I am trying to design a vented box for a DIYMA 12" sub using WinISD. I read threw the tutorial that this site offers which helped out alot. I have few questions that it did not cover though. I am trying to tune it at 30Hz and it seems to want 30L for volume. Which is really small, evidently these subs like small enclosures. So now for the port, I am wanting to build a rectangle rear port. I understand I want to keep the port resonance at or above 120Hz and the port air velocity needs to be no higher than 25 m/s. But I am not sure what the "cross area" should be at or what the "end correction" number means. Also why is it that the bigger W and H I make the vent the longer it has to be, you think it would be the opposite.