Yet another glass question

[JoeMilner]

Hi Steve,
Long time since last post, but almost done w/ the mix stage. My question is on the glass. You gave me a lot of info on my last post, but it appears that I'll be using the stage more as a mix stage than an ADR stage. So, I'll have the control room glass behind me (about 12 feet) more often than in front of me, but clients will be sitting with the backs of their heads within a foot or so of the glass. The 4x8 window will usually be covered by a thick velour curtain, although it IS an observation window during the mix so I'm not sure how often I'll actually get to close the drape

The final composition of the wall is: 2" 703 on stage side, two layers 5/8" dwall, 6" steel studs, 1 layer 5/8" on CR side, 1 1/2" mineral wool/fabric. 2x6 timber frame/header, 3/4" ply (12" wide) rough frame. Total thickness with 703/ etc. 12"

So, 2 questions:
1) Angle the stage side glass with the narrower gap at the top or bottom?
2) If I could angle just one side, should it be the CR or stage side? I'm not sure how close I can safely go to the edge of the plywood given the weight of the glass and the fact that the plywood isn't supported by actual framing all the way to the edge....
3) OK, three questions. You said the stops should be " minimum 1 1/4" solid wood"...does that mean hardwood like oak or something? And, 1 1/4" sure will take up a lot of space, is that a correct spec or a (doubtful) typo? So I'd be using 2x2's for the stops?

Thanks in advance, I'm so sick of building this thing, it'll be great to be done!!!

[knightfly]

Narrower at top is probably the best result, especially if you have the ceiling absorbed. That way, reflections off the glass will go toward the ceiling and be absorbed.

For that reason, you should tilt the side where people's heads will be within a short distance of the glass - even that will probably NOT be enough to keep them from getting some nasty reflections/comb filter effects.

Your 1/2" glass will weigh around 200 pounds or slightly under - you're right about the plywood and support clear to the edge. I'm not convinced that's a good window design, how much isolation do you think you'll need?

I think you could get away with full 1" stock for outside cleats if you used hardwood and drilled/countersunk screws @ 6" centers, but I don't like that unsupported plywood, or the single frame passing through the wall without a break - and I REALLY feel nervous about that glass so close to people's heads - not for safety, just sound quality... Steve

[JoeMilner]

Thanks Steve
Actually, I don't really like the wall design either, but such is life.
It seems that a single pane of 1/2" might be the best and safest way to go, angled as you said. This would allow the stops to be secured thru the plywood into the 2x6 rough frame. I could also make 2x6 "side-stops", I'm sure those would be more than enough to support the 200lb pane. No biggie about losing 3" out of an 8' run.
Isolation, well, as it's become more of a mix stage than an ADR stage it's not as critical, and as I'd be recording ADR (not a drum kit) I'm not too worried about transmission thru the glass. Most of the time the actors are talking, not screaming And, facing the screen not the glass.
Any final thoughts?

Thanks for everything, maybe I'll even post some pix someday!

[knightfly]

Yeah, maybe a couple - people seem to be divided as to the benefits of splaying glass - wider air gap of course improves TL, especially at lower frequencies (as does increased mass, laminated instead of plate too) - non-parallel reduces flutter but also reduces available air gap.

Having both glasses in the same frame can hurt isolation in a couple of ways - one, too rigid a coupling between the two surfaces - and two, hermetic sealing between the two surfaces increases rigidity of the unit so worsens the TL at resonance, which (using 3/8 and 1/2" plate glass w/4" gap ) works out to around 53 hZ, with an overall STC of about 45) - for non-drum usage, it should work pretty well.

One thing that would improve the window at lower frequencies (without hurting anything) would be to bore some 1-1/2" holes in the sill plate (between the glasses) spaced maybe 8" apart, so the inside of the window can vent to the inside-wall insulation - then cover with black cloth (helps reflections and hides the holes)

If you don't put the bottom of the glass where there is more than maybe 2" of 3/4" ply that's unsupported on the frame on each side, the weight should be no problem. For glare and acoustics, narrow at the top is better (if you don't leave too much unsupported ply with weight of glass on it) - For sheer strength, narrow at bottom is best but will degrade acoustics a little by reflecting back into the occupied portion of the room instead of into the (hopefully) absorbed ceiling.

By "single pane of 1/2", I hope you didn't mean only the one pane for the entire window assembly? That would drop your STC down to just under 30 dB.

Overall, if you have 12" plywood supported by 5-1/2" 2x6 in the center, use 1" cleats flush with the outer edge of the ply, then glass, it should work fine. It doesn't matter which glass is tilted from an acoustic standpoint, so if there's one side that should have LESS reflection problems I would tilt that side inward at the top (narrower gap), make both bottoms flush with the outside (minus cleats), and put the THINNER glass on the straight side. This will put less stress on the unsupported plywood and increase the angle the other glass can have (so echoes are reflected further away from people and more toward the ceiling) - you could tilt your heavier glass in at top by 4", which would give nearly 5 degrees splay for flutter control, and also do a fair job of deflecting unwanted reflections, both light and sound.

Hope that helped, I appear to be rambling (short sleep, 2 many graveyard shifts) - Let me know if I only confused things more... Steve

[JoeMilner]

awesome reply as always, Steve. I shall now proceed with style and grace....