simple window glass question

Hi all. I was wondering about weather I should just use one pane of glass in my new (floating 2 x 5/8" gyp on wood studs) room within a room wall. There is an existing window (2 seperate ones actually) in the original outside wall of my house. One of these is double glazed 1/8" (sealed) and the other is single 1/8" that opens. I was originaly thinking of putting 2 layers in the new inner wall, but this would form an triple leaf system right? So should I just use the one layer with maximum spacing? (I can afford about 9" on the one and about 12" on the other) Also I noticed in some threads here that 1/4" laminated glass is equal to 1/2" regular glass in TL. Would one layer of 1/4" laminated glass be sufficient in my application here? I was hoping to keep the TL of the windows close to the TL of my walls. 1/4" laminated would be nice because it is readily available here without having to special order it from the factory, and is also fairly affordable. Would this work? Thanks in advance for you replies, Les

I know you all are very busy and my question is probably not terribly interesting, but if there is a chance someone could let me know if I am on the right track with the window glass, I'd really appreciate it as I have to start building them in a couple of days and I don't want to make any more serious mistakes. Thanks very much, Les

Les, I apologize - You are headed on the right track - but are not quite there yet. Standard 1/8" glazing is pretty much useless in this respect - you don't mention the size of your window - but if you use 1/4" lami in one wall - and 3/8" lami in the other - you should be fine. This is unless your window is fairly large - and then I would have to rethink this. You do not want 2 panes of glazing that are the same thickness as each other. Rod

Thanks for your reply Rod. Actually one of my best friends is a Gervais. Probably a long lost cousin somewhere in the mists of time. To clarify, the previously exsisting outside wall has 2 windows still in it. One is 3' x 3' 1/8" double glazed and the other is 2' x 3' 1/8" single glazed opening. I realize that both of these are relatively useless and should be replaced (I was thinking of putting it off til later though after I see what kind of performance I get from the inside wall and window. The new inside wall has the same window spaces cut out and I am about to build the windows for them. I had read here that 1/4" laminate has the same TL properties as 1/2" regular glass, so being that 1/2" is more than 1/3rd the thickness of the 2 layers of 5/8" drywall, I was thinking that this would match my new inside wall's TL properties. Am I correct on this? The outside older walls I think I will replace (later) with 1/2" glass which I am guessing would have a different resonance being 2x as thick as the 1/4" laminate, but with equal TL properties. Also, I should stay with one layer on the inside wall, not 2 right? And then the other layer in the outside wall, thereby keeping it a 2 leaf system right?

I think you've got it pretty much; I'm not sure who told you that laminated glass was twice as good - it's better at higher frequencies by a few dB, but mass is the ticket at lower frequencies. Unless you need it for safety reasons or you find a good price, I'd go with thicker float plate. To stick with 2-leaf construction, this means mass, air, mass between you and whatevery you;re trying to isolate. If you have an insulated (thermopane) window, you're DONE. IF you add another leaf to that, it will be 3-leaf and will have (possibly) better TL at mid/high frequencies, but will have (DEFINITELY) WORSE TL at lower frequencies. For music, if you build for the lows the highs will take care of themselves 99.9% of the time. 1/3 thickness of glass (relative to the gypsum in that leaf) is MINIMUM for maintaining consistent TL of the assembly; more is better, but too much more and you're wasting money. Air gap is your friend, increasing that can help even 1/8" glass do a (semi) acceptable job, but it takes at least a foot. Hope this helps... Steve

thanks Steve. Do you mean that a foot is nessecary for 1/8" glass or for the thicker say 1/2" glass? I think I am going to go with the 1/2" on the inside and then possibly 3/8" on the outside. On one window I have about a foot of space, but the other one I only have about 8 inches. We are starting to build them tomorrow at 10AM PST (Tuesday Dec 14)

Actually both; it depends on what your walls are like whether the glass will be able to "keep up" or not. With identical air space, keeping glass thickness to a minimum of 1/3 of the gypsum thickness (40% to 50% is better) will keep the wall performance within a few dB of the window performance. What I meant was, if you ONLY have 1/8" glass, you can compensate somewhat by widening the air space. Case in point - 3/8 and 1/2" glass, 4" apart - roughly STC 46, with approx. 33 dB @ 63 hZ - 1/8" and 3/16" glass, same spacing - STC 45 ??!?, but with only 18 dB @ 63 hZ... Now, widen the air gap from 4" to 8", still the thinner glass; you get about STC 48, with TL of 23 dB @ 63 hZ - widen the gap again to 12", and you end up with about STC 50 and a TL of about 26 dB @ 50 and 63 hZ. Everything in THIS paragraph is using 1/8 and 3/16 glass. Keep the 12" air gap and make your panes 3/8 and 1/2", and you end up with about STC 50 (still??!?) but your low end TL raises to around 40 dB @ 50 hZ - This last example might sound a bit strange, because the TL isn't different enough from low to high frequency to sound "balanced" - that is, if the TL of a barrier doesn't block sound with a smooth rise in TL as frequency increases, you get some sounds that sound louder than other frequencies and it doesn't sound "natural" - because of this, the wall actually sounds like it's not doing its job as well as a flimsier wall might - assuming that the flimsy wall had a more even rise in TL as frequency rises. Hope that helped more than it confused; I probably should have left that last part off to avoid confusion, especially since you're probably already at it... Steve

really great points. thanks a lot for that Steve. Actually were starting tomorrow morning, Tues, Dec 14. I notice that the clock on this site is a day and a half ahead, so that's a bit confusing. one thing I don't understand is your statement about the double glazed pane that's in the one window now (I will be replacing it). I don't understand why if it's double glazed with a 1/2" airspace between the 2 glazes that that would make much difference than a single pane of 1/8". You're saying that it's way worse. I understand that if it was narrowing the airspace appreciably say, several inches in or something. But I don't quite get it why a simple standard double glazed window like that would be so much worse than a single pane of 1/8". I will keep the maximum airspace between the two windows and use 1/2" and 3/8". Thanks so much for the great explanations.

Are there measurements on these kinds of constructions? A thermopane like and a normal glase pane with large airgap compared to 2 normal panes with the same mass? Bert

Hi Bert, welcome - actually, 99% of what I know about glass I learned from Eric, so take it for what it's worth - here is a 3-part comparison using Insul demo (I know Eric hates this, but it's all I've had time to digest - still looking for enough time to digest all Eric's spreadsheets, etc) First example is two 1/8" panes like thermopane, second is a single 1/2" glass ( I "cheat" insul by setting properties to 2100 kg/cu meter instead of 690) and the third is 3/8 and 1/2 glass set 8" apart - beyond this, I have no test data whatever - it's all "smoke and mirrors", but seems to follow the "2-leaf " rule pretty well. Note the m-a-m resonance in the first example; this will be a weak area, and is right at the shell resonance for typical kick drum - not good. Gotta run but will get back to this hopefully later tonite... Steve

Steve, It would be sweet when this INSUL software could be modified to accept particular weightings, fe for a standard spectrum popmusic or discomusic. I am fully behind you concerning triple leafing (or eric would step over the border to kill me :D ), only wondering if a thermopane leaf with a few mm gap in a large gap system is devastating for the overall behaviour. We had this discussion on the other group, and sometimes it isn't to bad. I know some constructions used by professional insuation companies are triple leaf and they have worse low freq behaviour compared to double leaf, but still are overall superior for popmusic because the dips are so low frequent and the higher freq behaviour is superior. I'm asking because sometimes it is convinient to advise a person to keep the termopane and put another window in front on a large airgap in stead of also having to replace the thermopane. regards, Bert

Bert, I agree with that pretty much; as to keeping the thermopane and using a third leaf of heavy glass with large air gap, of course this can be done - it WILL reduce the LF TL at least a few dB (I have no way of calculating this, being completely self-taught with obvious gaps in the process) - my thought is, if you MUST do this, make the extra glass even heavier than you would and the new air gap as WIDE as you can to help overcome the weakening effect of the triple leaf. As you can see from the Insul predictions (which can't do triple leaf no matter HOW I cheat ) the thermopane is going to have a m-a-m weakness in the low mids, which will be passed on even when adding the new pane - I'm not saying that this setup couldn't work, I'm only saying that it is not optimum for the space (total thickness) available, and that for music we should design for best LF because it's the most difficult; then, the mids and highs will basically take care of themselves. This is admittedly a simplistic way of looking at it - for people who would rather argue fine points on alt.sci.acoustics for months instead of making and recording music, I'm a heretic for sure; for people who would rather be making music, probably not - I guided a young man with a metal band in Austin, completely thru building and treating a 20 x 24 foot free-standing building (on concrete pier blocks) for his band to practice/record in - all standard materials, spent several page thread keeping him from making serious mistakes, in a few months he posted and said he'd finished the door(s) and could not hear his double bass kitted drummer from 10 feet away from the door - After the first 3-4 posts, the entire thread was just him and me and thousands of lurkers. I figure from that (and several others since) that I must be doing things at least half-assed OK, but I'm still studying all I can... BTW, Insul (the demo) is quite old and the FULL version, which costs around $1000 or so, is several versions newer and much more complete - I have the files already downloaded, but haven't had time to email Keith at Marshall-Day for the "key" - he's willing to give me a month of active use instead of the usual week or so, once I find time to evaluate it. I asked if they intend to make a "personal" version of the new one, and he didn't think so (the older one like the demo, you could get either a "personal" or a "full" version, for either $200 or $1000 (roughly)) If/when I get to that point, I'll post some kind of "mini-review" - I've not given up on talking them into a "light" version for shallower pockets... Steve

Thanks, Steve. I like the simple/effective approach just like you, that's why I was asking. (I'm just a stupid guitar player/loudspreaker guy, not a professor on doppler distortion/name calling from the science group, although over there are a lot of good guys too :lol: ) Bert

very interesting post Steve, Rod and Bert. thanks so much for your input. I'll let you know how it goes. Les

This is a very interesting conversation. When I get home I'm going to read through it all. I need to bone up on my glass / window knowledge. I'm getting very close to finally putting the glass up in my studio and I've been wondering for a couple years if I'm heading in the right direction. I think I'll be fine. I was thinking about getting 1/4" and 3/8" for the two panes. Unless that would be insanely inpractical. The size of my control room window is roughly 3x6 feet. I know there's a nice way to calculate the weight of the glass, but I don't have it at my fingertips. The air gap between the two panes of glass is almost 2 feet!!! :D that's where I'm thanking myself for buidling the control room window around the support beam for my house. That gave me the width I needed and hid the beam all in one shot. If anyone sees anything stupid in anything I just said please let me know.

How are the walls around the windows constructed? How heavy are they?

They are as heavy as you could ever imagine them being. I used a 2x6 stud for the base and top plate of the walls, with 2x4 staggered studs 14 inches apart with a stringer between each vertical stud. Then I put 3 layers of 3/4" OSB on each side. Then capped that off with 2 layers of 1/2" drywall on each side mounted on Z channel. That's EACH wall. Then there is something like 7 inches of dead air between them with 2 inch studio foam facing each other to get rid of any sound that might get between the two plates of glass. The two plates of glass will end up being 22 1/2" apart from each other.

wouldn't it pay to calculate the weight of the wall leafs and use glass panes comparable in weight?

Clang, you're confusing me with your wall descriptions - can you state, in order, the material(or frame) a sound would encounter when trying to get from room A to room B? Also there is a VERY simple way to figure glass - just match the MASS, or go slightly heavier. Gypsum has close to 1/3 the mass per unit thickness of glass, MDF is similar, plywood is slightly lighter - so just generally, if you use 1/3 the glass thickness of your TOTAL panel thickness for EACH SIDE of a wall, then increase the glass thickness to the next available thickness, you'll be fine. 2 feet between glasses is good, it will lower the m-a-m resonance regardless of WHAT thickness glass you use. The thing I'm concerned about though, is your description; it almost sounds as if you've built TWO walls, EACH with TWO leaves of mass; if so, you've actually WORSENED your isolation at lower frequencies by several dB... Steve

I don't get how going with two separate rooms would worsen my sound isolation at low db. Ok, right now I'm in the control room so I can describe exactly how the walls are constructed. My memory was a little fuzzy before. I will now describe what materials/space sound has to travel through to get from the live room to the control room. 1) 2 layers of 1/2" drywall mounted on z channel to help prevent vibrations getting through to the OSB in #2. 2) 3 layers of 3/4" OSB. 3) 2X4 wall. 4) 1/2" drywall. 5) 7" of DEAD AIR with 2" studio foam facing each other. 6) 2 layers of 1/2" drywall. 7) 2x4 wall. 8) 2 layers of 3/4" OSB. 9) 2 layers of 1/2" OSB. 10) 2 layers of drywall mounted on z channel. All seems staggered and caulked/mudded. Does that help clarify? I still don't get how that would give me worse low db isolation than if I had a thinner wall. I even differed the thickness of the materials mounted on the 2x4 walls.

Steve means (if I may speak for him) that you have a wall consisting of 4 leafs with 3 airgaps, if I understand right. It is better to have a wall with two heavy leafs and a large airgap in between. Such a wall resonates on a very low freq and that is good, your wall has multiple resonances what is less positive for sound isolation. Bert

No there is only one dead air gap. I don't know how I could better describe my setup than I did in my previous post. Draw out exactly what I typed to see a visual.

Guys, I'm reading a report about sound isolation with double leaf constructions from the Leuven University(geluidsisolatieverbetering met voorzetwanden, G. Vermeir, P. Mees). Here is something about windows constructions with very high sound isolation: They advice a totally seperated construction where you have two brick walls, with in 'm a fixed window with a 12mm glass-12mm gap-6mm glass, then an air gap of 230mm, then a double sealed window that can be opened with a 12 mm glass pane in it. So in fact a triple leaf construction with a very narrow and a very large airgap. The R values (Hz-dB): 100 - 45 125 - 49 160 - 45 200 - 46 250 - 51 325 - 54 400 - 57 500 - 61 630 - 64 800 - 67 1000 - 68 1250 - 71 1600 - 73 2000 - 69 2500 - 69 3150 - 67 So: seems that when you have a good thermopane window you don't always have to replace it if you want to avoid a triple leaf construction. Bert

Looks like the third leaf may be responsible for that dip around 200 hZ... Steve

Yep, would take knowledge about the source spectrum and a calculation to really predict the effect, but it doesn't look to bad.....