Glass or Plexiglass

What is the downside to having 1/4" Plexiglas instead of class for the two control-room windows?

Plexi is not a good idea for several reasons. 1. In time, Plexi disclolors. 2. It scratches easier. 3. It is not 100 percent clear. There will be distortions off axis. 4. It is not as regid as glass. It will vibrate easier than a sandwich of glass.

I appreciate the post and all of it makes sense, except one point. In my ignorance, I thought that since Plexiglass is softer, it would not reflect sound as strongly as glass. Any comments on this? As for positives for Plexiglass, it is far cheaper than glass, something like $120 (total) for two pieces of 3'x6' size. I have used Plexiglass for a number of things and have not had trouble seeing clearly through it. It is also much safer than glass and not heavy. Also there is Lexan, but the is also pricey.

1/2" Laminated glass on control room side. 1/4" - 3/8" tempered plate for the live room side. CLICK HERE to be taken to school. :wink:

MErlewine wrote:

I appreciate the post and all of it makes sense, except one point. In my ignorance, I thought that since Plexiglass is softer, it would not reflect sound as strongly as glass. Any comments on this? As for positives for Plexiglass, it is far cheaper than glass, something like $120 (total) for two pieces of 3'x6' size. I have used Plexiglass for a number of things and have not had trouble seeing clearly through it. It is also much safer than glass and not heavy. Also there is Lexan, but the is also pricey.

This is now the weekest part of your wall (a hole) with thin layers to replace your stud and wall board system. It needs to be rigid and reflective acoustically. This is why you splay the top out and the bottom in, to keep the reflcetions closer to the floor. Plexi allows for the transmission of sound.

Plexi is roughly HALF the density of glass - so in order to reach the same TL through a plexi window, you'd need at least twice the thickness in each of two leaves, assuming the same air gap. Glass is 3 to 4 times the density of gypsum, so the glass in each side of a wall needs to be approx. 1/3 the thickness of the TOTAL gypsum (if that's what is used) - IOW, for a wall with 3 layers of 5/8 gypsum each side, you'd want glasses that were about 5/8" thick to match the TL of the wall - with one glass being slightly thicker to avoid identical coincidence frequency. If you were to use plexi, all other reasons against it notwithstanding, you'd need TWICE that thickness (approx. 1-1/4" each side for the above example) to maintain the wall's TL values... Steve

lowdbrent wrote:

This is why you splay the top out and the bottom in, to keep the reflcetions closer to the floor.

Apparently that is a myth, it only helps with light reflections through a 3rd panel in the middle of 2. So i read, dash this theory if you like, i was gonna do that but instead i'm opting for two panes of double glasing installed vertically, as this insulates better and there's no messing around building custom windows = price.

Paul wrote:

lowdbrent wrote:

This is why you splay the top out and the bottom in, to keep the reflcetions closer to the floor.

Apparently that is a myth, it only helps with light reflections through a 3rd panel in the middle of 2. So i read, dash this theory if you like, i was gonna do that but instead i'm opting for two panes of double glasing installed vertically, as this insulates better and there's no messing around building custom windows = price.

That's insane. Glass is reflective, and the angle of windows, walls, etc does affect the angle of reflection. Try it.

This is a subject I've been thinking a lot about lately. I'm getting to the point of looking at putting glass in my windows... finally, and I've been reading all about the subject. The thing is, I totally understand why people say to angle the peices of glass. What they say makes sense, about them vibrating together and standing waves and all of that. But the thing that I don't get is, why does Auralex, a company with many... many resources to do extensive testing on the STC of different methods, say that angling them is a waste of time and effort and they should be parrallel? Of course I'm of the school of thought of "keep it simple", and if Auralex says make them parrallel, I tend to want to do that. I mean, all the people who say to angle them, have they actually done testing with db meters to compare the two methods? Or are they only going off theory? Please set me straight if I'm wrong here. I'm looking to start building the sills soon for the glass to sit on. And plus if I'm going to angle them I'm going to need to make sure I get the correct dimensions as they'll change if they're angled.

Okay here is my 2 cents. I had a "home" studio for about 7 years here. When I built the studio I did so more for asthetics than for acoustics. This time around I have been doing the research. In my old studio I used 1/4" lucite plexi. Two 2' x 4' pieces seperated by a 1" x 1" frame. Essentially two like pieces of material with a small airgap. This was acoustically horrible and was PURE HELL to see through. It should be noted that quite a few bands that I recorded wanted to shoot video through the window which was a reflection nightmare for that as well. Now, being the common sense genius that I am, it would seem to me.. that if you used 2 pieces of glass. Both different thicknesses, which to me would have different resonant sympathies, they could effectively be mounted parallel with no problems. Acoustic problems that is. Reflection problems would depend on what's directly behind your site line at any given point. It is the same principal that you face with a television screen. If there is a sunlit window at the right angle from your viewing angle, the picture will be difficult to see. Shade the window and you are all set. In most studios, the lighting comes from above so there are problems with these reflections that are difficult to erradicate without angling the glass. From the auralex site I get the impression that they are saying if you angle less than 8 degs then you might as well forget it. So parallel or angled? That is the question. I would think that it depends on what kind of glass you are using, thickness of your wall(s), lighting, etc. This is something that I will be experimenting with as I get closer to my window installation. I will be posting pictures and my results as soon as I can. The wall that seperates the control and live rooms in my new studio is nearly 10" thick. My glass is 3/4" lami and 3/8" tempered. I should be able to angle the glass without any real loss in acoustic isolation. Yes it is more difficult to build, yes there maybe a very small difference in isolation but not enough to worry about. If I can get rid of those damn reflections, it would be worth it to me. As with the Helmholtz calc error, I tend to go with what John says works. Experience wins over theory in my book anyday.

And see, I completely see the visual reflections, or "mirror effect" we can call it (as to not confuse anyone... between acoustic reflections and light reflections). I can totally see a peice of glass, with the lights turned dim in the live room and on in the control room, or vise versa, turning into a mirror. If the glass is angled, bam, you don't see yourself. Well, unless you get down on your knees or something. But... that's not the explanation given. :P so I'm still wondering if anyone has any scientific proof with tests that shows that angleling the glass actually improves the acoustic seperation. I tend to agree with Auralex when they say you should maximize the space between the peices of glass. But, this is PROBABLY going to be an issue for me. You see, I built my control room wall around the floor beam. The beam is 7 inches thick. I went an inch off the beam for each wall, with 2X4 studs, with a crap load of layers of wood and drywall. So as it is now my wall is 22 inches thick. So my two peices of glass are going to be PLENTY far enough away from one another. But hey, if you say it was hell trying to look through the glass or shoot video through them, that's a pretty good reason as far as I'm concerned to angle them. I do have temporary plexy glass in place that I had laying around (I was going to make some drum sheilds that I never got to) and the reflections aren't bad. I don't know, I'll have to give it some serious thought. I'm anxious to hear what your findings are though. Keep me posted.

I'm no expert, but I would think that with an 18"+ air gap as you have, you could probably angle the glass a bit and it wouldn't make much of a difference isolation-wise. Then you wouldn't have you staring back at you. ;-)

With that large an air gap, and if your lighting isn't optimum (as in GLARE) you COULD angle the glass a bit and not see a lot of diff in isolation. The downside of angling glasses is that it reduces the air gap, thereby raising the mass-air-mass resonance and weakening the TL of the window - if your total wall thickness is under around 8-10 inches, you start getting into noticeable losses in isolation by tilting the glass. Angling glass does NOT improve isolation, it WORSENS it as I said above. Here's a "myth" paper written by Richard Schragg, who at one time worked for Walters-Storyk Design Group (not sure if that's still the case) - http://www.audio-muziek.nl/audiotechniek/acoustics.pdf HTH... Steve