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Discussion Starter · #1 ·
So, pursuant to my quest to design a loud sub to cover the 8-32 Hz octaves, I started looking at LLT sonotube subs, and also transmission lines, when I had this idea... would it be possible to build a folded transmission line using concentric sonotubes of different diameters, such that after the first "run" of the overall enclosure length, the waveguide would be the annular space between tubes?

With an 18" driver and inner tube, the tube area would be 254.47 in^2. The next tube size up would ideally be 25.46" for equal area (assuming zero wall thickness), and the one after that 31.18", which is about the width of a doorway and would represent the upper limit if mobility is an issue. Still, that gives three multiples of whatever length (height?) the space could accommodate, so a quarter-wave TL that gives great extension should be possible, provided this idea is sound - I don't know if the waveguide has to be a contiguous area or if it would still work in an annular space.

Thoughts?
 

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It is an interesting thought. I have not heard of anyone contemplating a quasi-folded horn, tube design. Are you subtracting the inner tubes volume from the outer tubes? I really like the idea as bracing would be so much less problematic. I can forsee cutting out 2, 3 or 4 sections of the ends of each inner tube which would in essence create legs that would accomodate mounting the the next tubes end cap. Or just mount blocks between the tubes (easier). May have to do a combination of both (best). That resulting space/volume will also have to be calculated. Very interesting. Suscribed!
 

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Discussion Starter · #3 · (Edited)
That was the idea - each successive ring has equal cross-sectional area to the centre tube, taking into account the area lost on the inside, so the enclosure would be analogous to a straight transmission line, as opposed to a tapered TL or expanding horn. I envisioned the ends incorporating a semi-toroidal shape for the smoothest possible transition from one tube to the next (laminated MDF & Bondo?) . With a low Fs driver (I was looking at an RE XXX 18" with a Fs of 15.6 Hz) this potentially gives a quarter wave TL that should extend to 8 Hz without too much trouble.

The only problem is that I don't know what the acoustic wave behaviour would be in such an annular space, or obstructed tube. I can draw a wave in an open tube. To do the same in an annular cross-section seems to require that the amplitude be limited to the radial distance between tubes, but then there's no reason why that wave would exist at any particular circumferential location, so that isn't valid either. I just don't understand the mechanics, which means if I can't answer that question, I may have to allocate some funds to an appropriate experiment.

Incidentally, if nobody has done this before, and it ends up being a valid idea, I'm officially claiming credit for the invention. ;)
 

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Discussion Starter · #4 ·
So I joined the quarter-wave mailing list, where transmission line guru Martin J King has assured me that the idea is valid. Now I am presented with two problems: selecting driver(s) which have a Fs close to 16 Hz, and coming up with an embodiment which will either fit through a standard residential doorway, or could be disassembled to do so. My infrasonic requirements necessitate a lot of volume.
 

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Discussion Starter · #6 · (Edited)
I have an initial concept sketch which uses six Dayton Titanic MK4 15"s in three pairs of cone in / basket in push-pull arrangement connected to the nested tubes above, starting with a 36" tube in the center and ending with a 60" tube on the outside. These are assembled in 24" sections so that the massive cabinet can be moved if necessary. I'm going to call it the "WAF: Zero" sub.
 
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