Getting my arse kicked! Any help?

[Sguirrelfeather]

I don't know if I'm over thinking things, if I'm doing it wrong, or what, but I just can't seem to get the hang of figuring out a port.

Normally I pay to have someone else design my boxes and then I'll build em, but my wife finally wanted a small enclosure in the back of her truck and since that's gotta be a custom built thing (due to varying measurements cuz it's under the seat and the seat was raised) I figured I'd give this a shot myself. I'm currently only at the estimation stage of how big the box will be. I did measurements four times to be sure I was accurate. I use Triticum's port area calculator to determine port size and then a port length calculator (tried several to make sure) and HOLY CRAP the port is bigger than the box!

I know I can go down in port area to get a shorter port length, but then I run the risk of getting port noise. I tried splitting the difference but the port is still just really long. The box originally was about 2.15 cubes and had an odd shape because it's an under rear seat box and there's a transmission hump. She'll be running 2 HDS 2.2 8s in it and will have about 1300 watts total going to them. When I did all the calcs, subtracted the port volume from the box volume, recalculated, etc it almost makes more sense to just do two separate boxes about .9 cubes each and connect them with a common port, but even then I'd wind up with something like four feet of 5" diameter port for each one, and I just don't have that much room. lol

So, what do the infinitely more experienced people on here have for me? Please educate this old, tired mind!

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[SlugButter]

I’m working on this box at the moment for 2 8s. 2 cubes tuned to 32 HZ after all displacements. The port is 3x10x35 inches long. If you’re using a round aeroport, you’ll need to go with 4 inches probably. If you have no flares on the ends you will certainly get port noise. Both of these boxes are 2 cubes tuned to the same frequency, just different subs. As you can see, I had to bring the port out of the box some even using a 4 incher. 5 is definitely too long. You can get more length by firing the port sideways if you can slip it into your design to work that way.

 

[Buck 5,000+ posts]

This is a tough one to tackle with ports speeds. There's rules of thumb, but IMO you actually need a program like winisd that gives you some idea of what port speeds are. A lot of times smaller woofers require bigger ports, vs a bigger woofer with a bigger box fundamentally tends to want a smaller port, and it's just due to air physics, and I mean inches squared per cube ratio of port area. I've had to do 18-20 inches squared per cube for some sa-8 woofers to avoid port noise with slot ports, in some cases, and I've done 3000w 18's with less port area than that, ratio-wise, just to give you an idea how port area can be complicated. How high or low you tune makes a big difference in port air velocity as well.

 

[Buck 5,000+ posts]

Here try this (I use simple programs like this as a long time box designer all of the time):

Subwoofer Port Calculator - Easily Determine Length of Port

This subwoofer port calculator will help you determine the length of the port you should use in your ported subwoofer enclosure. Subwoofer Port Calculator Why do you need to determine the length to use for your subwoofer port? There are several reasons, for example, the correct port length helps...

boomspeaker.com

It's a really simple to use calculator, just make sure you follow the instructions. For a slot port for that box, you'd be close to a great box doing your recommended airspace from manufacturer at 12-16 inches squared per cube (probably) of port area tuned anywhere from say 30-36 hz or so. It's hard to say exactly without designing it. I'd tune to 32-33 hz personally with those, I think. Round ports like aero's are a little different situation, but it's still a port. Round ports are hard to fit in 8" boxes or smaller boxes in general, because it's hard to bend a round port, easy to bend a slot and more finely controllable tuning with slot.

 

[Sguirrelfeather]

Appreciate the input! I'll check out the calc site and see how it compares. I'm in the beginning stages of building the box. Unfortunately with something like this it's so custom (lots of changes were made to her truck) that I have to build a piece, measure for the next, cut, glue, measure, and so on. lol Once I have a firm box size I'll be able to figure this out a bit better.

I do think I figured something out though. Tell me if I'm cracked? Ideally you want to determine desired box size, calculate port, and then expand the box accordingly, correct? I'm, of course, stuck with figuring out max box size and trying to determine port size, subtract that from the box, re-calc, etc. That really didn't work for me, but I then tried taking max box size, subtracted a little more than 1/3 the volume, and then used that to calculate port size and length. When I laid out the port it was a good match. I wound up with a little more volume left in the box though, but I figured if I simply install some bracing (being careful to measure and account for them) I figure I should be able to take down the volume until it reaches the level I calculated. While I know a larger box, to an extent, is probably desirable, but I don't know what effect it would have using a port calculated for a smaller box in a bigger box....if that makes sense.