Need Box/WinISD Advice

[lanearthinking]

Hello!

 

I've been planning a build for my '04 Silverado extended cab, but I have a few goals I'm trying to meet. I want to keep things as low key as possible without giving up too much in the way of cabin space or bed space. Initially, the plan was an under seat box, but it really seems to restrict my sub choices. I'm keen to doing a blow through, but I don't really want to have a tonneau cover or to have my box exposed.

 

That leaves me with one other idea: I have a truck bed tool box that I do not use whatsoever. It just happened to be installed on the truck when I got it. After doing some rough math and tinkering with WinISD, I came up with what appears to be a pretty decent 4th order box for a pair of CT Sounds Ozone 12s. I've tried to take vehicle resonant frequency into account taking some measurements and using the filter a YT video suggested (it's been so long ago now I can't even remember who the video was from).

 

Do these parameters even look right or can I trust the graph I'm getting at all? I've been plucking away at it for a few months and have a design together that theoretically would work, but I'd like a second opinion. Included pics of box, vent, and what filter I had running.

[Joe X]

Your cabin gain simulation seems ok with that linkwitz transform filter set to 60Hz so that seems good.

 

Your box will be good or not depending on your goals, if you are aiming for a not loud flat response box it could likely do the job.

 

If you are aiming for high performance like what is expected in car audio you need to check a lot of things to make sure.

 

Not sure if the box is going to be a blowthrough with the box mounted inside the bed toolbox and therefore a ported design needs to be excluded, if not model the ported box and compare, likely ported will be louder.

 

Next, your port area seems low, for 4th order bandpass designs one needs your peak vent airspeed to be low, like 15-18 meters per second or lower as all your output will go through the port. disable the filter to view this plot correctly.

 

Also the 1:1 chamber ratio is only adequate for flat response, most car audio designs use 2:1 for extra output, you need to model greater ratios to see what can be gained always checking not going over Xmax in your excursion plot at any frequency at the maximum power you intend to deliver. The excursion plot will only work with the filter off.

 

Before you build the box post your design to get feedback on it, bandpass designs need extra care to work correctly.

 

 

 

 

 

 

 

[lanearthinking]

On 3/13/2026 at 2:15 AM, Joe X said:

Your cabin gain simulation seems ok with that linkwitz transform filter set to 60Hz so that seems good.

 

Your box will be good or not depending on your goals, if you are aiming for a not loud flat response box it could likely do the job.

 

If you are aiming for high performance like what is expected in car audio you need to check a lot of things to make sure.

 

Not sure if the box is going to be a blowthrough with the box mounted inside the bed toolbox and therefore a ported design needs to be excluded, if not model the ported box and compare, likely ported will be louder.

 

Next, your port area seems low, for 4th order bandpass designs one needs your peak vent airspeed to be low, like 15-18 meters per second or lower as all your output will go through the port. disable the filter to view this plot correctly.

 

Also the 1:1 chamber ratio is only adequate for flat response, most car audio designs use 2:1 for extra output, you need to model greater ratios to see what can be gained always checking not going over Xmax in your excursion plot at any frequency at the maximum power you intend to deliver. The excursion plot will only work with the filter off.

 

Before you build the box post your design to get feedback on it, bandpass designs need extra care to work correctly.

 

 

 

 

 

 

 

Yeah the plan is a blowthrough, but the idea was to keep it stealthy. I really want a flat response that sounds good, but I also tried to go with as much cone area and wattage as I could to keep some volume. The plan for the front stage is a three way active setup with a dsp for TA/EQ/etc. It doesn't have to be a show stopper since I'm usually riding by myself, but it'd be nice to have a good sounding system for sure!

 

The port area has definitely been a concern. I've taken into account how much of it will fit inside the box and subtracted that from the volume. For some reason WinISD has no problem showing me port velocity on a ported box, but it will not give me front port velocity on a 4th order, filter on or off. I've searched all over but I can't find an answer for it. I downloaded the most recent version. Without knowing that, I've basically gotten the port as large as I can without having a port that is unrealistically long. From my math the 5x10in port would put me at about 1/3 cone area. I can fit a 5x15in port no problem and be at 1/2 cone area, but I'll have to do some more measuring on the truck to see if I'll end up with the correct amount externally going from the box to the cab.

 

I've tried to model a ported box that will fit under the seat with the most cone area/wattage that will fit and the SPL seems to be about to same if not a bit less, but it wouldn't be nearly as flat. I've heard to take the SPL graph with a grain of salt as far as what real world numbers you'll achieve, but that the response curve is usually pretty accurate. I appreciate the advice!

[lanearthinking]

Update:

 

After playing around with WinISD I finally got front port velocity to work. Turns out I must've made a mistake inputting the sub parameters. It actually changed the entire response curve I was getting, and I haven't been able to find anything that will work with the constraints I have for those subs and have a flat response... So I decided to mess around with a single Meso 12 in a 6th order (it DID NOT like a 4th order AT ALL) and came up with this. Pretty satisfied with what I've come up with in a couple of days, but I'm going to keep plugging away at it.

[Joe X]

I found that your issues you find with your fourth order bandpass are connected with your WinISD usage, So I install the program and fill the data and here is what I get:

 

 

Notice they give the CMS parameter in the wrong units, that can cause problems.

 

 

Frequency response:

 

 

Cone excursion:

 

 

Vent airspeed:

 

 

 

box specs are in my next post where I use a more professional software but honestly results are very similar, find what is wrong and proceed to build your box.

Edited March 18 by Joe X

[Joe X]

Did my own modeling with bass box pro and a 4th order did work fine, share you the results:

 

Box specs:

sealed chamber: 1 cubic feet

Ported chamber: 1.25 cubic feet, tuning: 48Hz, Port area: 34 square inches.

 

Frequency response:

 

 

Cone excursion:

 

 

 

Vent airspeed ( 48Hz, 34 square inches):

 

 

volumes and port area are for one sub, for two subs volumes and port area double.

 

The only issue is low Xmax of the sub, you would need to use your subsonic filter if intending to push the full 800WRMS to protect the sub.

 

 

Edited March 18 by Joe X