howiedrum wrote:One window or two? Originally I had one 5'0x3'-6" window on North side of building. Then when I resubmitted as second unit they required two of those windows. Then when I resubmitted again as just a studio, the draftsperson left both windows even though none are actually required. I want at least one for some natural light, but I am concerned that having two windows will not leave me enough wall surface for bass traps and other room treatments. What do you think?
You could go either way: one or two, doesn't really make much difference. You have plenty of wall surface area, so it's not going to have a terrible effect on acoustics.
The decision is likely going to be based on cost, not acoustics!
Don't forget that for EACH window you need TWO panes of glass (one in the outer leaf, the other in the inner leaf), and EACH of those has to be thick laminated glass... it ain't cheap!
I recently saw some very bad advice on another forum where a so-called "expert" stated unequivocally that ordinary window glass (commonly known as "float glass") is
better than laminated glass for isolation:
Totally wrong! Here's why:
- coincidence-normal-glass-vs-laminated-glass-CUT.gif (13.75 KiB) Viewed 33845 times
- coincidence-normal-glass-vs-laminated-glass-CUT.gif (13.75 KiB) Viewed 33845 times
That shows the difference in isolation for normal window glass (light blue line) and laminated glass (yellow line): you can see the very large difference in isolation in the mid range, referred to as the "coincidence dip". That's a rather complicated thing to explain, as it involves bending waves and the speed of sound and angles of incidence, but the graph above shows it clearly. In simple terms: it's a bad thing that can kill your isolation if you are not careful.
Every solid material suffers from this "coincidence dip" problem (even brick walls, drywall, plywood, MDF, OSB, concrete.. everything) at some frequency, but damping compounds can reduce it significantly. (That's why I designed your walls the way I did: to minimize coincidence (and other issues) and maximize isolation.) Ordinary glass happens to have that rather nasty coincidence dip, and in fact with THICKER glass the dip moves down to LOWER frequency (even though the thicker glass isolates better overall... its complicated!).
But laminated glass is much better for this, as it has a thin "interlayer" in the middle: it is made up from two thin panes that are bonded together with the interlayer, which is usually made from PVB. That interlayer greatly improves the isolation, as you can see in that graph. So no, the guy telling people that float glass is better than laminated glass is absolutely wrong.
There are actually different types of interlayer, that can improved things even more. The normal PVB interlayer does a reasonably decent job, but there's also "acoustic PVB" that does a better job, and there are also some more exotic compounds that does a very much better job, as you can see in this image:
- coincidence-dip-plot-for-several-glass-types-GOOD-BGR.jpg (52.57 KiB) Viewed 33845 times
- coincidence-dip-plot-for-several-glass-types-GOOD-BGR.jpg (52.57 KiB) Viewed 33845 times
There's several curves on that graph: blue is normal float glass, black is laminated glass with normal PVB, green is with acoustic PVB, and red is with something called "Sound Acoustic Film". There's pretty much no coincidence dip at all with that stuff. Of course, as you go up the product range here, the price increases! Acoustic PVB is about the best cost/benefit balance, but even then it's not cheap. So ordinary laminated glass is fine if you are on a budget, but if you can spare a bit extra, then go with acosutic PVB laminated.
About thickness: the good thing about glass is that it is very high density: roughly the same as concrete, in fact. The density of most types of glass is around 2500 kg/m3 (give or take a bit), so it doesn't need to be nearly as thick as your leaf material to have the same surface density. But there's also the issue that you can't have insulation in between the two panes of glass (!), so you need to compensate for that by increasing the thickness a bit. In you case, for your isolation needs, I'd recommend AT LEAST 16mm acoustic PVB laminated glass. That's about 5/8" thick. It's made up from two panes of 8mm (5/16") glass with the PVB interlayer bonding them together. But it doesn't have to be symmetric: it could also be made from one layer of 10mm (3/8") + one layer of 6mm (1/4"), or even more unusually one of 11mm (7/16") and one of 5mm (3/16")... any combination that makes up the total 5/8" thickness is fine. If your budget allows it, then go thicker for sure, but not thinner. 19mm (3/4") acoustic PVB glass would be great. 16mm is bare minimum.
So I'd suggest that you get some quotes for different types and sizes of 5/8" laminated glass, to see what fits your budget, then based on that decide if you want one window, or two.
Of course, windows in the non-isolated area can just be ordinary glass: no special needs there. So if you wanted to have a window in the bathroom, or even in the front door, that's no problem: ordinary float glass is fine for that. You only need the expensive stuff for the isolated studio area.
- Stuart -