Originally Posted by Aural_Sex
Can you expand on that? A single 8" port would need to be 178" inches long and cuts the box volume down to internal 2.25 ft^3 from 6.6 ft^3 - unless I'm missing something obvious (entirely possible).
Aha, and thus the dilemma of the vented subwoofer design! You're not missing anything, I go through this same struggle with every sub I design, battling a reasonable cabinet size with port volume to minimize port air speed. The more you attempt to achieve the ideal port speed, whatever you think that is, 20 m/s, 30 m/s, the more you fight an ever-increasing cabinet size to make up the volume consumed by using such a massive port. In the end you have to ask yourself, does a little port noise really matter that much to you? When you have to explain to your wife that this monstrosity of a subwoofer just doubled in size because that's the only way to achieve less than 5% port air speed, she's not going to be super sympathetic to your plea. I agree with the comments made though, dual 4" ports is probably too small for that sub. I'm seeing 45 m/s modeled in Unibox. You might want to rear-mount them anyway just to try and mask how much noise they're going to make.
Minimum should be (3) 4" ports, (2) 6" ports or (1) 8" port to stay below 26 m/s (according to Unibox, though not sure why all the port air speed discrepancies among the programs).
If you can't afford the volume, then you may want to consider going sealed. UM18-22 models great in a 6 cube sealed box with only a few dB less bottom end at 20 Hz. But still pretty close to your vented design. Note that I do not own this driver, so my experience with it is merely academic. Others can maybe speak to the benefit of this driver in a sealed application. I've built a handful of both ported and sealed subs and they both have their merits. But you stumbled upon one of the major drawbacks to a ported design and that is that if you truly care about port noise, the port area has to be large, which means the ports have to be long for a low tune, which means the boxes have to be big. Most designers/manufacturers ultimately will compromise somewhere along the way. I've got a dual passive radiator design in the works to combat this very issue (like Chris mentioned above) - small box, low tune, high WAF, no port noise, but it does come at a cost, to your wallet!
You gotta choose your battles.