so Ive been thinking, I remember when I had my old Fi IB3 18" in my Aztek, 21.5 cubic feet, 7.5" diameter port, 15Hz tune...and I experimented with it like I did the towers and cut it down to a 20Hz tune, then a 25Hz tune, then I ripped the port completely out..therefor I had a 3/4" port length..the thickness of the wall. I still don't think that 3/4" port length tuned the enclosure past a certain point.
I think the port volume acts as a slug of air, Fallen once explained it best that way...
that slug of air is just like a speaker cone in that it moves in and out, has excursion. when you have a 16.5" cone moving in and out 3.25" inside a given volume of air, the enclosure, you are moving the air in and out of the port. How much does the air slug move? What is it's excursion? The same as the driver? No... 1st off.. the driver area is bigger than the port area, so if it were this simple... you'd have to think volume. If the 17" cone moves 3", then the 7.5" slug must move:
well, 17/2= 8.5*8.5=72.25*3.14=226.865*3=680.596
about 681 cubic inches?
The air slug in the port would have to move 15.5" for the same Vd... 5 times as far
to double check,
but, as we move further from tuning we get less gain... and at tuning we dont get exactly double the output... we get a lot more, depending on the enclosure.
So really IDK what I'm talking about, but, I don't think a 3/4" long port 7.5" in diameter will fully handle an 18" subwoofer moving full excursion 3.25" p-p.. whether it's in 2 cubic feet (less port gain) or 22 cubic feet (more gain).
Just as well, I don't think my 12" diameter port 5" long fully supports my 18" in 128 cubic feet...even more port gain.
For my VAN build, I plan on using about a 20" port for the 4 18's.... I am thinking about ordering 1 long 20" sonotube and using the scraps for the Two Towers. 127 cubic feet, 20" port, to tune to 14hz needs to only be 22" long... drivers remain under Xmax with only 600 watts, showing an airspeed of 8 m/s at tuning!