If you’re interested in building a 4th order enclosure and need some help, hopefully this page is for you. I have put together a bunch of tips and how-to’s that should help you put together a 4th order enclosure. Check it out.
There are 3 major steps you’ll need to do in order to design your enclosure.
1. Choose a Driver(s) – This would be the subwoofer – anything from a 6.5″ to a 18″ will do. You can even go smaller or larger. The most common would probably be 8″ and 10″ subs, due to the massive enclosure size of a 4th order bandpass enclosure. I currently have 4 10″ subs in a medium sized 4th order and it is roughly 17 cu/ft before displacements.
2. Choose a location for the driver. Is it for a home or a car? I assume most people will want it for a car, but they can be used anywhere really. Once you have a location you’ll need to measure the size of the area you’re working with. This will ultimately decide how many woofers and how large you can go.
3. Calculate total available area and choose what type of response you want. Do you want as much SPL as possible but at the same time creating a very peaky box or do you want a less peaky curve with the sacrifice of a few db? This will determine the ratio and size of your sections.
To determine the internal sq/ft you simply multiply length x width x height (in inches) then divide by 1,728. If you do 20″ x 20″ x 10″ for example you have 4,000″ then divide by 1,728 and you get 2.31 cu/ft, so that would be a pretty small box. If you have an non square area then you can break that up into 2 or 3 smaller square sections to make measuring simpler, then add the sections to get a total.
If you’re not super familiar with what a 4th order is, it’s basically just a sealed box and a ported box stuck together. The woofer is sandwiched in between and all of the sound output comes from a port. The 4th order design will yield a very large peak in spl at or near the tuning frequency, the width of the peak will depend on the ratio of the box. You can try different tuning frequencies in WinIsd or any box design program to see what it does to the response curve but on average 45hz is a safe bet for tuning. It allows the box to still dip into the 20’s on the low end while still having output around 60hz up top, giving it a fairly wide pass band. You don’t have to rune a 4th order super low to get into the 25-30hz territory like you would a normal ported box. The sealed section of the 4th order determines low end output for the most part, the larger you go the lower it will usually play but the port tuning frequency plays a role as well. If you tune higher than 45hz it will start to cut off some low end response. If you tune lower than 45hz it will then lose some upper end response.
I built my most recent 4th order very large on the sealed side, 1.0 cu/ft per 10″ sub. Then about 2.5 cu/ft each ported. This gives me 4 cu/ft sealed and 10 cu/ft ported total volume, since I used 4 subs. It plays under 30hz no problem. I went with a 2.5:1 ratio of ported to sealed which gives me a pretty good peak in output. It narrows the response a small amount but it’s not that big of a deal for me, I wanted the best output I could get with my budget woofers. You also need to remember to subtract the displacement of the woofers, any baffles or braces, and the port from any calculations you do.
The box ratio is simply the sealed airspace vs. the ported airspace, the ported side will almost always be the same size or larger than the sealed side. If you have a 1 cu/ft sealed section and a 1 cu/ft ported section, you have a 1:1 ratio, this will yield a more flat response but with less peak spl. If you have a 1 cu/ft sealed section and a 3 cu/ft ported section you’re at a 3:1 ratio, this will result in a more peaky curve but the total spl will be higher. The average boxes you’ll see are usually 1:1 all the way up to 3:1. You will see some competition vehicles as much as 10:1 or more though with a very high tuning, this results in what is called a burp box. It will play one frequency really loud, the rest will be lacking severely. They are usually used for competition only. I like to have low bass too, so I built mine to play from about 28-65hz.
The port volume is calculated using the airspace of the ported section only. You calculate it the same way you do for a normal ported box. I use WinIsd to input my preferred response and tuning frequency with my given airspace and see if I can make that port length work. You want to make sure you have enough port surface area though, so you multiply the length times the width of the port, for example a 12″ x 12″ port would be 144 sq/in. If you have 4 10″ subs which are about 300 sq/in of cone area (cone area chart at the bottom of the post) that port would be good sized since 144 is about half of 300. You can also divide the sq/in of port, 144sq/in in our example by the ported section of the box, we’ll use 10 cu/ft as an example. This gives us 14.4 sq/in of port per cu/ft of ported airspace. This is good enough. I try to shoot for 10-20 sq/in of port per cu/ft. If you go lower you may have port noise but if you’re limited you can try it, sometimes it works. You want to keep the port as close to square as possible, you don’t want a 15″ x 1″ port, it is too long and narrow with too much port velocity. If you do a rectangle port just make sure you’ve got a good amount of port width vs. port height.
You’ll see many people fire the woofers into the sealed section of the enclosure, I did this as well. In my case and the same as many others it is done so you can smell the voice coils if you begin the over heat them. I have gotten mine hot a few times and had to turn it down, if I hadn’t had them mounted this way, I probably wouldn’t have known it and fried a woofer.
In the construction aspect of the box you’re basically just building a large ported enclosure. The only difference being that the woofers are mounted inside of the box. You can do it in any fashion you want really, I built mine pretty simply as you can see below. You can see the full build by clicking here. You can build yours in any shape but squares/rectangles are easier to calculate, so keep that in mind.
Sealed section on the left and ported on the right. You have to make sure the woofers are all connected right or make one of the panels removable. I sealed the entire box up after mounting/connecting the woofers, so if something ever blows it will have to be cut open. I did test it all before sealing it up though, make sure you do the same unless you have a removable panel.
You’ll see some people put plexiglass/acrylic panels in, if I had the budget for that I might have done it. This allows you to access the woofers and looks cool. I did not brace my box much aside from the center panel. It is 2 sheets of 3/4″ MDF where the woofers mount. So that is a 1.5″ baffle in the box, it made it pretty solid. If I had massive power and stronger woofers I probably would have but for this setup I didn’t deem it necessary. You can also coat the box in fiberglass and resin for strength. Wooden dowels do not take up much air space but add a good amount of rigidity if mounted properly and glued or better yet fiberglass them in place.
I bolted my port in place, I could have probably used screws but I was originally going to make it removable. That plan changes though so it ended up being bolted in place. I sanded the edges of the port to round it over a small amount but a router would have been better. You can also kerf the edges but this is even more work, a lot of work. It can result in a small spl gain though, so if you have the tools and time it can be worth it. If you do kerf the port or round it over with a router you need to account for that in the length. The length will be measured from the half way point of the kerf or roundover. This usually adds only a small amount of length (1/2″ or so) but it will change the tuning frequency slightly (it will be slightly higher) if you do not account for it.
Hopefully this page has helped you on your quest to build a 4th order bandpass enclosure. If I missed anything or you have any questions feel free to drop a comment below.
Subwoofer Cone Area Chart (You can subtract a few percent for the surround, larger width surrounds taking up more cone area than smaller ones)
- One 5.5″ = 24
- One 6.5″ = 33
- One 7″ = 38
- One 8″ = 50.27
- One 10″ = 78.54
- One 12″ = 113.1
- One 15″ = 176.71
- One 18″ = 254.47
- Two 8″ = 100.53
- Two 10″ = 157.08
- Two 12″ = 226.19
- Two 15″ = 353.43
- Two 18″ = 508.94
If you’ve got 4 woofers just double the amount listed under pair and so on for more.