Why does port length increase when two ports replace one?

fusseli said:

I wouldn't ever sacrifice tuning frequency for a shorter port, unless onyl absolutely necessary. Another hugely important note is that the cross-sectional area of the port will allow enough airflow for the sub(s) of the given design. I think air velocity should not exceed 30 m/s. Anybody correct me if I'm wrong here...

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You are correct there. I typically try and use 15-16 sq in of port area per cubic ft of air space. It keeps the air velocity down to a min.

Yeah, I used 3 3" ports on my 18hz tuned enclosure. It worked out perfectly. BUT my enclosure started out as a 7 cu. ft. enclosure. By the time you take into account for port, sub and various other displacements, it worked out to where the usable volume was 5 cu. ft.

I use bass box pro 6 to model my projects. Not sure if that makes a difference or not, but it might.