Since the woofer will be completely inside the box the more port area the better! That's where your sound will come from! I try to aim around 15sqin PER CUBIC FOOT! I say it nice and big because I myself forget sometimes and have to go back and redesign lol! From what I have read The shorter the port the better but that's not a big issue since tuning high normally means a short port. Your subwoofer needs to come from the sealed section facing into the ported section. As stated above with a ratio of 2:1 for ported section should be double the air space of the sealed section! Unlike normal ported boxes your not going to tune this low! In fact kind of high. The second section of the box is the ported witch plays the higher end bass. 8 and 1.5 cubic ft in the sealed section! remember the sealed portion makes the low end bass so the lower the f3 the better lol but normally I set it near the fs of the subwoofer! I normally design the sealed section around the recommended specs from the subwoofer manufacture! A 12" will normally need between. I start the design with the sealed section of the box. Some people do 3:1 but most commonly you'll find 2:1 works out best. So if your sub needs 1.5 cubic ft sealed then your ported section is going to be 3 cubic ft. Most 4th orders are a 2:1 ratio this means for every 2 cubic ft of ported section there is 1 cubic ft of sealed. Main thing to remember is the ratio of the box. If you do a lot of car audio or home audio this is well worth the investment!!! There are a couple of awesome programs out there that help with the design of a 4th order. The advantage to this type of box is you get the tight clean bass everyone loves from a sealed box but with a wider range of frequencies and higher output! You have a sealed section witch develops the lower frequencies and a ported section that develops the high end frequencies. I wouldn't mine if someone has more to add!Ī 4th order is simpler then it seems. I want to keep it simple and sweet! Hope this helps a lot of new people!!! I'm not a expert but from my research this is what I believe is best. This is my quick write up on 4th order bandpass boxes.