I try look into the Orion design in my own way.
First, it is a true 3-way dipole speaker to be used in a normal living room. From his analysis, normal living room has a RT60 about 500ms, Schroeder frequency is about 150Hz and the fundamental mode is about 20Hz.
The 20Hz fundamental mode is good since a dipole will not radiate energy in such low frequency thus we don't need to worry that non-audible low frequencies will shake the house. I like the fact that he uses two closed packed back-to-back woofer because the front and back sides of a cone radiate differently. Such arrangement can still be regard as a single point source as long as the distance between the two woofers is much smaller than the operating wavelength.
In terms of room modes, the longitudinal modes are not a problem if you sit at the optimal position. Such mode can be regard as the multiple reflections between the front and back walls. When you sit at the optimal position, all the front/back reflections form a dipole will cancel. The two speaker setup also keeps the beauty of symmetry in the room thus only even order modes between the side walls may be excited. Let's say the fundamental frequency of the mode between the side walls is 25Hz, this one will not be excited. The modes that will show up are 50Hz and 100Hz. The quality factor for these higher order modes are usually smaller, which means the peaks and dips are much smoother than the fundamental one. Moreover, the dipole excited the sidewall weakly thus such mode is less a problem.
In terms of mid-range and tweeter. Dipole simply shines. The dipole acoustic radiation pattern has nulls on the two sides. We can toe-in the speakers such that the null is pointed toward the specular points on the side walls. In such arrangement, no reflection will reach our ears! In reality we cannot toe-in that much otherwise the left and right channels will cross over in front of the listening position but the flavor of this technique is kept. Another smart move is to use tweeters. There will be a comb filter effect between the cone and the magnet of the mid-range driver. The null frequency will be around 1.7kHz assuming they are 5cm apart. The Orion is cross-over to the tweeter at 1.5kHz thus the comb filter of the mid-range is not a problem. The two back-to-back tweeters with 5cm part also keep the true dipole radiation pattern till 3.4kHz. I believe anything above will be direct path dominant in a normal living room.
The three-way dipole seems to be a really good design. However, the dipole frequency response is not a flat one and equalization is a must. It is also a less efficient design. It will require a lot of effort to tune them correctly. I believe buying the build plan will be a smarter move than start from scratch.
