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..

 

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

 

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?

 

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

 

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.

 

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 *** 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.

 

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

 

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.

 

I'll ask around too...

 

I know this post is old, but as I've been researching, this information is new to me, so I'm digging up old posts and I wanted to question this:

If you aren't accounting for port displacement, shouldn't the port be 100% in the larger chamber? Here's my rationale.

Box is divided 2/3 & 1/3.
There are 3 ports of all equal length.
There for, I suggest 2 ports fully in the large chamber 1 port fully in the small chamber
This would mean that the large chamber contains 2/3 of the ports volume, and the small chamber has 1/3 of the ports volume.

One port in the large chamber could take up 10% of the volume, and the same size port in the small chamber would proportionately take up 20% of the small chamber since we are working with a 2/3 & 1/3 ratio. Therefore, I'll be so bold to say that the basic box builders trick should be to place 2 ports fully in the 2/3 chamber and one port fully in the 1/3 chamber.