OK, they aren't planars but they are
dipoles and I'm using them with planars...
The speaker building psychosis got the best of me again after becoming intrigued by Ripole subs. I see a Riplole as basically a compact folded-baffle dipole that plays very low (below 20hz) and because it projects a “figure-eight” sound pattern that nulls the output off-axis, it should excite fewer of the room resonances that can give bass the dreaded “one-note boom”. Ripoles are reported to be clean and musical but, being dipoles, they can’t pressurize a room with the kind of chest-thumping bass that’s preferred for HT sound effects.
I chose the Peerless SLS 12” woofers for their optimal Ripole parameters and nice price. The Ripole patent by Axel Rithadler is in German and the math is over my head anyway so I just followed some basic guidelines gleaned from posts by some smart guys over at the DIY Forum:
· Area of front chamber opening should be 1/3 to 1/4 of woofers’ piston area (SD). For woofers
with > 10mm X-max, use 1/3 SD minimum. For woofers with < 10mm 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 chamber resonance of up to 20db to develop between 200hz-300hz, so either test
and build a compensating *notch filter or choose a crossover frequency and slope that keeps
the output below the chamber resonance.
*Approximate values of notch filter for 4 ohm subs:
L serial 3.3 mH, 0.4 Ohm
Parallel LCR: 365uF, 1mH, 0.28 Ohm
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.
Sheathing and trimming the boxes is probably more work than most people would want to endure so I’ve attached a drawing and cut list for a simpler box using only 3/4 MDF or plywood. The woofer cutouts in the baffle boars 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 side panels allow .090" of clearance for the speaker magnets and I used foam speaker gasket tape to seal around the woofer magnets. Of course, I also have a CAD drawing of the box with the oak sheathing and trim if anyone wants it. I also have more photos but I guess I reached the limit on this post.
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 will likely opt for a low crossover point so the subs aren't stepping on the TL woofers in my hybrid stats (which have a gorgeous tone).
I prefer to wait until I've had more time to experiment and tweak the setup before I comment too much on the sound of my new Ripoles. For now I will only say that they are “unobtrusive” and play really, really low. I confess that I’m a little taken aback by how much they move the diaphragms in my electrostats sitting next to them!
Parts List for two Ripole subs with connecting 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
skill saw for first cut on MDF sheet
router with circle jig
3/4 Forstner bit for counterbores
1/8 & 1/4 drill bits
clamps for gluing