Bill: Measurements? V-Coupling will only extend the horn by a little over 2 feet, and increase the mouth area significantly. Won't that increase Fc? I remember modeling it once for someone who was thinking of using THTs outdoors for an outdoor HT scenario....you get more output, but not 5-7 Hz of extension, IIRC....that would be almost a 1/2 octave.....not easy to get from a 22Hz Fc to 15Hz Fc with only 2 more feet of horn....but I could be wrong.....
Crabra: after modeling and folding several 15-20Hz large folded horns, not one ever got built, so no development on that front. The horns were just too damned big. You think you can build a 700-900L horn until you have to fit it through doorways, hallways, and up stairs..... For comparison, the 24.5" standard THT is a 520 liter horn when folded, and is a 22Hz horn when corner loaded, and it is as big as can be comfortable for 2 people to move (esp with a lacquer finish)... To get into the sub-20Hz realm with power, you need a huge horn, or multiple smaller ones. I have been working on the multiple smaller route lately, to cancel room modes as well as get good spl for the $$ spent. The TB horns I am working on should get extension in room to just under 20Hz, possibly 17Hz, then response falls. Unless you have crazy room gain, a THT is simply not going to give you good, clean response far into the teens, because the horn's rising response into the passband amplifies any harmonics created by the driver, artificially driving up distortion. This is why horns have to be highpassed, or at least tested well before driving them with significant power below Fc (horn cutoff frequency). The THT with the Dayton Reference Driver can be driven pretty hard below Fc, as it is a very low distortion driver. I'm glad it is the go-to choice now for the THT. It is not as sensitive in the THT as the old DVC, but it works great in my THT.
Antripodean: That would be great. I need real figures to make sure these models are worth folding up and making drawings for. I have 6 basic models I am working with:
1. ~22Hz 115 liter
2. ~17Hz 160 liter
3. ~20Hz 200 liter - more efficient
4. ~30Hz 115 liter
5. ~30Hz 150 liter - more efficient
6. ~20Hz between 115-160 liter
I also have been playing with a 250 liter 15Hz version, but I'm not happy with it.
All of these liter numbers are prior to folding, you can add about 50% to these values for volume of the cab after folding....
All of them need multiples to work well in room, unless you are only listening at -15dB levels, in that case, 1 or 2 can be used. To get reference level, you need 4 of them, and preferably 8, all corner loaded. The 17Hz variant has a chance to reach 15Hz (and maybe lower) in room.
My goal is to design them to be wall-mountable, and use the ceiling/wall corners to load the horns, to use ZERO floorspace.....paint them the same color as the wall, and they could almost fit into the decor of the room....