Incidentally, I angled those long strips so that I could provide a continuous 1/2" wide ledge of wood for the speaker frames to butt up against on one side, while minimizing the internal volume consumed on the other side. I couldn't do it exactly the same way on the back side of the box. I wanted all the braces there to come to within 1/2" of the surface. This allows for a stronger rear mounting with many more anchor / screw points when a single long piece of plywood is used for the back cover. Incidentally, those inner braces are placed at slightly uneven distances. I put one centered at 11-1/2" from the center of one end brace, the next one centered 13" from that one, the next one 12-1/2", leaving that last one about 9-3/4" from the other end. You want them uneven because we are already going to have to mitigate standing wave resonances from the distances between these braces, and it is always helpful when you don't have multiple resonances to deal with all at the exact same frequency.
For the back panel, I'll be cutting a piece of the 4 foot long ply wood, slightly less than the 3-1/2" inner width, to fit into the frame and up against all the recessed braces. I'll want the sealed cabinet to be reasonably air tight, so in the rear I used similar angled wood on the walls between the braces. This time tough, I had to cut them into shorter pieces of varied lengths, to tightly fit between the braces. Due to a few lost photos, you won't see that detail here, but its plainly visible in some of the latter photos where the wiring is shown. In addition to the usual gluing, screwing, and clamping for those pieces, some silicon caulking was used at the ends wherever there were gaps.
I said i wasn't going to offer a step by step guide, but at this point, let me mention a few construction details and materials that won't be obvious. You're going to need phillips head screw fasteners in several different lengths. 2", 1-1/2" and 1", the latter in both #6 and #8 widths would be a good start, and some smaller 3/8" ones later for mounting the speakers. You'll also want to pre-drill all your screw holes to ensure against splitting. (The smaller holes for the actual speaker drivers are the only exception.) I find it very worthwhile to have two electric hand drills ready, one with a phillips driver bit, and the other with an appropriate sized bit for pre-drilling holes. I'd also recommend you get some Elmer's Carpenters wood glue (not the typical white), and a caulking gun with a tube of silicone sealant, as you'd typically use for weatherproofing windows. You'll want to screw and glue every point where you put wood pieces together, and its always a good idea to clamp long pieces in place (as you can see from some of my photos, I ran short of clamps and had to get a bit creative.).
Some other misc things you're going to need very soon. First, as part of our anti resonance strategy, many speaker boxes need some kind of padding along the inside surfaces to help discourage reflections. In this case we don't have a lot of inside volume to lose, so my solution was to obtain a roll of craft foam, which you can get at Michael's and several other craft stores. Its thin but has a good damping effect on sound. Please forgive this awful blurred image on the left. It's the only photo I took when I first added this craft foam to the inner walls, and didn't realize the focus was bad. The craft foam is cut into reasonably well fitting pieces, overlapping where pieces join, and glued to the edges with 3M general purpose spray adhesive. Despite the blurry image, this will all become clearer in the next photos. I won't go too much into details on this next point, but you're going to want to plan on adding some vinyl covering fabric to the outside of this speaker for both durability and cosmetic enhancement, and you're probably going to want to spray paint pieces like the speaker frames, since these will be partially visible in the end. For simplicity and to blend with most other stage equipment, I just used black paint and coverings. Here are some photos to show various stages of subsequent construction.
So here, left to right, you see...
Obviously there is a whole lot of important detail in that 4th frame! First, I wanted a good seal for the speaker frames, but wanted to reserve the ability to disassemble too. So all the contact surfaces on the inside were coated first with silicone caulking, and the inside edges of those frames that would contact each other were coated too. Thats still a pretty firm hold, but it would be possible to separate them if necessary. Using the carpenters wood glue would make any such future revisions impossible. Next, in addition to the silicon caulk, notice that each frame is also screwed into place from the outside. This was done with 1" fastener screws, each with a recess washer, with each screw hole located to catch wood at 1/2" from the end of the frame. And of course you really don't want to forget to properly pre-drill those holes!!
The speakers have also been mounted at this point. There are three important steps here. First, you'll want to solder about a foot or so of wire to each speaker terminal. I used Red for all the positive terminals and white for all the others, to avoid errors in speaker phasing later. Next, you'll want to cut out a piece of that craft foam to the exact outline of each speaker, along with a 2-1/2" inch hole in the middle. These will allow each speaker to have a secure and air tight fit, and prevent any unwanted vibration. the craft foam is easy to work with and cuts easily with a safety razor. Finally, each speaker is situated into its frame along with the craft-foam inserts, and screwed down. I used #4 x 3/8" black flat head sheet metal screws, obtained from MicroFasteners.com , part number SMFPK0406". You'll have no trouble pushing the sharp screws right through the craft foam. Tighten these all with a hand screwdriver. Be mindful that the craft foam flexes, and so you want to tighten the screws a little at a time all around, as you would a car tire, and don't over tighten. The speaker frame metal should flex slightly, which is OK, but don't over-do it.
The next thing we'll want to address is the wiring, which means a lot of working from the back. But first, although its pretty safe at this point to rest the whole assembly speaker-side-down on a totally flat surface, I found it worthwhile to cut some pieces of wood to build a couple of 'U' shaped holders, that would allow the speaker to rest its weight on the edges of the frame.
So back to the wiring! There are many possible and equally valid ways to wire the speakers to form different total system impedances and power outputs. There are also variations for different weighting of power (sometimes called shading), such as wiring top or middle speakers to be louder then the lower ones. That said, trust me... you'll want to take the extra step of wiring all the speakers to terminal mounting strips, to make it easy to change your mind. All it takes are some small cut pieces of wood glued to the edges, to screw in the terminal strips. You cant see it, but I took the extra step of drilling small holes for the wires to pass through near the walls. It holds the wires in place better and makes the whole design neater.
In these two photos, you can also see the cut strips of angled wood mentioned earlier, fastened between the inside braces. This will allow the back to make a good seal all around. You can't see the angles of those wooden strips, but you can imagine how cutting those angles cuts the volume they take up by half of what they would, if simply cut straight. Notice also that the vinyl fabric covering is looped around the back areas. The back piece of plywood should be 48" x 3-1/2", but since it too will be covered and wrapped with this same vinyl fabric, I ultimately found I had to trim the back to about 3-1/4" to fit.
So now I'm prepared to wire and re-wire several different ways, without ever disturbing the wires actually soldered to the speakers. But the next step, before the actual wiring to combine the speakers, we have to plan for "stuffing" the insides of these speakers. Often speakers are stuffed with pillow stuffing or similar substances, because doing so helps to defeat resonances, while also giving the effect of a slightly larger box. But in this case, due to the short distances, we need to go some extra miles to defeat those resonances, and standing waves. So here are some materials needed.
Again, left to right, you see...
So what's going on here? The first frame shows the poster board cut into 3" strips, the exact same depth as the inner braces. The middle frame shows a layer of the firm but flexible packing material placed over a speaker group, followed by an insert of poster board, carefully cut to match the inner diagonal angle. You can see this repeated in the next frame. (sorry about the wiring mess, but I was already beginning to experiment with the wiring to combine speakers here.) These angled pieces of poster board are hugely important! They cause the standing wave resonances that would normally occur between all the walls and inner braces to be replaced by a continuous infinite band of resonances, covering all frequencies, since there are now infinitely more wavelengths accounted for. I'll explain more about that later.
The last step would be to stuff all the open spaces with that pillow foam. I don't have a great photo of that, but the rightmost photo gives you an idea. You might notice from that shot where I had added some holes between the inner braces, to allow me to more easily pass wires around without looping all the way under the foam.
Let me talk about the wiring a little more, because this again is a part of the project I didn't document well with photos. The terminal strips make it easy to prepare many wiring arrangements. You'll want to do all your wiring with small jumpers along with larger pieces of wire, all prepared with crimp on connectors, to easily route and re-route the wiring many different ways. The simplest connection is to wire 3 sets of 5 speakers, so that you have 5 speakers in series (5 x 4 ohms = 20 ohms), with all 3 groups of 5 wired in parallel. This will yield a 6.7 ohm total, with all speakers outputting equal power. Many modern amplifiers can offer greater amounts of power into lower impedances though. Some like my Behringer NU1000 can offer 500 watts/channel into 2 ohms! With such an amplifier, you could do the opposite... wire 5 groups of speakers in parallel, with each group having 3 speakers in series. 3 4 ohm speakers in series is 12 ohms, and 5 such groups in parallel would yield 2.4 ohms. Then there are wirings where some "shading" is done. One such arrangement I personally like is to combine the bottom 12 speakers into three groups, each with 4 speakers in series (20 ohms). Then, the top three speakers are wired in series (12 ohms) and all 4 groups are then wired in parallel. This yields approximately 4.3 ohms total, which my Behringer amplifier will drive to approx. 250 watts. What is interesting about this kind of arrangement is that the top three speakers output a bit more power per driver then the lower 12. that can be an advantage if you consider that the main use of the top three speakers will be to send sound to the people farthest away. And since the top three in this array are actually pointed in three different directions, the extra energy is well distributed.
Thats not the end of the possibilities either. Though there is no way to exactly balance 15 speakers left and right, a very interesting effect can be obtained with a stereo wiring arrangement with the 7 left and 7 right pointed speakers connected together separately, and brought out to a stereo source. (perhaps 4 in series, parallelled with 3 in series for each side). While some compromise would have to be reached for the topmost speaker (perhaps the old cross wired 'surround channel' trick), the end effect is pretty amazing and it can offer the illusion of stereo speakers, from a single box!
So to wrap this up, here are a few of the additions I ultimately made, to make this speaker ready for performance use.
So here, left to right, row by row, you see...
Before wrapping this up, I'd like to mention that the addition of those angled baffles (made of the poster board) was a really major breakthrough in the evolution of this speaker array. Despite sounding good during initial tests, an actual frequency sweep test pointed out some noticeable peaks and bumps at several middle frequencies. A little calculation of the wavelengths of those frequencies revealed that they matched up exactly to the distances between my inner braces and walls. Of course it is a little difficult to find the best microphone placement for these tests, given that the speakers point in many directions. But after reaching what I felt was a reasonably representative test setup, I went back and added those angular baffles and re-packed the foam. Eureka! It made a very significant difference in a positive way. No longer were there obvious and extreme midrange peaks and valleys, and I now had what I felt was a reasonable response. The graph here was the final un-equalized response, without my bass box speaker connected. There is still a bit of a dip around 12Khz, but this turned out to be very easy to correct with a graphic equalizer. Coupled with a separate floor speaker to cover the bass and lower mids, I can emphatically say this system has offered me the clearest and most detailed sound I've ever had the pleasure of playing through.
On that last point, it is important to understand that even the BOSE system has a very custom tailored equalization network for their speakers. When building an array, a good result is one like this, where the response is mostly flat between the frequency region it was designed for, where the number of corrections needed is reasonably small. But some corrections and equalization should always be expected.
Finally, despite the lack of full step by step detail, I hope enough essentials have been provided here to allow a seasoned DIY builder to have a good starting point, if an array with a similar purpose is what they are after. Good luck, and don't hesitate to ask if you have a question. I'm sure I've left a lot of details unsaid.