4th Order Help!!

[Goblenator]

I'm building a 4th order for 2 Chaos 12's. The problem is whenever i see a design for a 4th order it is at least a 2:1 ported to sealed. My design is 1.5:1 sealed to ported at least that's what the design programs come up with. I need some help designing my first 4th! I'm looking for a flat response from 30-60hz with a slight low end bump.

[Triticum Agricolam]

What vehicle is this for? What amp are you going to run? What design programs have you been using?

[Goblenator]

Bmw 330ci, Helix Spxl 1750 rms, Bass box pro 6 and win isd

[Triticum Agricolam]

OK, so here is my general design philsophy for 4th order bandpass boxes:

First start with the rear chamber. The rear chamber determines what output you are going to get and controls cone excursion below the tuning frequency of the front chamber. The bigger you make the rear chamber, the more low end output you will get and the lower your sealed resonant frequency will be, but as you make it bigger your cone excursion goes up too. It's a balancing act to try to make the rear chamber as big as you can to get the most output, but without having it be so big your subs destroy themselves the first time you play a low note at war volume. The manufactured recommended sealed box size is a decent starting point for rear chamber sizing.

The front chamber size determines what output you will get around the tuning frequency. The bigger you make it the more output you get, however you only get output over a limited frequency range. Make the front chamber too big and you get a one-note-wonder. Just how big you should make it depends on the parameters of your subs. Some subs will give you wider bandwidth than others will using the same size front chamber. Subs with lower QTS and/or Vas will be more peaky than subs with higher QTS and/or Vas, this will make some subs inappropriate for 4th order bandpass box usage. Just how peaky you want the output to be depends on your personal goals and listening tastes. Everything is a trade off.

When it comes to the tuning of the front chamber you can move the tuning around a bit to get the frequency response you want, but generally you will get best performance with it being near the sealed chamber resonance, this is another reason why its a good idea to keep an eye on your sealed chamber resonance when sizing your front chamber. Its a good idea to make your front chamber port adjustable so you can tweak the tuning a bit when you get it in the vehicle. Tuning frequency on paper doesn't always work out to what you get in practice and you may want to shift your passband up or down a bit to fit your listening tastes.

In terms of port area for the front chamber, you need to have a lot. All of the output has to come out of that port, so you don't want to lose output to air resistance due to an undersized port. I try to keep port velocity under 20 m/sec. Depending on how much power is going to be used, this can require ports as big as 1/2 the cone area.

So for your particular case, a rear chamber of around 1 - 1.1 cu ft per sub should work well, this will keep excursion reasonable and give you a sealed resonance of about 45 hz, which is just about perfect. For the front chamber size, it depends on how much bandwidth you want to have vs how much you want to maximize output. A 2 cu ft front chamber (total for both subs) looks pretty good to me. You could go a bit bigger for more output, at the expense of bandwidth, or a bit smaller for smoother frequency response, but then the port is going to get awkward. A front chamber tuning around 45 Hz should work well, though I always recommend making the port adjustable, around 60 sq in of port area should be sufficient, but its going to make for a fairly long port.

[Tattojackson]

Response to your box building advise - I'm also trying to build a 4th order band pass for my fosgate T2 15 and will have a crescendo BC3500 for power. I'm only running 1 sub and idk really crap about the numbers I'm more of a guy that needs basic instructions like cut top board 42inch bottom 40 and sides 22 and glue this to that and so on. Are you able to help me with this ? I would like to tune it to the frequency that grabs you by the throat. Lol it doesn't bother me that it can't get to low like a 18 would but from the way it sounds you know box building well and I've been searching for months trying to get box help.