Ripol Subwoofers:

The speaker building psychosis got the best of me again after becoming intrigued by the unique dipolar subwoofers patented by German speaker builder Axel Ridtahler. The name "Ripol" is short for "Ridtahler dipole".

Unlike most other subs the woofers in a Ripol are in an opposing push/push configuration. The resulting compression in the front chamber actually lowers the woofers' fundamental resonant frequency by as much as 10Hz (depending on chamber volume and opening area). Thus; the Peerless 12" SLS woofers, which normally have a resonance of 27Hz in free air, can potentially extend to 17Hz in a Ripol configuration.

A Ripol is basically a compact, folded-baffle dipole. It can play very low, and because it projects a “figure-eight” sound pattern that nulls the output off-axis, it excites fewer of the room resonances that can give bass the dreaded “one-note boom”. Ripoles are clean and musical but, being dipoles, they can’t pressurize a room with the kind of chest-thumping bass that many prefer for HT sound effects. I would say that Ripoles are great for music but not so much for theater earthquake effects.

The Peerless SLS 12” woofers are favored by Ripole gurus and they are fairly inexpensive.  Ridtahler's patent is in German and I had no idea how to model a Ripole anyway so I just followed some basic guidelines gleaned from posts by some smart guys at the DIY Forum:

Area of front chamber opening should be 1/3 to 1/4 of woofers’ piston area (SD). For woofers having more than 10mm X-max, use 1/3 SD minimum.For woofers with 10mm or less X-max, chamber area can be 1/4 SD.

Area of Rear chamber opening should be 1/2 to 1 SD.

Chamber depth need not be greater than necessary to fit the woofer.

Expect a powerful chamber resonance up to 20db to develop between 200hz-300hz-- so either test and build a compensating *notch filter or choose a crossover frequency and slope significantly below the chamber resonance.
*Approximate values of notch filter with 4 ohm subs: L serial 3.3 mH, 0.4 Ohm Parallel LCR: 365uF, 1mH, 0.28 Ohm. Take these values with a grain of salt because you would likely need to do some measuring to determine the exact chamber resonance and then tweak the filter component values to exactly match up the notch to the resonance.
Rather than using a passive filter as described above, I used a Behringer DCX 2496 digital crossover, with the crossover frequency set below the resonance using a steep slope and then used the parametric EQ to fine tune the sound to my liking.

The boxes are 3/4 MDF, sheathed with 5mm red oak plywood, and edge-trimmed with quarter-round oak molding to match my other living room furnishings. The box center section pieces are solid white oak planks stained in a contrasting color for effect and indexed to the mating box halves with (4) 1/4 oak dowel pins on each side.

The box is assembled with 1/4-20 all-thread rods and cap nuts as shown in the photos.

Gluing on the plywood sheathing and edge-trimming the boxes is probably more work than most people would want to endure so I’ve also attached a drawing and cut list for a simpler version, using only 3/4 MDF or plywood.

The woofer cutouts in the baffle boards are cut with a bit of extra clearance to allow the woofer magnets to center themselves in the holes in the outer panel. The holes in the outer panels allow .090" of clearance for the speaker magnets and I used foam speaker gasket tape to seal around the woofer magnets.
The woofers are wired in parallel for 4-ohm load and I’m using a Behringer DCX2496 crossover and TFM-25 amp. For the initial setup I’m using a 48db L-R slope and experimenting with various X-over frequencies.
Some people prefer to play their subs into the midbass region and others take the view that subs should never actually be heard, only felt. Certainly the latter view is best for subs that have heavy woofers but the SLS’s are lighter than most so would offer a wider clean-operating band. I prefer a lower crossover point so the subs aren't stepping on the TL woofers in my hybrid stats (which have a gorgeous tone).

Parts List for two Ripol subs (MDF version), including speaker cables:
(4) Peerless SLS 12" woofers (Parts Express) $320
(2) Pair binding posts (Parts Express 091-1245) $18
(1) Sheet 3/4 MDF (Home Depot) $25
(4) 1/4-20 x 36 all-thread rod (Home Depot) $7
(16) 1/4-20 furniture cap nuts (Home Depot) $16
(8) 1/4-20 threaded wood-inserts (Home Depot) $8
(8) Speaker spike feet (Parts Express 249-727) $4
(1) 1/4 x 36 oak dowel $2
(32) #8 x 1” cap screws (Parts Express 081-425) $3
(1) bottle yellow wood glue $4
(8) Banana plugs (Parts Express 091-356) $11
(1) Roll speaker gasket tape (Parts Express 260-542) $6
(1) 50ft- 14ga flat-braid copper speaker cable (Rat Shack) $25
----- Total $443

Tool List:

skill saw for first cut on MDF sheet
table saw
router with circle jig
3/4 Forstner bit for counterbores
1/8 & 1/4 drill bits
clamps for gluing

Click to enlarge photos:

Below: The version shown in the photo above

Below: An easier to build version using MDF