Ok, here it comes one time! I only explain these in detail once a year (I make $$ on them )
For that DVC 15. Build a box with 2 chambers, one at 3 cuft and the other one EXACTLY half of that, ie 1.5 cuft. Next cut 3 ports all IDENTICAL in length. Put one in the face of each chamber and the third in the wall separating the two chambers. In this instance each port would be either 12.5" long for a little deeper response or 8" long for a little more SPL.
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 woofer also mounts in the large chamber.
The way they work is that, at low frequencies, it all functions as one big box with 2 ports, and is tuned to 30Hz (12.5") or 35HZ (8"). At higher frequencies the middle port starts to restrict 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 , ie. either at 57Hz (12.5") or 67Hz (8").
This is similar to a vented bandpass but has more output and smoother sound due to the woofer not being tucked inside the box. The results are that it plays both low and loud, while being relatively nice sounding. The drawback is the box size.
Did one 2 years ago in a sedan with 1 12 on 400 watts and pulled a 138 on music. There's a guy around here with 2 DVC 15s like this in a Daytona off a single 800a2 pulling 149s.
They work but can be intense to build. They are the ONLY box I take care to factor exactly both port displacement and woofer displacement. Mess one of these up and they work very poorly....