Three leaf problem, again

Hi again, Got my first ever post whacked by the crash, so I'll try again. I'm designing a stuido to be built inside of a 200 year old church - big spaces, high ceilings, big windows. I've read about the 3 three leaf problem here and elsewhere but am still slightly ( ok, really) confused. The outside walls of the church are wide cedar clapboards over 1x pine loosly spaced, over studs, with plaster and lath on the inside. I figure my first step is to build a second wall inside of this one, floated and isolated from the outside wall. Is that outside wall considered one leaf? And the windows, which I would like to frame my new wall with glass openings to allow natural light - the church wall has old glass, 12 over 12 and maybe 20 year old storm windows outside - not airtight. One leaf or two? How does this leaf thing work? Are four even worse than two, or can you build them in pairs? Thanks again - Ollie

Hey Ollie, welcome to the board - I'll see if I can clear up the "leaf" thang just a bit - Mass, air, mass is what you want for optimum use of materials when sound proofing (a relative term, BTW) Mass is basically anything you can NOT blow through - even styrofoam insulation, by dint of the fact that it's closed cell, is technically mass; just not very GOOD mass, since it would take several FEET of it to = a typical gypsum wall. Air is basically anything you CAN blow through - rigid fiberglass insulation, actual air, rockwool or mineral wool insulation, fluffy fiberglass insulation, blown in cellulose; again, if you can BLOW through it, it's "air". A "leaf" is ALL the layers of mass on one side of a frame (if there IS a frame), these layers MUST be tightly spaced (no air gaps, not even 1/16") - otherwise you set up a mass-spring-mass resonance (air is a spring, and a pretty stiff one) - this will add another resonant point to the assembly, creating a weak spot in the frequency range at resonance. So, a typical "leaf" would consist of (say) 2 or 3 layers of gypsum wallboard, each at right angles to the last layer, fastened at frame members and fully mudded and taped before the next layer - screws are the best fastener, and the ONLY fastener for ceiling work. Your church walls, starting from the outside, would be one (very leaky) leaf consisting of the "wide cedar clapboards over 1x pine loosly spaced" - this will be a problem for sound isolation already; each "leaf" of a wall needs to be hermetically sealed, or you lose a LOT of isolation, especially at lower frequencies. Then, you have studs - this means an air cavity, whether it has insulation or not - if someone has retrofitted newer stuff here, there might be closed cell insulation in the cavities - if so, that will act as a leaf of mass. if not, you still have your leaky outer leaf, your air gap, and your "plaster and lath on the inside" - With the inside being lath and plaster, I doubt very much if you have the "wrong" kind of insulation; you might have NONE if the building's that old, or the walls could be filled with old newspaper, rat droppings, old socks, etc... Anyway, technically you already HAVE two leaves and an air gap; but it sounds like it's a pretty leaky setup, so depending on your degree of daring, you may be able to test for what resonant frequency your walls already have, and then design a NEW double leaf partition inside, making sure that the new partition's resonant points are NOWHERE NEAR the old ones; this wouldn't be a perfect solution, but could work pretty well if you don't want to (or cant) remove the lath and plaster and start over. " And the windows, which I would like to frame my new wall with glass openings to allow natural light - the church wall has old glass, 12 over 12 and maybe 20 year old storm windows outside - not airtight. One leaf or two?" - following the comments above, even a 1/16" gap between layers makes it 2 leaves - I have no way of calculating the frequency of resonance with that narrow a gap, but it WILL be in the audible range; very likely right where the crack of the snare drum will bleed through. Things that improve a mass-air-mass system - more mass, slightly DIFFERENT mass in each leaf, wider air gap, insulation in the gap (not for windows :? ), decoupling of the two leaves (staggered studs, resilient mounts, separate inner and outer frames, in order of effectiveness) ; I've probably forgotten a point or two, I'm sure one of our regulars will point it out :wink: Hope that helps... Steve

Steve, OK thanks, that's starting to help. These walls, by the way, are only exterior walls - I'm trying to deal with traffic noise outside, then I'll move on to isolation between control room and studio, plus good room acoustics. But it sounds like, if I read you correctly, my outer church wall is a (leaky) two leaf system - so my choices are: build an additional one leaf wall inside, with insulation between it and the outer wall, but this would be bad because then I've got three leaves, or build a new two leaf wall but make sure its resonant weaknesses don't match the outer church walls's. And the same with the glass - go with two layers of different thickness, spaced as much as possible? Many thanks in advance - this is going to be a pretty big project - (I might need to consider hiring John although I'm not sure how that works) so I suspect you will hear from me many times. Hope I don't drive you nuts. -Ollie

Yeah, sounds like you're starting to get it - if you want more personal help, you can PM John, and he'll explain how everything works... Steve

isolating traffic noise would mean that you don't have to worry about low frequencies as much as with music noise. Maybe you could seal the inner wall and then built a new leaf in front of it? As with the glass issue: I'm intrigued by the question: when does it pay to rip the whole leaky construction down and rebuilt, and when could you just put a new leaf in front and that would be sufficient? Would be nice if you could calculate that.

Extra factor - the local Historical society does not want me to change anything on the exterior of the building, and that would include the windows, so I'll have to work with what I've got there. I have a feeling these windows are so loose and thin and full of leaks that they might almost be considered to not be there at all, for design purposes. Maybe I could just get the town to move the road? Thanks, Ollie

bert Stoltenborg wrote:

isolating traffic noise would mean that you don't have to worry about low frequencies as much as with music noise. .

Could you explain this please? The OITC noise curve PEAKS in the low frwquencies. Always learning; Andre

I think that since human hearing has no ABSOLUTE reference levels but instead gets accustomed to whatever ambient levels are present, that most of us get inured to the constant rumble of traffic; I live about 10 miles from the nearest town, however, and REALLY notice when a large truck rumbles by(since I'm NOT being constantly inundated with it) Ollie, before we get much further into this we REALLY need to know what your existing floor construction is; if you are to add massive inner walls (to please the town fathers in their quest for history) then your floor construction becomes VERY important for safety... Steve

A little hard to describe this place, except to say- overbuilt, in general. The beams in the tower, for instance, are about 10x12 inches, posts almost as big. The second floor of the church was originally a balcony with an opening in the center about 25 feet square. This balcony was supported every 12 feet by posts running down to piers in the basement. Around 1900 someone filled this center square in to make two separate floors, with the sanctuary now exclusively on the second floor. New posts on the first floor support this added floorspan. Presumably, all of this was designed to hold up to several hundred people, but I will have an engineer figure the live load figures for me before I build. Right now, I'm strictly at the gather information and try some designs on paper stage. Once I have a workable design, I figure I'll add up all the weights and show them to the structural guy. I'll be asking you about floated floor types, with the idea of balancing all of this in terms of effectiveness, money, weight and any other factors all of you tell me to watch out for, too. Thanks for looking out for me. This is my first big project like this and I'm grateful for the help. -Ollie

Hey André, over here we weight for sound isolation: standard spectrums noise Dutch normalisation standard NEN 5077: 125-[removed]-2000 Hz respectively: road traffic -14, -10, -6, -5, -7 dB train -27, -17, -9, -4, -4 dB air -21, -11, -7, -4,5, -6 (normal outside sound = like road traffic.) music -14, -9, -6, -5, -6 disco -8, -8, -7, -7, -9 Eric Desart has developed spectrums in third octave bands based on these; he also includes lower frequencies. F.e. the university of Leuven uses integrated (one number) ratings for measured constructions based on these standards. The standard spectrum music is developed in 1988, as we from the music scene know, there is since then more emphasis on low frequencies, so it would be wise to use the disco spectrum to calculate isolation performance. Looking at the isolation realized by some constructions measured by the Leuven uni you see rather large differences between music/disco and road traffic numbers; the music numbers are far worse (depending on construction, but always worse). Hope I didn;t make to many mistakes :D

Thanks for the clarification Bert. The OITC I referrd to goes down to 80 HZ in 1/3 octave bands. Eric, or you (I forget which) wrote abotu the Dutch standards being leading edge at any given time, but as knowledge grows, the leading edge of development changes, so yesteredays best becomes today's sort of acceptable. BTW for people not catching all the subtleties, the ASTM standards are worse in applicability, with there being NO music related spectrum! Happy New Year! Andre

AVare wrote:

bert Stoltenborg wrote:

isolating traffic noise would mean that you don't have to worry about low frequencies as much as with music noise. .

Could you explain this please? The OITC noise curve PEAKS in the low frwquencies. Always learning; Andre

That traffic noise peaks in low frequencies does not by itself tell us anything, as human hearing is less sensitive down there. Distance matters. A road 20 feet away will be a fairly balanced spectrum, whereas a road less than 10 feet away, with a decent mix of trucks, will be heavier in the low freqs. If the road is a decent distance from Ollie's church, I'd be inclined to agree with Bert. PS: Hello everyone, and thank you for this excellent forum, which I have just discovered.

The road is about 35 feet from the front wall of the church. A bell and clock tower block a lot of the front, and probably some of the sound. From inside, I can hear a modest amount of the whoosh part of the spectrum, but not much on the lower frequencies. More worrisome though, I can feel a little vibration in the floor (and this is the second floor where the studio will be) so I'm resigned to the idea that I'll have to float the floors. Hope someone has some tricks for keeping cost down - it's about 2,500 square feet up there. (but I guess that'll be another thread) By the way, so far my experience is that this forum seems to attract only thoughtful helpful people - hope that doesn't sound too dippy. -Ollie

Floating floors are always difficult and expensive. Maybe you should look at the Paul Woodlock studio build diary at the www.studiotips.com forum. He has created a well documented (to say the least) diary about building a floating room. :D

I agree on both the book-length and validity of Paul's labor of love - but keep in mind that if your second floor is ALREADY vibrating, you REALLY want to know what's there (and what you can add, if anything) before you start (that's why I'm really glad you're analyzing the crap outa this) Steve

Yes, Steve. What I meant was that a vibrating second floor means that the whole structure could be vibrating. And as vibrating constructions (walls roof) emit sound, you would have to be sure that floating the floor is sufficient to make things better. Else you have to create a box-in-a-box like Paul did. In this case that would probably be a ridiculous enterprise, IMHO.