So I've implemented a folding form of the horn using a pair of large door hinges and two spring loaded latches
There's a minuscule amount of play to the hinge and a small a thin gap between the speaker enclosure and the horn proper, but that can easily be solved by attaching felt to the horn side of the joint. Red felt ought to be a great antique looking effect. Theghorn is begging for decoration now, and o think it'd look great with some simple graphic pattern. If it's some sort of floral arrangement, let it be. Red colored flora would look fine and match the felt, wood stain or not.
Unfortunately, the transmission line speaker is hardly portable. It's I'd say, more "luggable." It's a piece of furniture, not very different from just gluing two bass reflex speakers back to back and calling it a "boombox." The weight of the whole horn is now approximating the weight of the Mk I Victorian Boombox. So given the size, I'm projecting that Mk. I still wins over Mk. III in the portable design category, no matter what, although Mk. III has an open potential for much better sound (I'm not done developing that yet, but the frequency response of Mk III above 1 KHz is orders of magnitude better than Mk. I, so the only thing I need to beat Mk I is to give Mk III the same bass frequency response - totally doable, and very close to doing precisely that in the near future)
The Mk III in its current iteration, does make for a very fancy sound bar for your living room, like those found under flat screen televisions, but bigger and with full hi-fi performance without need for a Subwoofer and satellite speakers.. Perhaps that will be the destiny of this horn, an I intend to bring it to fruition, probably along that path. I'm just doubting that I'd ever carry this thing for blocks like I did the Mk. I for events and such. About the only way you'll see me carry this, is if I put wheels on it - which by the way, sounds like a good idea!
While I will finish Mark III with no defined purpose yet, I think it's about time to start thinking what I can do to apply what I've learned from this system while I built it. Outside of the last problem with the side placement of the subwoofer driver, I think I have a pretty good understanding of the design steisi need to take to come up with with an acoustically successful design.
First and foremost, the work told me that in a slender configuration, a horn will need to be hibridized with a two pipe Helmholtz resonator. The port design is done by complete octaves, with the lowest frequency being two full octaves below the resonance frequency of the main set of drivers. In other words, overtones are an energy transport system you can use to extend the bass two octaves below to the fundamental frequency of a given driver.
The inverted horn is designed to overlap with the first overtone, one octave above the lowest frequency and one half of the driver resonance frequency. This design constraint can be used for any set of drivers I can find. Interior volume defines Helmholtz resonance, length defines ¼ wave resonance. You can overlap both methods of achieving resonance, and for a thin waveguide you may have to. If I use a subwoofer driver it's best to use a very stiff driver at the end of the horn, not on the side. Maximum resonance is always achieved by placing the speaker in line with the axis of the horn, as long as the driver is not too soft, otherwise the driver may "clip" the sound and possibly deform.
The "start of the line," where standing waves form is critical; its where you will notice energy loss and sound distortion the most. The waveguide must be clean, uncluttered, smooth. No sharp bends if possible. The end of the line, or neck must be wide enough to get some sound. Unlike a regular horn, the inverted horn's job is to kill sound at all frequencies, except for one frequency you wish to enhance, so at the end of the line you have very little pressure to work with, that's why the narrow end of the horn must be large enough.
No parallel surfaces anywhere if possible, try to approximate a conical cross section. Did I mention no corners? Avoid corners because they garble the sound and make the speaker sound "boxy." Use ramps and reflectors to smooth the interior of the speaker for clean sound, and so you may avoid the use of batting - the inverted horn shape should be enough if it's conical enough. Batting is for square boxes only, and basically can make a volume seen larger, because it slows the speed of sound depending on frequency; by the same token its that frequency dependence in the batting that causes sound to be muffled - so I discourage the use of batting.
*whew*
Next, while I finish this Black Forest sound contraption, I will run some numbers to see if I can translate this design into a circular cross section pipe system with more folds. It's probably the only way I could get to better portability for the same drivers. Large diameter PVC pipe fittings can get expensive, though. And practically the full range speaker would definitely have to be smaller than 5 inches, because PVC pipes can get very heavy too. So I could design a system adapted to smaller drivers and with more folds.
