Revisiting the "Cubo"...

Over on a subwoofer design group on Facebook, one of the members recently drew my attention to the Cubo design, specifically the Cubo 18 Extended. He was not happy with how they sounded and while it was claimed that they have a range of 40 Hz to 120 Hz, he was getting basically 60 Hz and not much else.

Now, I had come across this design a few years ago, and I didn't really give it much thought at the time, primarily because there were no real performance measurements (impedance curves, frequency response), but mostly because a cursory glance at it layout suggested that it would not be possible to sim it in Hornresp because the horn expansion looked a bit off, and I have a "sim before building" rule that's become more and more strict as the years go by.

However, the member did indicate he would be interested in if it could be improved, so I decided to have a further look.
 
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The original Cubo 18 Extended

The attached shows the original plans for the Cub 18 Extended bass bin. If you take a closer look at the layout of the horn's path, it's easy to see that there's a "discontinuity" in the upper left section, as the csa at the start of the last section of the fold is greater than the csa at the end of the previous section. The overall path length also looks a bit short.
 

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...and here's where I tried to emulate the original Cubo 18 Extended in one of my BOXPLAN workbooks. As you can see from the "Horn Expansion" graph, it's not a smooth expansion - there's a discontinuity in the first bend (starting from the throat). The reason for that is clear - a single expansion is used for the horn's path, and the baffle board for the driver is too long to support a smooth single expansion in the box, given the box's dimensions.

This suggests to me that changing the design to a dual-expansion type and running the design through my usual "optimization" process might produce not only a smoother expansion graph, but perhaps even better results.
 

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So, I put together a BOXPLAN Optimization workbook around the design and this is what I came up with. The "improved" version now features a dual-expansion fold and extends the first segment (starting from the throat) to improve the low-frequency performance of the Cubo. And of course, now that the layout is all described by an Excel workbook, it's now possible to try out different external dimensions for the box and "re-fold" the horn using the optimization routine.

Note #1. There seems to be a very minor volumetric calculation in the workbook (the corresponding Hornresp sim seems to be slightly bigger than the calculated net volume of the box). I'll eventually find it, but I doubt that it would make any noticeable impact on the sim.

Note #2. A few assumptions were made about the Vtc/Atc and S1-S2 range of the TH sim because of the layout of the Cubo. Because of this the actual response above about 120 Hz or so might differ from what the sim suggests it would be, but who uses these things about 100 Hz anyway? :)
 

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Hello.
I have this experience with Cubo designs too. Did not match 18“ ported box in direct comparison between 30-45Hz.
It's nice to see improvements. Nice job and for free for all of us. In the light of all these new constructions (ROAR, THs, Paraflex builds) it's good to see this one got some bump too.
I would not recommend to do this with Cubo12 extended and Cubo15 designs though. Experimented with it, and it seems to me that efficiency at around 40Hz cannot be pushed much)(if at all) with low volume short path horn.

Will it be possible to rebuild the original box, or is it rather easier to build it from ground up?
 
I would not recommend to do this with Cubo12 extended and Cubo15 designs though. Experimented with it, and it seems to me that efficiency at around 40Hz cannot be pushed much)(if at all) with low volume short path horn.

Attached is a sim of the Dayton PA310 in a modified Cubo arrangement, box dimensions 24" x 24' x 15". Looks like 40 Hz is possible - you just end up losing a bit of sensitivity in the passband. I'll see if I can find the original dimensions for the Cubo 12 extended to see if there's a solution for a box with those dimensions. I think there probably would be - it helps to think of the Cubo as some strange Big Vent Reflex with the driver mounted in the vent. Reducing the csa of the vent will drop Fb, as would increasing the length.


Will it be possible to rebuild the original box, or is it rather easier to build it from ground up?

The example I gave uses the external dimensions of the Cubo 18 extended. All of the internal panels are different lengths and located in different spots in the box. If I was going to try and "improve" an existing and built Cubo 19 extended, I'd first try just adding an extra internal panel, joining it to the last one and extending like a shelf vent until it's closer to the corner, experimenting with its length until I get as I can to the target Fb.
 

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Horn noob here - Would extending the cubo with an extra section work/ be useful?
With a hing on the bottom of the mouth you could fold a 4 sided box over the entire cubo. For transport, folded it would be a tiny bit bigger and unfolded about twice as big.
4 Thumbscrews/bolts are probably enough to make a solid whole when unfolded. A piece of cloth can be glued over the hinged side and foamstrip for the 2 other sides to make it (almost?) air tight. I estimate the path-length to be 1.5x as long.
 
I'll see if I can find the original dimensions for the Cubo 12 extended to see if there's a solution for a box with those dimensions.

Well, I found the dimensions of the original Cubo 12 and, at 45 cm x 45 cm x 48 cm, it's just too small to get Fb down to 40 Hz. 50 Hz looks more practical.

FWIW, I compared the predicted response of a 50 Hz capable modified Cubo 12 with my POC6, which is a 50 Hz MLTL. The Cubo 12 is 97 l gross. The POC6 is just slightly smaller than that, at 96 l. The Cubo is a bit more sensitive above 50 Hz, but the POC6 matches it at 50 Hz. The Iron Law remains intact :)
 

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Horn noob here - Would extending the cubo with an extra section work/ be useful?

An extension to the Cubo 12 will improve its response at lower frequencies and Fb is lowered a little. I've attached an example of what the predicted response looks like if an approx. 32 cm extension is added to the front of the modified Cubo 12.
 

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Brian Steele: I have played with my own iterations, and that´s exactly how I approached the bin - as a long bassreflex bin with some additional horn properties. Well, 12" has only so much displacement and capabilities in small volume, and while I did not expect to keep up with those big boys, I was not happy with the results. It cannot be pushed or equalized much, because one runs out of available cone excursion very soon. And low tuned ~20l bassreflex box is not what is going to make a lot of bass, while horn element is not as long as your full sized model. It kept up with the 18" BR around 80-100Hz though. 55Hz tuning would be more practical. I also used 12TBX100 in 60Hz tuned ported box, and it still sounded good and musical. That led me to the impression that it is counter productive to push these 12" low, if you don´t give them enough of accoustical loading in adequate box. Like, DOH. :)
Some good 15" in the same volume but with proper port would be preferred.

I still have unused Hifonics MXZ 12D2 on hand, as a competition to 12TBX100 due to better excursion capabilities. I´m still inclined to play with it and make some other than BR/BP bin for it. Some type that can fully utilize the driver. ROAR will be done, and some other ideas are born. Box size up to 220l of outer volume.
 
An extension to the Cubo 12 will improve its response at lower frequencies and Fb is lowered a little. I've attached an example of what the predicted response looks like if an approx. 32 cm extension is added to the front of the modified Cubo 12.

Is it correct that the extension only works 1 way for the low end? So effectively half of the extra length.

The result is not to bad for 4 extra pieces of wood and about the same pack space.


I have seen extensions like barn doors and the 'Vplate' of the BFM T39/48 FLH's extending the horn by putting the boxes at 90 degrees and adding a top.

Are there any 'fold out' horn designs or other methods used for extending the horn?
 
Strange because on the cubo thread on speakerplans people seem to rave about them. Is it the choice of driver that's the issue rather than the design?
Ive recently been looking at these as a possible upgrade in future for my current subs. On paper, one of these cubo 18s fitted with a B&C 18DS115 driver (1700rms, 98db) would possibly keep up with 3 of my current subs (1000rms 15", 97db) in a box that would still go in the back of my car (assuming the cubo design increases output by about 2db over my 15 boxes)
 
I cannot agree on the output of the Cubo below 50Hz. It will not outdo basic ported box, I don´t care what presentations and reviews say... Tried that myself, and NO.... It´s interesting and nice box, but the hype looks to be too much. Speakerplans is plagued by some "hype waves" If you thoroughly read trhough Scoops, Tapped horns, and now Paraflex (not that much presence at speakerplans), you would see that people are going crazy. In real world, in time, bins evolve relatively slowly, and no "night and day" improvements are to be seen. Keep it cool, do not panic, do not fall for the new hype wave...

I´m trying to put most SPL output (PA) to transport in my car, in bins I can move, and it seems ported box with some advancements still wins. So I´m most probably going 4x18", each in 76l (internal volume) box!
 
Fair enough. I'm plenty happy with my (4, soon to be 6) 15s which are in a fairly small vented enclosure (58cm by 50 by 43) weighing around 30kgs each so very portable. But if there was an option to have a slightly bigger box with an 18" with almost double the power that could be an attractive option.
Some of the tapped horn designs around look great (like the TH118) but they are just too big / heavy for me. Like you I need this PA to be portable and be able t lift them in and out of the car.
 
Box size and high SPL low frequency performance go hand in hand. Difficult to have one without the other.

The BOXPLAN workbooks allow you to "tweak" a design to make the best use of your trunk space. The Cubo, because of its shape, may not fit the need, but the THAM and SS-style folded THs might just do it because of their rectangular shapes. In my car for example I should be able to fit in two 15" 40 Hz THs side by side, with the back seats folded down.
 
Damo s: right now, I´m "trafficking" two 21DS115 in advanced ported box. In the same car. It has 100mm wheels in it´s base, and it is relatively easy to load them into the car and out, by just me (not a big guy, just little crazy). Outer box size is 58x58x73cm. Weight 47kg. Works very well, but as I said, being crazy, I´m striving for even more power density and even easier loading. This box can be dragged and loaded, but stairs are still a little problem. Tested them against some other enclosures, and I didn´t see single one that would outdo this box with the same frequency range. Only bigger box can do that, or higher tuned box. The less bass you need under 40Hz, the easier is to find suitable box for you. In certain "modes", my bin must do 25Hz. Compact TH´s and hybrids will not do that for me.
 
47kgs is just too heavy for me. Id say max 40. My current boxes are actually pretty good I guess IM just being bass greedy. IIRC they drop off only 3db at 40hz. Ive had them reproducing C1 32.7hz which was definitely very audible. Perhaps I would get more overall output from the THAM15 above 45 but they do look like they drop a fair bit at 40. A lot of the stuff played through my system is using that range (techno and tech house and some deep house)