title^. one website says subtract the length of the front chamber port from the length of the rear chamber port and that gives you the inner vent length, is this true?
If they are the same diameter, then supposedly that's the case, yes.
In reality, you may find yourself having to adjust the vents a bit to hit the target resonance frequencies, so design your subwoofer accordingly to allow for this.
That really depends on factors like how close the vents are to the side of the box, or how close the internal opening of the vent is to the rear of the box. Usually though if you go with the design program's suggestions, you may find that the ports have to be trimmed to achieve the target resonance points (which is a lot better than finding out they have to be extended).
In my experience, no that is not true. It was would nice if it did work out that way, in practice tuning seems to be much more complicated.
One interesting (and frustrating) aspect of series tuned boxes is the chamber tuning interact with each other. If you raise the tuning of one chamber, it will raise the tuning of the other, and vice-versa.
HornResp can simulate series tuned 6th order boxes, you might want to play around with it a bit.
Actually Thomas Danley had indicated to me (here?) some time ago that using that approach would work. It might be possible to find the thread in question with a search.
Of course experience always trumps theory, and there's always the issue of the vent lengths being somewhat dependent on other factors as well, including proximity to the side walls. Even vent SIZE wrt compartment size can impact Fb, as I found out with one bandpass build. So, it's a matter of build, test and trim.
For a series-tuned system, my approach would be to trim the "outside" vent to hit the target upper resonance point, then trim the other vent to hit the lower resonance point.
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