Port area can be complicated. Depends on if it's a ported box, or bandpass. Or if it's in a car or in a house. Too big of port, you lose some quality, maybe will sound muddy, but the biggest loss is in bandwidth. Too small and you get port noise and decreased output/SPL.
It's really situation dependent. Even with ported boxes, say with my 2 18's. I have 12.3 in^2 per cube port area @ 29 hz. But if I was to go for a SPL it'd be more like 16+ in^2 per cube. So port area can vary tremendously. Some aero boxes I've done only have like 4-5 in^2 per cube and performed very well.
When there is more air in the box for the woofer to move, there is more air coming out of the box...think of it that way and that's why you have to have a bigger port.
Also, as you tune lower, your air speed in the port will lower as well given all other variables stay the same.
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