Ported Box Parameters

To help you decipher that info:

• Net air space, inside the box, dedicated for use by the woofer.  This is the volume of air after all displacements are subtracted.
• Two 4" round ports, each at 10.5" long.  This design apparently calls for two ports for a couple of reasons:  one, to increase the port opening area to eliminate port noise, and two, to increase the length of the port.  Two 4's often fit better than one 6" on the box.
• Probably velocity of the port air measured in feet per second.  If so, this is a good low number.  If it is meters/sec, it is high (but within reason).  Low is better.  Whatever it means, I'd say that with the two 4" ports you will be fine as far as port noise goes.  (It's odd that this is shown with a decimal place.  Usually whole numbers are used when speaking of calculated port velocity.)
• The actual displacement of the driver when "top-mounted" through 3/4" construction.  This is the normal mounting for subs; not recessed.  The driver part that sticks into the box takes up airspace, and that is what is shown.  There's a lot of junk on that sub by the looks of that number (it's high).
• The displacement of the two 4" ports when mounted through the baffle.  These displacements are calculated into the total inside airspace so that you finally end up, when all is considered, with the net air space shown in the top line.
• Total volume = 4 makes you scratch your head when you do the math, but that is what they are telling you to start off with, in interior cubic feet volume.  By the time you install the ports, the driver, and maybe whatever bracing they have detailed in the design, you should end up with 3.5 cu ft net.

The tuning frequency appears to be approximately 36 to 38 Hz..

Use a pencil and paper and do a lot of rough sketching.  Start by measuring the available space in your car where you want to put the enclosure.  Use a steel tape measure and get the measurements in inches.  You'll have to know width or side-to-side, depth (front to back), and available height.  These three meausrements multiplied will give an answer in cubic.  If you multiply inches, you'll end up with cubic inches;  if you multiply by feet you'll have cubic feet.  12 x 12 x 12 = 1728........the cubic inches in one cubic foot.

When you have a good idea of how much room you have to work with, go to the help section of this site:

https://www.the12volt.com/caraudio/boxes.asp   and   https://www.the12volt.com/caraudio/boxcalcs.asp

If you intend to use 3/4" MDF for the box, enter .75 in the box that calls for "thickness".  Play around with some numbers in these boxes that are within the parameters of space that you already measured in your car;  that is, each time you try a combination, be sure it will fit into your available space.  See what meaurements you can arrive at that will work in your situation and make the box size appropriate for your project. 

And because you want to learn, here is a list I've compiled over a couple of years of online help topics dealing with DIY audio:

Information links, calculators, and other online help:

http://www.bcae1.com/

http://sound.westhost.com/articles/cscaling.htm

http://www.thielesmall.com/database.asp

http://www.carstereo.com/help2/Articles.cfm

http://www.soundandvisionmag.com/idealbb/view.asp?topicID=37023

http://www.avsforum.com/avs-vb/printthread.php?t=759083&p=9024043

http://www.users.bigpond.com/bcolliso/flare-it.htm

http://www.opamplabs.com/cfl.htm

http://www.hometheatershack.com/roomeq/

http://www.cvs1.uklinux.net/calculators/