Raven,

Consider this, and then tell me what you think:

From my calculations for the 36 x 36 x 12 in interior depth, the volumes for the vented chamber is 1456 cu in, which is somewhat close (17% higher) to the calculated volume I gave you at Fl =30 Hz which was 1245 cu in. You said you were going to brace it to 'heck', so I suspect the bracing volume displacement can bring the net "real" volume closer to the 1245 cu in. I say close enough...

And...similarly, the volume of the 36 x36 x 12 in sealed chamber is 2054 cu in, without any reduction for speaker magnet, and the volume I calculated at the Fl =30 Hz, is 1726 cu in, about a 328 cu in difference. Again, if you estimate the volume of your speaker magnet and cone inside the chamber to be close to that 328 cu in, I'd say that's an ideal design already. Both chambers appear to be at the design volumes for the assumptions I made - 12 inch depth (I'm assuming for your 10 in speakers), S=0.7 and Fl=30Hz.

Remember, these calc'd volumes are for a true 4th order bandpass, but this cabinet adds the horn component, which may inherently extend the tuned frequency to below the 30 Hz cutoff. Anyone comment to that statement??

But that's really beyond my understanding. :-?

Actually, I don't completely understand all of the theory behind these design formulas either, but the math is easy for me to follow, and I can build a sheet to compare numbers on 'paper', making changes and corrections simple. I love Excel. I also cheat using Autocad to sketch the layout in a jiffy, and then it can automatically calculate the areas (and volumes) for each chamber.

But, if you want to

*really* to see what's required at Fl=20 Hz...

Vf (volume of vented chamber) = 20.4 liters = 0.721 cu ft = 1245 cu in (same as before)

Vr (volume of sealed chamber) = 118.7 liters = 4.19 cu ft = 7242.5 cu in (HOLY COW...a little too large?? :-/)

Qbp (Qtc of sealed chamber) = 0.61

Fb (resonant frequency of vented chamber) = 34.9 Hz

Pa (gain) = -2.65 dB (not ideal to drop into negative here...)

It appears to me that your speaker drivers don't work well for Fl of 20 Hz. I dont even have to rerun the calcs to show that 15 Hz wont work either...

However, Fl=25 Hz gives a Vr volume of 2885 cu in, which you could achieve by increasing the interior depth to 16.85 inches. You can decide if that make sense for you or not...

Let me know what you end up doing. Take care.