4th order subwoofer box for home theater or for your car

This guide will show you how to make a DIY 4th order bandpass subwoofer for home. It’s an active subwoofer as it uses a plate amplifier. However, if you want to use it for your car, you can ditch the plate amplifier and place some binding posts on that panel. This article is actually complementing this Youtube video. So, if you didn’t come here because of that, make sure to watch that video first. Also, if you don’t know nothing about BP4 enclosures, you can check out this article.

Parts list

There are a few components which you are going to need. I’m going to list them over here and also give you some links on where to buy them. These are all affiliate paid linksI get a small fee if you make the purchase at no extra cost to you:

Few remarks on the parts list. The Monacor amp will be super hard to find in the US market. That’s why I have a Bash amp alternative. Be advised that the amp cutout will be different. I would use a Dayton Audio plate amplifier but those leak air if you don’t place them in a separate chamber (No, thanks!).

The binding posts are optional. I use them on the inside, to quickly connect the amp. However, you can drill some holes to pass some wire and seal the holes. That will work also. Furthermore, the binding posts are mandatory if you plan to use this DIY 4th order bandpass subwoofer box in a car. On the panel where the amp will be, just place the binding posts there and you’re done.

Box dimensions

For building this box I used 18 mm thick MDF. Here are the panel dimensions:

  • Front / Back : 639 x 286 mm (2 pieces)
  • Top / Bot : 639 x 303 mm (2 pieces)
  • Sides : 303 x 250 mm  (2 pieces)
  • Baffle : 250 x 250 mm (1 piece)
  • Brace : 250 x 250 mm (1 piece)
  • Port short panel : 250 x 268 mm (1 piece)
  • Port long panel : 250 x 439 mm (1 piece)
  • Brace material : 250 x XXX mm (1 piece)

The brace material is just a long 250 mm wide board, which you can use to cut smaller pieces with 45 degree edges, which we can use to brace the box.

This is how the finished box should look like. A slot port subwoofer with the plate amplifier on the back.

Box brace

The reason for the above image is to illustrate those trapezoidal braces. Basically you need to set you saw at 45 degree angle and cut the pieces in such a manner that the top part has only 7 mm and the bottom part 43 mm. 3 of these are necessary for this DIY 4th order bandpass box.

How do the panels go together

Here is a top view of the box so you can better understand which panel goes where.

DIY 4th order bandpass

You can see from this image that I rounded the edges at the port bend and also at the port intake. Not shown here but I also rounded the port exit. Try not to skip this step as it does lower the air turbulence and improves sound quality.

panel positions plate amplifier cutout

Just above you can find the position of each panel along the length of the box. Furthermore, the dimensions for the amp cutout is also present. Remember, that those numbers are for the Monacor amp. If you buy the Bash amplifier or use some other plate amp, make sure to check what the correct cutout is. Again, if you plan to use this DIY 4th order bandpass subwoofer for your car, you can leave this panel be and simply drill two 7 mm holes for the binding posts.


This subwoofer does use damping material. It’s the dense kind. Furthermore, for ease of use, try to find some self adhesive mats.

DIY 4th order bandpass

As you can see damping material is present in the sealed chamber but also in the ported chamber. Be careful not to place too much in the ported chamber and restrict the airflow through the port. Go nuts with the sealed chamber though.

DIY 4th order bandpass – finished box

This sub sounds amazing. It has very low distortion, goes deep, and the volume is sufficient. The port is large compared to the size of the woofer therefore the port air velocity is low. Sounds awesome, and I do recommend to build one if you want to. It’s just an 8″, so it has its limitations but it does pack a punch.

4th order bandpass activesubwoofer

I used a glossy white paint for the finish, but you can do whatever you like to the box. Enjoy your new BP4 sub box.