Tuning a cabinet to extend response beyond & below resonance of the driver

You always want to go with a sealed cabinet. You limit your low end reach by using a port. The trick is to lower the Q and control the driver by using a digital crossover with EQ capability Then you can make the driver do anything you want as long as you have the power to do it. I would make an enclosure around 6 cubic feet plus the volume of the driver. The problem is building an enclosure that does not resonate. Most enclosures are musical instruments. Large flat sides will resonate the most. With bracing, you just raise the frequency. The stiffest structure is a sphere, but a sphere is very difficult to work with. Next is a cylinder. You can easily place a driver in the end of a cylinder. You can get 20 - 24" pipe in any number of materials, the best would be aluminum. With a driver that large you want an enclosure that weighs at least 200 lb or you could place a driver in both ends and make a balanced force subwoofer. I made the cylinder with a decadron cross section using 1.5" plywood placing a 12" driver in both ends. 18" drivers would require a huge enclosure, double the volume of a single driver enclosure. My guess is that would be impractical. How you make the enclosure also depends on the shop capability you have available. 

Good luck. If you have any questions message me.  Mike

I have followed your advice, except with a Bi-Horn cabinet to get the efficiency hi enough that the 24 dB Xover and 31-band EQ allows me adjust the Below-Fs response to keep it flat to 24. The cabinet weighs about 235 lbs. (Sits on casters), the two 12" drivers are used dipole with a 12" Tractrix horn in front and a rear folded Tractrix horn 6' 6", with the horn using the corners of the room. The drivers have a high Qes .81, a Qts .71, Cms .12. I had them built 4 ohm so I could wire them in series, giving them BL 17. They have an extreme Xmax capability of 23 mm, enabling the two 12" to move plenty of air, providing cushion to the low end, able to handle the EQ. With the combined effects of the 2 drivers on both right & left channels, dipole, with horns, it uses little power. Top cutoff is 120 Hz where AMT-Planars, 2 each side, handle the mids. The cabinets are 38" wide by 38" deep by 30" high, but that’s mostly due to the horns. The horns are heavy because the horns have 2" thick walls of solid MDF cut by laser, covered in walnut veneer. The size and weight do not include the upper dipole bi-horns of the midranges and tweeter, which are similar in outer size though the horn measurements are quite different. They give a spacious sound, tight, highly defined, sharp, but mellow. (Using a 300B tube amp.) I recognize that the big difference is the weight killing those resonances and the EQ. I’m thanking you for your help, and always curious/ interested in any feedback?

@mijostyn Re: my post above. I've read so many posts on this site that have been helpful. So total I have four 12" dipole subs covering 24-120 Hz, which I felt may be necessary for the low and sustained organ notes, as I listen to a lot of choir music. There are two midrange dipole AMT-planars each side that handle 120 to 1.5 kHz - 6 kHz, and AMT-planar tweeters dipole from 1.5 kHz -6 kHz up to 20 kHz. With the Linkowitz active Xovers I can easily vary the Xover frequency and I'm still playing with them to see what sounds best. My big focus was to get as much as possible of all voice frequencies handled by the Planars and the tube amp. I guess the speakers are modified Open Baffles. The midrange, which covers 6 octaves, are driven by a 300B tube amp, the subs by a SS amp that uses the TDA7498E chip, and the tweeters by a SS amp that uses the TPA3255 chip. The SS amps enable some operating chip rolling, so I am still looking for what might be the best OPA chips. The subs and tweeters' amps use a tube preamp. I got a hold of a Dual 1218 TT and a decent stylus, a 3 x 7 mil elliptical that tracks at 1.25 g. Any suggestions?