The "ABC", Series Tuned or 8th Order enclosure is similar to 6th order bandpass enclosure, as it uses two precisely sized and tuned ported boxes, but has a third port joining both enclosures. Unlike a traditional 6th order where the subwoofer is between two enclosures vented to atmosphere, one of the vents fires into the other enclosure which is then vented into the atmosphere.
The narrow band width and substantial peak at a higher frequency makes this a great enclosure type for SPL competition. Like all bandpass boxes, this one is pretty finicky about air volumes. Make sure you're measurements are spot on; otherwise you'll end up with a horrible sounding, badly performing enclosure.
Still interested? Of course you are! Here’s a simple design to build your own ABC. If you're doing this for SPL, then you'll probably be using a 15" woofer, so that’s what these sizes are based on.
1. Build a box with 2 chambers, one at 3 ft^3 and the other one EXACTLY half of that, 1.5 ft^3. Make sure the dividing wall has been cut as your baffle, with a hole for the subwoofer and the port between the chambers.
2. Next cut 3 ports of the same length, each from 6" pipe. Use 12.5" long for a deep rumbling response or 8" for SPL.
3. Put one port in the front of each chamber and the third in the wall separating the two chambers.
Now you have a box with a big chamber with a vent in it, a small chamber with a vent in it, and a third vent in the wall between the two. The woofers basket & motor should be inside the larger of the two enclosures.
4. That’s it!
When building your boxes, don’t forget to factor in the displacement of the subs motor and the ports. So that’s 3 ft^3 in the larger chamber AFTER subwoofer & port displacement, etc.
The way this enclosure works is, at low frequencies, it all functions as one big box with 2 ports that is tuned to 30Hz (12.5") or 35HZ (8"). At higher frequencies the middle port restricts air movement and the second chamber begins to act more as a resonance chamber but not as part of the box. At this point a second tuning frequency develops 1.9 times (almost an octave) higher than the lower one, i.e. either at 57Hz (12.5") or 67Hz (8").
The results are that it plays both low and loud, while being relatively nice sounding. The drawback is the box size and complexity.