Alright.. I think I got one figured out.. but the only way to find out is to build it.. That may be a little way from now..

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I would suspect that the author of that page has never actually built a 'Dual Chamber' box (ABC is my term eh) A 30hz design tuning will theoretically result in a 57hz secondary tune if the box is spot on.

For the vents: Take the total volume of the box and design it to have two vents. Then cut 3 of that length.

One can play around with the second tuning by altering the size of the small chamber and/or the middle vent. My experience is that it pretty much always sounds worse though.

Feel free to put up your design here and we'll help out as needed

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Paul Niwranski
1966 Cadillac Superior Crown Royale Limousine Hearse

If you're mirrors ain't shakin...you got taken...
RIP Member #8
Be careful what you wish for...

good timing there dukky.. I just did some drawings in sketch up.
The only thing that kinda scares me is how close the ports going into chamber 2 are to the side.. I think they are about 2 inches or so from the side. Which I have a feeling might create some port noise. Also, this is for an RE SX18, so it has to be BIG.

chamber 1 is approx 7.6 cubes after displacements, and 9.05 before. The ports are 5 inches in diameter and 20 inches long. According to bassbox the tuned frequency is 33.27 hertz (this is including the displacement of the connecting ports). The second chamber is 3.8 cubic feet, and I have no idea what it's tuned to. If I somewhat did the calculations correctly, then my educated guess would be 47. Anyone else?

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here's a better close up on how close the ports are to the side of the box

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Looks good. 5" diameter vents? Where are you getting 5" diameter pipe - it is hard to find. Why box programs spit out 5" diameter all the time is beyond me.

Oh and the middle vents don't have to be 50% in each chamber so you can slide them into the bigger one if you are having a clearance issue. Technically if you aren't directly accounting for the port displacements then the middle vent(s) should be 2/3 in the larger chamber to keep the volume ratio even (basic builders trick). Yeah enclosure volumes can be that critical. Along the same lines - remember to account for driver displacement in the large chamber.

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Paul Niwranski
1966 Cadillac Superior Crown Royale Limousine Hearse

If you're mirrors ain't shakin...you got taken...
RIP Member #8
Be careful what you wish for...

I chose 5" diameter piping... I should be able to find it somewhere

And all volumes are accounted for, so moving the ports farther into the other chamber would be a pain in the ass now. Actually it was pretty easy. I just accounted for one full pipe in each chamber. I guess now I just have to hunt for 5" piping.. I picked up some 3" yesterday for my home towers, and it was only 11 bucks, so I'm hoping that 5" isnt more than 20 bucks a length.. If I can find it.

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I wish you luck - I was a piping estimator for a mechanical contractor for a while and I've never seen it

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Paul Niwranski
1966 Cadillac Superior Crown Royale Limousine Hearse

If you're mirrors ain't shakin...you got taken...
RIP Member #8
Be careful what you wish for...

uh-oh... haha, well I may have to do some modifications.. I wish you'd never told me that..

Well it looks like I'll probably do it hopefully sometime soon.

But who knows.

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I'll ask around too...