Re: two 15" alpine type r's some ported box questions
In short, larger ports equal less possibility for port noise but too large a port lowers power handling and makes the sub(s) more prone to damage when played below Fb. With respect to box tuning, more port area causes you to need a longer port. i.e., a 3" port tuned to 30 Hz will be shorter than a 4" port tuned to 30 Hz, all other things being equal. Larger box volumes will allow for shorter port lengths to achieve the same tuning frequency.
Bent ports won't change increase SPL but they can decrease it if not constructed properly. When you do a bent port, you need to maintain port dimensions as closely as possible throughout the turns, which can be sufficiently achieved using 45° wave guides in the corners.
Lastly, if we assume about 3 cubes per sub for the subs you mentioned, you need ~100inČ of port, which isn't too far off from the 120inČ of port area you mentioned. And in the end, I'd rather see a little more port area than needed than a little less, as turbulence in a small port can ruin the sound quality of the box.
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