In this article I talk about Summed SD.
SD = cone area of your woofer in inches square (it's in the TS specs of your subs)
Summed SD = total cone area, in inches square. EG. If you run 10" subs, and they have an SD of 55 sq.inch, and you've got 4 of them. The Summed SD = 4x 55 = 220 sq.inch
To perform these tests you will need to use an accurate SPL meter. When you do the tests, test over a range of tones. Don’t just get in your head "around 57hz is loudest" (or whatever your cars note may be) and just test around that, the changes you’re making may be finding gains up higher or down lower. I like to test 1 Hz either side of the "Mad Note”, roughly where the system should peak. I also test 5 Hz lower (IE. 49 Hz) and 5 Hz higher (IE. 61 Hz), just to make sure I'm not missing anything important around the 40 and 60 Hz regions. If a test shows that 56 Hz is louder than 55 Hz, then I'll try 1 Hz above that until it gets quieter. There are some good gains (up to 2dB) to be found just from doing this alone!
The key is to start with BIG changes, then slowly work down to small ones... Big changes result is big difference in numbers, which makes it easy to see which way you should be heading.
Don’t forget to include SPL gaining tricks in your enclosure design BEFORE testing. Like making the rear wave path 3x the distance of the front wave path if you can (sub to port + port length + distance to mic point = 3x distance from cone to mic point). Or putting in 45 degree angles on each corner & joint to prevent dead zones. Or bracing the enclosure with fibreglass, steel rods or timber. Or bracing with fibreglass. Make sure you run your complete system as well, there's no point tuning for 1 sub when you want to use 2 or 4, or using a 500WATT amp if you intend to compete with a 2000WATT amp. These tests are meant to be done on the enclosure & equipment you'll be using in competitions!
Stage 1: Enclosure Volume.
For this stage, use a test port that’s 13" deep, and around the Summed SD in area (as explained above).
The way to test the enclosure volume is to make a big box (prolly bigger than ideal) and slowly reduce the space inside the box whilst blowing tones... Use bricks or blocks of solid timber, phonebooks won't work they're not dense enough, and reduce the volume 1/2 a cubic foot each time. If the numbers are going down, then the box is prolly ideal (or could stand to be bigger). If the numbers slowly go up, then the box is too big, keep reducing space until the numbers start to drop, now you can work out the ideal enclosure volume (internal volume of your enclosure minus the volume of bricks).
Stage 2: Port Face Area.
For this stage, keep the ports 13" long.
You're definitely on the right track with the ports.
Try 3 different sized ports... One that looks about right (summed SD), one that’s WAY too big (2x the summed SD, or close enough) and one that’s WAY too small (1/2 the summed SD).
Just chuck the ports in and blow tones, see what happens. Once you have an idea of around about what size is best, you can start adjusting around that size (IE. try a slightly larger and slight smaller port against the initial test size, etc until you find the ideal size. Ideal = loudest)
Stage 3: Port tune.
This is the easiest part. Now you've got your enclosure volume to port area to cone area ratio (IE. You know what volume + what port area + your subs are best), you just need to adjust down the port length to get the tune spot on. Make two new ports, same size as the one you decided was best in Stage 2, but make one about 18" deep (or however deep you can fit), make the other 8" deep. Again, run tones to find which the better length is, then make some more testers around that length.
The trick (and what takes the most time) is finding the ideal box size & port size for your configuration. The best thing about doing it this way is that it’s applicable for every SPL system, not just walls :). You'll notice I made no mention of WinISD, Enclosure Calc, and the 1/4 wave tuning frequency method or anything else... Those are the theoretical ways of doing all this. This is the hands on way of doing the tuning, and it seems to work the best, and as we know not everything works in practice.
Oh and one final thing. Make sure you write down EVERYTHING you measure in a note book! Document everything you're doing with the car, because I assure you WILL forget most of it and you'll kick yourself later (I know I do). It’s also nice to have a log book of results from the tests, so you can see where you started and how far you've come! :)
Some other considerations for you will be:
Largest enclosure size you can fit
Largest port you can fit
If you have limits on size you can use because of your class, or needing to use the car for other purposes than SPL comps, figure out how big a box you can fit and work with it instead.
Because using a Summed SD might not be practical (or possible) for you use the old classic formula: 15-16 sq.inch of port face for every cubic foot of enclosure space. Length remains the same at 13".
But the testing method remains the same!