60/90 are good guesses based on my lame sketches... You're on the money actually, John.
Because the slanted ceiling, um, slants at a 30 degree angle rather than, a more comfortable 60 degree angle, I have no choice but to run the monitors on their sides. Do know that all four monitors aren't in the monitoring circuit at the same time, but rather as inner and outer pairs. I have a homemade switchbox that handles the sub switching in and out of the circuit, as well as up to 16 pairs of monitors, of which I'll have two pairs as you know.
I fully understand your thoughts regarding tweeter/woofer phasing, however mounting them horizontally is out of the question, I just can't do it. The ceiling slants too far. I either move the monitors and the console out from the wall, and interfere with the electrical box and the side door (same wall), or, I pick a different wall. Now because of other quirks of the room, even though the room is about 13x20, its not truly 13x20, there are some things that "hang in" that are structural that I cannot move. So putting the console against the back wall (stairwell in the way), or the right wall (slanted cove next to dormer in the way) or the left wall (entry door!) that leaves me with the slanted ceiling wall. If the console were narrower, fine, I could do this. but its difficult to slap in a 11'9" wide console table in a 13x20 room with a dormer cove, two radiators, a ceiling slant, a stair well, and a shower stall that indents into the room but is accessed from outside the studio.
Now here is where I truly fucked up.
When I measured the space the monitors would take up, I measured them exactly. I didn't take into consideration that when I turn the monitors at 60 degrees and 90 degrees, they stick into the slanted ceiling. In fact, based on the original location I had engineered into this space, they back top inner corners of the monitors would be touching the slate roof if I kept their center axis where it was.
I know, that was really stupid and a major oversight, but this put me in this pickle forcing me to deal with it now.
Anyway.... steel tubing and cages.
The steel is 3/4" rectangular tubing, 1/8" in thickness, simply because I have a lot of it in the garage I won't be using for a car project because its too thick (the 1/8" measurement), and its strong enough if triangulated properly, to support the weight of both monitors on each side.
Because I have an understanding of vehicle dynamics, road generated low-frequency oscillations and handling related issues, I was hoping to draw a parallel into the construction of these monitor cages.
By triangulating the structure on all sides but the face (so the monitors can blow out without tubes in the way), I achieve two things - I make the structure significantly stronger of course, but also I significantly lower the resonant frequency of the structure, as it will then vibrate as an entire structure. Because no two pieces in the structure are the identical length, there is a lot of cancellation of intermediate harmonics and the fundimental frequency is the one I have to seriously pay attention to.
This, in road/rally cars, are significantly reduced by injecting at high pressure, an expanding foam into the tubes through a nozzle. This is done by boxing (sealing) the exposed ends of the square tubing with 1/8" thick .75" square plates. Each enough to punch out of 1020 plate. I've already made about 20 such punches. Anyway, those get welded on, and they each have a 1/8" hole in them. This way, foam can be injected in one end, air can escape out the other. Once its filled, a razor slices down the holes to remove any excess foam, and I do a quick butt weld over the hole to seal it up with the foam in it. With a mig welder, this is a quick zap, thus the foam isn't bothered that much, and I don't have slag fly out the other end. The weld is a little contaminated by the expanded foam, but its a 1/8" hole so it does seal up just fine. Its a sealing weld, not a structural weld, so it doesn't matter as long as it does in fact get sealed.
Tubes that will be butted together, thus one or more tubes will have their ends sealed against the side of another square tube, will have to be pre-drilled with a 1/2" hole so that the foam can force its way easily into the tubes without their ends exposed. High pressure application of the foam enables the foam to go anywhere and everywhere no problem. We've done this with 1.75" tube chassis's before from one end of the vehicle, and had foam leak out at the other diagonal corner, no problem.
The monitor cages aren't rectangular in shape, they are mroe or less a boomerang type of structure for lack of a better description, so that too significantly hampers vibrations in the structure itself because of the odd shape. Its measurable side benefit, but an accidental discovery rather than deliberate engineering. its a "fit under the slant" thing
On the top sides of the bottom of the cage, to which the monitors would rest on, I intended to use butyl rubber glued to the cage, or neoprene, I've already purchased both so I can experiment as to which dampens vibrations the most. Same for the sides of the cage that surrounds the two monitors, and the top. Whether I build a true soffit or simply keep them as a cage, I intended to make the monitors fit somewhat snug so they don't "walk" on the cages, resting on the butyl or neoprene.
So I'm hoping there is a parallel that I can draw on racing car construction to monitor cage construction. If so, I can use the above experience to make this work. But I am very worried about phasing issues, like you said.
I chose steel for several reasons - I can use .75" square tubing where as a .75" square or round dowel would snap with the monitors on them. Second, if there are any miscuts, I can fix that by zapping it with the MIG welder. Once you cut wood, well, you've cut it and doweling and gluing is not always a good thing with heavy monitors on them. I could have used 2x4's, however thats just plain ugly. even if I sheet rocked over them. They take up too much space, anyway. I can make the cage physically smaller and closer to the slanted ceiling out of steel than I can with 2x4's. Another reason is welding allowed me to "fasten" the different pieces of the structure without regard for having screw access throughout the structure. Zap and let cool and its over with. Now that I have a nice miter saw, I can do complex miters in multiple directions. I also have the steel, the mig welder, the flux-core welding wire ready to go. In fact, I just finished cutting out all the pieces, both sides, ready for welding over the weekend if I can get away from hanging the gutter that fell down *again*. Pulled the front soffit off this time friggen (#@*$&@(# wind.
Then of course, the initial soffit discussion as to whether partial soffits are any advantage over no soffits, or equal, or worse. Its something I'm just not grasping well, and thats okay. I am of the opinion that an open cage will be very similar to mounting the monitors on a pole, or a bracket, or a hinge of some kind. Its still open. I can easily attach plywood around the monitor faces and see if that sounds better, or worse, or no change. Maybe making a full sized soffit around the three sides, and hanging down a 2" soffit might be enough. Really, I barely understand the whole soffit theory, dispite your excellent explainations. I understand it in a simple sense, I do realize its purpose is to prevent back wall cancellations by keeping waves going forward, in phase. I understand this conceptionally due to my understanding of anti-reverberation headers on race cars, which essentially achieves the same thing, for different purposes. For audio reasons its to eliminate wave cancellations, and for racing purposes, its to encourage exhaust scavanging, but the principle is the same.
And regarding what pro studios do in magazines, you're right. Just because a pro does it doesn't mean its theoretically correct or optimum, or even in practice. I'm unfortunately faced with a lot of compromises, which I'm really okay with considering this space is a major improvement over my last home studio space, by about 1,000%. I'm trying to balance "correct" with "practical", and I'll be the first to admit I've already made a shitload of compromises. I'm trying to do the best I can, with what I got. I probably should have redone the dormer half of the room to make it a complete dormer rather than have this stupid 3' slanted cove against the one wall, and put the console across the side wall with the single window. In fact, I wish I did. Unfortunately... I can't do that at this point based on the money thats been allocated and spent, and the fact that its getting really friggen cold outside - not the time to tear off dormer walls
Anyway, thanks for allowing the core dump. Not sure if that helps, or not. I don't think i addressed the soffit discussion at all, but rather just explained in more detail about why I chose steel, how it was to be constructted, and some of the reasoning behind both. If this is getting out of context or boring, let me know. I know sometimes I'm a bit dense at times.
If you have a sketch, I'd like to see it... Steve
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