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I am trying to design a vented box for a DIYMA 12" sub using WinISD. I read threw the tutorial on HTS 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.
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"Cross area" must be the cross sectional area. A round port has an area determined by the diameter, a slot port can have the same area in a unlimited combinations of width and height. Guess the area is what you have to go with for an overall "size" dimension, rather than the one and only diameter of a round tube.
Edge correction is the factor that a flared port adds to the length. If you roundover the edges of the port, it would change the length a bit (for the same tuning frequency) compared to a flat edged port. I'm not sure if a slot port gets the same change factor a round one does, but probably so.
You might want to round the slot port edges, on any kind of port a flare helps reduce turbulence and the resulting noises with higher airflow. Other general airflow ideas, don't go too thin on the slot, i.e. try not to get extreme with the width to thickness. You could probably imagine how 1/4"h x 16"w port would have more friction with the walls than a 1"h x 4"w one, even though they have the same area. I'd start in WinISD with a round port, of reasonable size for good airflow numbers, then convert to a slot with the same area.
Lastly, that's the way it goes with the tuning. Kind of counter intuitive, I know... And that's why it can be a real challenge to do a vented sub in a small box. Very low in a small box, you might have to go the "smokestack" approach since the port starts getting too big to put in it. Passive radiators could be a way to go then... And of course, the slotted port can just make your box bigger, with the port going around one or two sides and the actual box a smaller part of it.
Hope that helps...
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