Sean, actually from all the reading that I've done on the subject of the Voigt Pipe(TQWP), the general consensus is that it is a "mix" of transmission line, rear loaded horn, and bass-reflex designs, that "more or less" blend together.
The taper of the Voigt pipe gets wider at the port end, where traditional tapered transmission lines get narrower at the port end.
So this widening toward the port end contributes to the "rear loaded horn" part of the concept.
Also, the driver is located at the approximate middle of the cabinet, and is not located at one end, like either a horn or transmission line would normally be.
So this driver location contributes to the bass reflex part of the concept.
The waves develop in the full tuned length of the cabinet like a transmission line(tuned to quarter-wave), they are somewhat amplified by the moderate widening toward the port (like a horn), and the port does behave to some degree as a bass-reflex port at the frequencies below the horn lower cutoff point.
The top half of the cabinet is well-stuffed, to reduce the midrange and treble frequencies that otherwise would try to come out the port, but allows the bass frequencies to fully develop. The bottom half of the cabinet is hollow, to allow the horn part to work on the lower midrange, and bass frequencies(which is needed to augment the inherent Lowther low-end slow rolloff starting around 500Hz and going down). In the lowest bass frequencies that it can produce, the Lowther excursion is very short(compared with most drivers), and the bass-reflex(and transmission line) parts of the design helps the driver with this.
The cabinet design, with the shape that it has, works like a rear horn for the lower midrange and down, to the point where the length and mouth would normally cause a lower cutoff point. Bass-reflex conditions present then help the ranges below the horn lower cutoff, and the transmission line characteristics keep the driver "loaded" down to the area of the port tuning, so that the driver stays tight and well-controlled with tight bass response, and doesn't "flop around" unless alot of very low frequency energy is "pumped into it" and the "unloaded" driver then flops around(happens on record warps and such). The mids and treble are restricted from the port output in large part by the stuffing in the upper portion of the cabinet, but some small amount makes it out the port(basically inaudible even at short range). The bass-reflex behavior causes a rapid rolloff below the tuning point, and this combined with a very high impedance spike at the F3 of the Lowther driver in the mid 30Hz range, combine to limit the usefulness of the system to about 40Hz on the low end with most typical amplifiers that would be used with these. Since the cabinet is quite shallow behind the driver, and the driver has a very thin paper cone, damping of the back wall of the cabinet behind the driver is needed to prevent "smear" caused by the reflections of the back wave coming back thru the cone face.
So, as you can see, this is a "hodgepodge" of various designs that have aspects of each design playing a role. It is far from a "perfect cabinet design", and nobody really views it as such, even the designer P. Voigt(designer of the Lowther driver), who considered it to be not one of his best efforts(from the 1930's).
However, it remains to be a very workable package, which actually can produce good results(with a bit of tuning work), and has advantages of deeper bass than most of the "true rear horn" designs for Lowthers(or single drivers). The normal lower-cutoff frequency that plagues horn designs because of the shortening of length and reduction of mouth area(in order to fit into the room), and thereby limiting the bass response to around 60Hz or higher, is not nearly as limited by the TQWP design, because it relies on the bass-reflex and transmission line aspects of its design to effectively produce the frequencies lower than the predicted lower horn cutoff point. So the TQWP will play deeper into the bass than a similarly-sized rear horn cabinet would play. And it will do so in a controlled manner which is not bloated, like many pure bass-reflex systems do. You actually get almost an additional 1/2 octave into the bass regions(down to about 40Hz, and maybe a couple Hz lower if you're lucky).
Yes, of course, as with any speaker cabinet with a front radiator and a front port, comb-filtering can(and probably will) take place, to a degree. And, as with any speaker with a narrow baffle-face, the baffle-step losses will occur predictably at the frequencies which can be supported by the baffle-face.
As you know, I have taken steps to correct these things in my cabinets, and the results have been good.
Nobody ever claims these cabinets/speakers as "perfect". But, we do claim them to be very good, and at a very reasonable cost, compared to other speakers which cost significantly more and don't perform as well overall(due to a number of factors involved - taste included).
They do what they do quite well, given the inherent restrictions and drawbacks of the drivers and cabinet design employed. It is a minimalist design, and is very simple, yet complex, at the same time.
The best way to describe it(IMO) is that the corrections needed to bring the system into good fullrange performance are done in the acoustical realm, and not in the electronic realm. Acoustic "filters" and acoustic "boosters" are used, instead of electrical ones. This allows a much simpler signal path, with better efficiency, and fits with very low amplifier power better than using electrical counterparts. We use the basic laws of acoustics to do the work, instead of "forcing" the thing to do what we want through electronic manipulation(that soaks up power, and does other things we don't want).
With a view toward keeping that basic design philosophy alive in my system, I took steps to improve the baffle-step problem by using acoustical boundary reinforcement principles to overcome the previously existing baffle-step losses that were present in the basic design of this cabinet, instead of using an electrical compensating filter network. This enlarged the frontal area(and visual impact) of my speaker system considerably, but worked very well. These are commonly called the "wings mod" or the "swinging-doors mod" for the Voigt Pipe. While I didn't invent this idea, as far as I know I'm the first one to use it on Voigt Pipes. This mod really does the job well, and keeps all my(very limited) amplifier power for making music.
As a result, the sound produced is very "effortless" and "natural" and "unforced" sounding, since there is nothing in the way, and the natural acoustic forces of nature are doing my work for me. Additionally, I keep the "direct drive" feature of my SET amp going directly to each driver with no intervening electrical parts(which definitely is a plus, as you know).
I'm very pleased with this, although I certainly realize that the lowest octave of bass is not reproduced, there are still some minor anomalies in the driver at the upper midrange area(but significantly mitigated by another mod), and also some(relatively minor) anomalies that occur as a consequence of the interactions of the direct radiators and the ports, and a smallish "sweet spot" which results from collapsing dispersion field in the upper frequencies due to driver diameter and beaming characteristics.
As a package, its strengths are good enough to make me feel that the drawbacks are worth it. I think it was a very good project, that was very easy to make, costs a fraction of what a "sonically competing" speaker would cost, and makes a very pleasing musical result,and is efficient enough to work with my 2 watt SET amplifier, which is what I wanted.
Whether others would want to take this route, I can't say.
But it gives me what I want, and so it's good for me.
PS - I should note that when I first constructed these cabinets, there was a hollow, chesty, "boxy" resonance in the lower mids/upper bass(low male voice area), which was due to the cabinet vibrational resonances(unacceptable). When I placed Sistrum Platforms(SP-004) under the cabinets, the cabinet resonance problem totally disappeared, and made the speakers a viable project. Without these platforms, I cannot recommend this project, because there will be that unacceptable resonance there. I know that I sell these platforms, but without having them under there, I would not be listening to these speakers today. I would have moved on to another cabinet design. Take it for what it's worth, but I know this cabinet design inside and out from actual experience. The platforms are "a must" for these. I first tried a set of regular Audiopoints, and that helped, but they weren't enough to do it. The SP-004 platforms did the job admirably, and it was like a miracle cure.
