A 4th order is simpler then it seems... You have a sealed section witch develops the lower frequencies and a ported section that develops the high end frequencies. 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!
There are a couple of awesome programs out there that help with the design of a 4th order. My personal favorite is bass box pro 6. You can get it for around $150 at parts-express.com. If you do a lot of car audio or home audio this is well worth the investment!!!
Main thing to remember is the ratio of the box. 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. So if your sub needs 1.5 cubic ft sealed then your ported section is going to be 3 cubic ft.. Some people do 3:1 but most commonly you'll find 2:1 works out best. At least I have so far.
I start the design with the sealed section of the box. I normally design the sealed section around the recommended specs from the subwoofer manufacture! A 12" will normally need between .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!
The second section of the box is the ported witch plays the higher end bass. 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 best (IMO) is between 45hrz-50hrz.
Now here is where it gets fun... Your subwoofer needs to come from the sealed section facing into the ported section. 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.
Since your subwoofer needs to be inside the enclosure the best thing is to include a removable front panel in your design. I did mine with the face of the box. Another thing you may want to do is face the woofer into the sealed section so the motor and basket are in the ported section so you can smell if the coils burning! I trust my self not to fry anything so I leave it facing the ported section plus IMO its easier to calculate the displacement in the sealed and it keeps the ported section a little smaller!
Hope I covered everything and it helps the new comers!
one i built
Edited by donellk, 01 December 2011 - 12:26 AM.