Jeep Grand Cherokee 4th order help

[Brad Majors]

Okay guys selling my 2 hds3 12s and getting 2 hds3 15s and wanting to put them in a 4th order. I have never built a 4th before. Always just ported so this is new to me. I have drawn up specs myself but would like to see what y'all come up with as again I have never done this before. This is going in a 2001 Jeep Grand Cherokee for 2 hds3 15s pushed my fsd 2600. Dimensions are 40w 30h 30d. Thanks in advance.

[Brad Majors]

Oh sry to del post but wanted to say that in my design I double walled the entire box and did a triple baffle.

[Triticum Agricolam]

So why do you want to use a bandpass box?

[Bayuk89]

Oh sry to del post but wanted to say that in my design I double walled the entire box and did a triple baffle.

Tripple baffle the face is pointless IMO, those HDS300 15" arent super heavy and the air ur gonna be pushing wont have enough pressure to need a tripple baffle, double baffle would be better IMO. Also to double baffle the WHOLE box is pointless IMO aswell for the subs and power applied.

[Brad Majors]

So why do you want to use a bandpass box?

Ive never done one before and always wanted to try one.

Oh sry to del post but wanted to say that in my design I double walled the entire box and did a triple baffle.

Tripple baffle the face is pointless IMO, those HDS300 15" arent super heavy and the air ur gonna be pushing wont have enough pressure to need a tripple baffle, double baffle would be better IMO. Also to double baffle the WHOLE box is pointless IMO aswell for the subs and power applied.

well thats why im here asking since again ive never done one before. i would assume that baffle itself would need to be at least doubled but figured a box that size would flex so thats why i was going to double all the walls. that and when i did the full measurements it landed perfect. but again all input is greatly appreciated. would love to see what others could come up with sketch wise for a 4th.

[Triticum Agricolam]

So why do you want to use a bandpass box?

Ive never done one before and always wanted to try one.

OK, if you are looking to try something different and want to experiment, that's as good of a reason as any to build a bandpass box. Just understand that the way most car audio bandpass boxes are done these days, which large front chambers, you can get the same or better performance out of a ported box that will take up less space.

To expand on what what Bayuk89 said, its important to build a box that's stiff and strong, doubling up the walls is one way to help reach that goal, but a good internal bracing strategy is usually a better way. It uses less material, adds less weight, and if done correctly will be stronger anyway.

A lot of people try to really over-simplify bandpass box design, this leads to shortcuts like using "ratios" that will bite you in the ass. In my opinion, acoustic modeling software (WinISD, BassBoxPro, TermPro, HornResp, etc) is a must for bandpass box design. Otherwise you are just taking shots in the dark, you may be know which direction you are shooting towards, but without software you can't see where you are hitting. It takes some experience to use the software well and to know how to interpret the results, but this is where other people can help point you in the right direction. Here is my general philosophy on bandpass box design, I'm happy to help you with any of this if you need:

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, but you still need to check and see what you get.

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. At 30+ m/sec you can lose half, or more, of your output to port compression 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.

[Brad Majors]

well as of right now i went for 4 cubes sealed and right at 1.5 cubes ported before sub and port displacement. was going off of what Soundqubeds site says and went with 144 sq in for the port so went a 12 x 12 and 11 inches deep and i come out that it would put me at 45ish hz which from what i read 44-47 is good daily. also from what i read that i should go a 2:1 ratio so thats how i landed with the number i have. i thought about going up to 4.5 sealed and just go double baffle instead of triple.

[Triticum Agricolam]

Was the 1.5 cubes ported a typo? Or is that 1.5 cubes per sub?

Your subs have significantly higher QTS than most comparable 15"s, this a good thing for using them in a 4th order bandpass box, but it means they are going to need a larger than normal sealed chamber. Try to forget anything you read involving ratios, they are no guarantee than you will get any level of performance.

Do you plan on ever running more power in the future or is your FSD2600 what you plan on sticking with? What kind of usable bandwidth do you want? Bandpass boxes are always a tradeoff, if you want a larger bandwidth it will cost you output and vice versa.

[Brad Majors]

oh god lol mean 10.5 ported and as far as performance the fsd 2600 will be it for a long time. if i was to ever go bigger amp i would change the subs out for HDC which according to Soundqubed they are interchangeable in boxes. but for now and a long time i think i will be good with the 2 hds3 15s and the fsd 2600. i dont have my notepad in front of me but i had it like 10.6 before displacements and like right at 9 cubes after displacements... again this is all triple baffle double walled. so if i go single wall and just double baffle ill gain more. can put more to sealed if need be but right now 4 sealed and little under 9 ported after all displacements.

[Triticum Agricolam]

OK. For some reason I was thinking you were going to use four subs and that kinda threw off some of my thinking, my bad.

Like I said earlier, your subs have a high QTS, which is going to make them want a larger sealed section. However they don't have a huge Xmax, which is going to necessitate making the sealed section smaller to keep cone excursion under control. So there are two conflicting requirements. Your planned 4 cube sealed chamber looks good from an output perspective but is a bit concerning when it comes to cone excursion. There are a couple things you could do. You could leave it at 4 cubes and use a 27 Hz subsonic filter on your amp, or you could cut it down to 3 cubes and stuff it pretty heavily with polyfil. Both options will decrease your low frequency output a little.

For your front chamber size, all that matters is net volume, so that's what's left after you take out sub, bracing, and port displacements. At 9 cubes you are going to have a moderately significant peak in output around your tuning frequency. Whether that's good or bad is really up to your music listening tastes and what you want to accomplish with this box. If you are trying to compete for numbers, that peak is probably a good thing, however if you want a smooth sounding system for playing music, the peak is probably undesirable. You can lessen the peak by shrinking down the front chamber if you desire. It takes a pretty big change it make a noticeable difference. For example if you want from 9 cubes down to 6 (or even less) it would cost you about 2 db of output, but it would increase your bandwidth by several Hz on both the top and bottom end. I can't tell you what's right for you, I can just help you make a more informed decision.

I think your plan for a 12" x 12" port will work out great.