Stereo & Electronics - Port tube questions

I have a JL Audio 12w3v2 and a sub box with 1.8 cu ft of space that fits in the trunk area of my TA. I would like to port the box and tune it to 30.3 Hz. The JL book shows a 1.75 cu ft. sub box with a rectangular port that is 1.375" tall x 12.5" high and 21.5" long for a total of 369 cubic inches tuned to 30.3 Hz. I can't find a port tube that has that much internal volume, so I was considering the possibility of using 2 port tubes to total 370 cubic inches. The only thing I'm not sure of is what determines the frequency of the port and how it would need to be changed to achieve 30.3 Hz with a box that is .05 cu ft larger. Would I simply use 2 ports of half of the required volume... and how do I know how much port volume I need to get 30.3 Hz in a 1.8 cu ft box?

search for and download winisd .
port tuning is an exact science.. even small variances can greatly effect the tuning freq.

I may be wrong, been awhile since I calculated a ported box, but I thought it was surface area of the port, not internal volume that determined its tuning freq.

search for and download winisd .
port tuning is an exact science.. even small variances can greatly effect the tuning freq.

I may be wrong, been awhile since I calculated a ported box, but I thought it was surface area of the port, not internal volume that determined its tuning freq.
Its kinda both that will effect tuning. The internal volume effects tuning because of the air the port will be displacing, bigger port = less air inside the box and vise versa. Now a change in internal volume with effect the tuning.

Evenrything needs to be perfect when it comes to porting. You had it right when u said its an exact science... Designing gives me head aches sometimes :bang:

I found a website with a calculator for port length and diameter.

http://www.carstereo.com/help/Articles.cfm?id=31

Download winisd from linearteam.org(i think, you might have to search google) and READ THE GETTING STARTED GUIDE!!!!! You will be a tunin' machine if you can understand everything...it gets pretty technical

don't forget also to find the displacement of the port and take that out of the box dispalcement.

don't forget also to find the displacement of the port and take that out of the box dispalcement.
as well as driver displacement

I found a website with a calculator for port length and diameter.

http://www.carstereo.com/help/Articles.cfm?id=31
thats what i use :)