DIY 4th Order Bandpass Build for Project Fit 4 10″ Subs

I recently designed and built a 4th order bandpass enclosure for Project Fit so I am sharing that build here with you. If you’re interested in building a 4th order bandpass enclosure but are intimidated by the complexity of one, don’t be. It’s really not all that complex, people make them out to be this super difficult process but it’s hardly much more work than a normal box aside from a bit more math and building time. You are basically just building a sealed and ported box at the same time.


Looking into the port of my 4th order bandpass.

I used the same 4 10″ Lanzar MAX10D subs I had in my previous ported box build in this build. I wish I could get some DC, Sundown, or Fi subs but they just aren’t in the budget right now, so I had to stick with the budget subs. I wasn’t sure how impressive they would be but let me tell you, I am super impressed with how it came out now that I’ve had a good amount of listening time.

I used WinIsd to design the box, although it doesn’t give you box dimensions and wood cutouts like some fancier programs, I don’t need that. I just use it to view the response curve of the box. I then design the size of the box myself based on the cu/ft I end with in WinIsd. I ended up with a roughly 2.5:1 ratio of ported to sealed. Meaning the box has 2.5 cu/ft of ported section for every 1 cu/ft of sealed section. You can go as high as you want for ratio,  keep in mind the lower you are the less peaky the box will be and the higher you go the more peaky it will be. I wanted as much SPL from these budget woofers as possible so I went as high as I could. If you go with a 1:1 ratio, you can get a pretty flat response and if you go 3:1 you will get a pretty peaky (but louder) box. I have seen some people go 4:1 and higher too, it all depends on what kind of results you’re after.

As for the port, there is often some confusion on that too. Some people are under the impression you can’t have too much sq/in of port but frankly that is wrong in my opinion. There is a point where returns are diminished and also a point where you’ll start to lose output when you go too high on square inches of port. To figure out how many sq/in of port you have you simply multiply port width by port height. So if your port is 12″ x 12″, you have 144 sq/in of port. So how much port area do you need you ask? Well I like to stick to 12-15″ of port per cu/ft of box in a ported enclosure. Some like less, some like more but that is what I prefer for good results. In a 4th order you only count the ported section of the enclosure when figuring how much port area you need. So if you’ve got 10 sq/ft of ported section in your bandpass and you used the 12″ x 12″ port above (which was 144 sq/in of port) you’d end up with 14.4 sq/in of port per sq/ft of ported section. In order to do the math you simply divide the total square inches of port by the cu/ft of ported section. So in this example you do 144 / 10 = 14.4 sq/in if port per sq/ft of ported enclosure.

In designing a 4th order bandpass, you can use the sealed box requirements for whatever woofer you choose. If they ask for 1 cu/ft per woofer sealed, that is usually going to be about what you’ll want in the sealed section of the 4th order. Then you can use as little as 1 cu/ft ported or as much as 3 cu/ft or more depending on the response you want.  A 4th order bandpass is a big enclosure for that reason though, it’s basically a sealed and ported enclosure put together. Well actually, that is what it is. The woofers are mounted into the sealed section and all of the output comes out of the port itself. The woofers are sealed into the box, so make sure you make a panel removable or mount them before putting the top on like I did.


Here is the initial test to make sure the box would fit in the car, I just set the panels around my old ported box in this picture.

The specs on this box are roughly 40″ long x 35″ wide x 25″ tall. It is  about 17 cu/ft total without any baffle, woofer and port displacement. The sealed section is about 4 cu/ft and the ported section is about 10 cu/ft. The port is 12″ x 12″ and the box is tuned to about 45 hertz. I designed it to have very good low output and to have a pretty hefty peak as well, which I succeeded at. Some would say I used way to much sealed volume for 4 10’s but the low output this box puts out would prove them wrong.


Here is the start of the box itself and the speaker baffle. I double layered two sheets of 3/4″ MDF for the baffle. So it’s 1.5″ thick and helps brace the box itself. The box is built of 3/4″ MDF otherwise.

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Here is the start of the box, the bottom, back and side panels are put together in this picture.


Here you can see the baffle mounted in the box, it is starting to take shape. You can also see how much larger the ported section is than the sealed section.


Here is the port itself, not completely assembled yet.


I then finished building the ported and bolted it to the front panel.


The back side of the port.


Another shot of the ported panel.


Here is the ported panel installed in the box.


Here is the box getting closer to done, the speaker pictured is a 6.5″ mid bass driver, just to give a scale of how large the box is.


Here I’ve drilled the holes for the speaker wire and used silicon to seal it up.


I didn’t mount the top panel in this photo, just checking fitment.


I’ve mounted the speakers in this photo and wired them up.


The speakers fire into the sealed section. With the back sides in the ported area, I did it this way so I can smell the voice coils if I ever begin to over heat them.





All of these shots are just what it looks like inside the box after painting. I also sanded the sharp edges of the port to help reduce port noise. If I had a router I would have used a 3/4″ round over bit. I will get one before my next build for sure.


Here is what the box looks like finished, it is one huge enclosure.


Another shot looking into the port.


Tagged the box with PolishSpec, that’s me obviously.


Here it is in project fit, it’s tight but made it in there just fine. This is why you measure 5 times before starting a build that will be tight of a fit.


Up against the front seat, very tight.


It is also a close fit at the amp.

You can see a demo video of the 4th order here. 

So that is it, my 4th order bandpass build. If you’ve got any questions feel free to post a comment.