After much research, I chose 1.5" extruded aluminum tubing because of the stiffness of the metal, and the flexibility in design options I had with this type of tubing. I wanted the frame for my open baffle speaker to accommodate a large variety of drivers, and be wide enough to sound good.
But, if I do want to turn up the volume I can have a second crossover programmed in to the MiniDSP that would cut off the low frequencies to the full range driver for listening at higher levels.
I won't be able to play this configuration very loud due to the full range driver operating in free air, with no box to mitigate extreme cone excursion. In using the full range driver, I hope to avoid most of these issues by only using an active crossover on the low frequency drivers, allowing the full range driver to run with no crossover. I am using the device as the DAC, which precludes me from using a more premium DAC, and there are the issues associated with active crossovers such as phase coherance. Of course there are compromises when going this route as well. I chose the active route, using a MiniDSP crossover/processor box and my laptop because it would allow me to quickly adjust crossover points, slopes and correct for phase issues as well as room issues. Not only that, inserting extra electronic components in between the amp and speaker creates all sorts of sonic issues and loss of signal.
When designing a passive loudspeaker, much time and expense can be spent on the crossover and getting everything to sound just right. Active Design - Everything in speaker design involves a compromise.
The full range driver will of course be augmented by drivers covering the lower octaves.
This makes building an open baffle speaker much easier and less expensive than conventional speaker designs. Just a rectangular piece of wood large enough to house the drivers, and of course drivers well suited to open baffle designs. Mostly due to the fact that one does not have to build an elaborate box, or a box at all to enclose their speakers. Open baffle loudspeaker designs are very attractive to the DIY community as well. Note the separate baffles for each driver. They have crafted some excellent sounding and good looking products that, in audiophile terms, are quite affordable. Open baffle speaker designs have been gaining in popularity recently due to the successful designs and efforts of small audio manufacturers like The Pure Audio Project and Spatial Audio. One can change drivers easily, and one can experiment with all kinds of speaker, wire, and vibration damping tweaks without too much difficulty. This is the perfect speaker project for the curious DIY'er, and the tweaker.
I want to share my experience in building this project in hopes that someone else will use the design, improve upon it and share it here so others can benefit. I have for a few years now been fascinated by open baffle loudspeakers, and also fascinated more recently by full range drivers (especially field coils like the Voxativ AC-X).