Help with a 4th

[tayodell41]

Hey guys, I'm gonna be picking up two sa12s in the next couple of days for a good price, the only problem is they're in a shitty box. So if I'm gonna build the box, I thought I'd try to build a 4th in my trunk. My box building and designing skills are limited (I've built one box for my 15" BTL using Joe X's design for me). Sa12s need 1 cf sealed and 2 cf ported so my box would have to have about 6 cf (right?) And my max dimensions are 40" wide, 26" deep, and 16" high. Those are dimensions I can slide in and out of my trunk, I'd rather not build inside. Its going in a 2001 Toyota Camry on a Hifonics Brutus 1600 amp (upgrade coming soon). I'd like to build a box tuned between 30 and 33. I would like the port along the side so that it will fire into the car through the area where my seats fold down. Any help is appreciated.

[tayodell41]

I'm thinking something like this where the port will fire into the car

[RooTxBeeR]

Before you tackle designing a 4th you need to learn A LOT more lol. When you build bandpass enlcosures. You figure the airspace on ratios. For a 4th order you do it as sealed:ported airspace. So what ever you driver calls for sealed, build that, then you decide what ratio you want. Then build accordingly. Most people do 1:2 ratio's, so in your case, yes, it would be 6cuft net. Then as far as tuning the ported side, in 4th order bandpass enclosures, you tune it to the resonant frequency of the sealed section. Usual it is between 45-50hz. I said this yesterday I believe in another topic about 4th order bandpasses. That as far as my knowledge goes with them, so as far as how much port area they should need on the ported section, I have no idea. With all that being said though, if you can get a design that is good. You've already built a box before so I would say try it and and see if you like it. One thing to keep in mind is it will take a lot more wood than the average box. Also, if you do one, invert the subs so they magnets are in the ported section. That way if you are to get them to hot, or clip them. You'll have a much greater chance of smelling them quickly to turn them down so you don't harm your equipment.

[tayodell41]

Okay thank you. I know I have a lot to learn that's why I wanted to see if I could get some help with a design before I attempted to build one

[ChatraN]

use drivers with low FS, average to low qts and high vas. Drivers with wigh qts (0.7 and up) may perform not very well in BP.

[tayodell41]

use drivers with low FS, average to low qts and high vas. Drivers with wigh qts (0.7 and up) may perform not very well in BP.

So would the sa12s be good in a 4th?

[Raptorman]

SA series performs great in 4ths.

And you don't determine whether a driver is good for a 4th or not based just off the Qts. You use the EBP of the driver. Which is Fs / Qes.

EBP of the SA-12 is ~58.

As a general rule:

EBP < 50 - Sealed box

EBP 50-100 - Works in both

EBP > 100 - Ported box


You'll want to tune in the 40-45hz range. 4ths can be very musical up until around 3:1 ratio, then the bandwidth will start to get unpleasant and it'll peak like a motherfucker. Start with half the cone area of port area. Notice I'm saying "start", most 4th bandpasses, you will have to tweak.

Two 12's is like 226 sq in of cone area. Something like that. Starting around 80-100 square inches of port would be good.

Give each woofer 1 cube net sealed, then 2-3 cubes ported, and you'll be happy.

[tayodell41]

SA series performs great in 4ths.

And you don't determine whether a driver is good for a 4th or not based just off the Qts. You use the EBP of the driver. Which is Fs / Qes.

EBP of the SA-12 is ~58.

As a general rule:

EBP < 50 - Sealed box

EBP 50-100 - Works in both

EBP > 100 - Ported box

You'll want to tune in the 40-45hz range. 4ths can be very musical up until around 3:1 ratio. Start with half the cone area of port area.

Two 12's is like 226 sq in of cone area. Something like that. Starting around 80-100 square inches of port would be good.

Give each woofer 1 cube net sealed, then 2-3 cubes ported, and you'll be happy.

Oh so they won't need as much volume as if I was just building a regular sealed or ported box? And I would like the box to be loud but I like my low notes too, will tuning it at 40-45 hz still hit low notes?

[Raptorman]

4th BP's take more volume.

You're looking at about 7-9 cubes gross.

Yes tuning 40-45 will retain low notes.

Your tuning doesn't determine how low you can get for 4th orders, it works a bit different for 4th bandpasses, as the tuning point is actually the low-pass point of the enclosure, NOT where it will peak.

The sealed section will give you your low end.

[tayodell41]

Oh I'm sorry you said 1 cube sealed and 2-3 cubes ported EACH. My bad. Anyway, how do you tune the sealed part?