port design

Thank you once again stevdart, and everyone else, for your helpful information. I am running into a bit of a problem designing MY next box, and I suppose it would be prudent to post the question here instead of making a new thread. I have a pair of MA Audio MA120XL 12" subs. They are currently in a ported box, similar in design to the last picture posted; 3.5 cu. feet (total - about 3.25 w/o ports, sub, etc included), two 4" ports 13.75" long. Note - this is per sub, not for both subs. This box was actually recommended by MA Audio as "optimal". Optimal for what, I don't know. I would like to build a new box, tuned lower in order to achieve frequencies I don't believe my subs reproduce well. In my opinion the box sounds great most of the time. However on certain songs with what seems like really low bass notes, the subs move back-and-forth quite wildly and sound not so good. I believe this is referred to as "unloading". My real problem is I don't have the T/S parameters for the subwoofers; MA Audio does not have them (so they said), and I have been unable to find them on the internet. I have actually asked people that have posted on forums that have my same subs, but to no avail. What would be the best method for designing a box for these subwoofers? Should I just stick with the size of the box and re-tune it? Note - My vehicle is a Ford Explorer. Amp is HK401SX, running at 1 ohm.

Well, MA Audio speakers will never be confused with higher-end component, so I'm not surprised they do not publish the specs.  If your subs are unloading you can quickly destroy them, so first off I'd suggest a HP (subsonic) filter set at your tuning frequency.  MA Audio subs probably have an fs up around 40Hz, so you've never likely to get really deep low bass out of them.Support the12volt.com