'm sure you'd be familiar with - Bradford Anticon
https://www.bradfordinsulation.com.au/h ... ng/anticon. It obviously has the aluminium foil on the backing which theoretically adds to the isolation equation but I've pulled that apart and the 2 aluminium layers are tissue paper thin - hence my adding the term "theoretical". However that is classified as glasswool insulation - I'd have to check first whether that is what you call fibreglass.
Andrew