DIY Access Cab Ported Sub Box Help

So I am making a custom sub box for my access cab tacoma. I am planning on putting the box in between the seats and becoming an arm rest kind of thing. So I am going to make it rear firing so it is going off the back wall. My question is how long does my port need to be. I see on JL's website they have a bend and I dont know if that just needs to be total length of the port or what. I would just like some clarification. Below I attached my idea how the port would go in the green. and also a few other designs that I want the box to be similar too.

 

the reason they have a bend in it is to meet the required tuning length of the port.

there's a few main factors in adding a port to get to the appropriate tuning frequency, but the tuning frequency is really what sets the requirement you're asking. port length is the variable that sets the frequency, while the diameter effects the amount of airflow through the port-- a larger port requires longer length to meet the frequency, while a thinner port can meet the frequency faster, but too thin can introduce more noise in teh form of airflow movement through the port.

round tube ports only need to deal with diameter and length, though smaller ports or a lack of flaring can introduce audible 'chuffing', or port noise.

square/rectangular ports, the width, height, and length alter the tuning frequency. but the same rules apply-- a 1/4"x 3" port will also result in chuffing.

the next step here would be to start playing with a subwoofer box building programs. while jl's spec's work for their box, you will likely need to alter one dimension or another to better suit you needs, or potentially alter the port dimensions-- they're using a large rectangular port, it's possible to meet the tuning frequency with a smaller round port.

 

I am just getting into the car audio thing and I feel like there isnt really a straight forward answer on port length/size. I have 1.218 cubic feet of space without a port but I dont know what size I need for it to not sound like crap.

 

there's no definitive answer on port sizing because there really isn't one. most common is going to be a 3-4" round port, or use a rectangular port similar to jl's design that utilizes one of the box walls. generally, you want 1-3" continuous width on that. it's really all a balance between between narrow and short, the the compromise of increased air velocity, or large and extended port length.

winisd is a free frequency response program, it doesn't give box dimensions, but will allow you to set up the overall box volume, and determine port size/length for the specific tuning frequency needed for a desired response within that specified volume.

definitely start looking into other box building programs. they do a lot of the heavy lifting. of course, there's plenty of resources for the mathematical formula's and rules of thumb needed (diymobileaudio, caraudiojunkies, stevemeadedesigns, or the12volt), but most programs take all that into account already.

the biggest thing about ports is that it takes away internal space from the box, and that must be appropriately accounted for with the box volume calculations. this is where 'external port' boxes are derived from, though they tend to end up looking a really funky.

 

I called JL Audio and asked them how long a circular port would be and they told me 4" diameter and 20.5" long. That is for the same internal volume which works out well for me. It is crazy how much space a circular one saves compared to a rectangular port. I will post some pictures this weekend when I finish up my box. Updates later this month how everything sounds when I get time to wire everything up since I will be on vacation.

 

Slot ports occupy more volume because of the materials used. If you're using 3/4" material, you have to add all that to total box volume. Then consider that your port volume is separate from the enclosure volume - which will factor in regardless of which port type you decide to use.

Pre-made cylindrical ports are certainly easier to design and implement, but their relative elegance is questionable as well as rigidity. Slot ports double as bracing & most enclosures don't have enough of that.

I'd suggest that since you're just getting into car audio that you consider a sealed enclosure instead. What you're trying to do is fairly advanced - especially for a nOObie. Building the perfect enclosure isn't the end of the puzzle. They're also trickier to implement & not everyone appreciates the group delay. Then there's cabin gain, which is really tough to predict.

I suspect you'd have maybe enough room for an 8" like you're planning. Maybe. Ported enclosures always wind up taking more space than you planned. Going with sealed you could use a 10 or 12 and enjoy the SQ benefits of less cone movement for the same SPL - which generally translates to lower distortion. Then again, if you just wanna make as much noise as possible, stick with the ported.

For reference, here's an enclosure I built for my 1st gen for a 10, tuned to about 29 Hz. There's no way that would fit in the armrest spot accounting for angles and the space required. And this is a simple rectangle.



Mock up pre-assembly gives you an idea of how much space the slot port needs. It also got a double baffle - consuming even more space. The finished product is well north of 2 cubic feet.

 

Last weekend I ended up finishing my ported box I won’t have time to get it wired for a couple weeks since I’m going on vacation but the box itself is done with a 10” in it. I managed to fit the 20.5” port tube in there with 3 inches in the back to spare

 

I got the sub and amp all wired up and I am beyond happy with how it sounds. It really made a world of difference and I smile every time I get in.