How to make a 6th order bandpass enclosure?

Choosing a 6th order bandpass box design for your woofer is definitely a more exotic approach to enclosure building. These types of boxes are pretty rare, because of obvious reasons. First of all, there isn’t much information about this design. Mainly because Bose has a patent on this type of enclosure. You cannot build a 6th order bandpass box commercially. However, there is no problem if you build one for personal use.  This might be the reason why I haven’t found any BP6 alignments in the audio literature (I was hoping for something similar to bass reflex alignments). Nevertheless, you can use box design software to model the frequency response (I used SoundEasy).

Let me think about other reasons why you seldom see this kind of speaker boxes. They have little tolerance for error. If you have a tendency to do designing or building mistakes, you are better off looking somewhere else. Slight errors can translate into serious misalignment. Some might be put off by the sheer size of the cabinet. While a large box is impressive, and it’s nice to show it off to your friends, no one wants to put up with the hassle of owning one. The transient response of bandpass enclosures isn’t great also. This is pretty much true, regardless of how you tune them.

However, on the plus side, the 6th order bandpass box design has a very high efficiency rating. It has the potential to hit really hard and go really low in frequency. And who doesn’t like loud, deep bass? The whole point of this article is to show you how to do that using a JL 10W6v3 (Amazon affiliate paid link) woofer.

6th order bandpass box design for the 10″ JL woofer

First of all, like any bandpass enclosure you need to set your priorities. You have to choose between efficiency, linearity and frequency response bandwidth. To give you an example : If you want the box to be able to play a broad frequency spectrum, it will not be as loud. As a result, if you gain something in one department, you have to make a compromise in some other department.

The design philosophy of this enclosure design : I want it to be loud, with linear response and go very deep (below 30 Hz). Hopefully, the response will go decently high in frequency. Because this is where we will pay our tribute, to the playable frequency spectrum. It will go up to at least 60 Hz, perfectly flat. In conclusion, it will play the lowest notes, really loud. A true “sub”-woofer.

Let’s take a look at the modeled frequency response. The actual measured response looks even better. That will be at the end of the article.

6th order bandpass box design

So we can see the numbers here :

  • Front chamber volume : 24 liters.
  • Front chamber tuning frequency : 47 Hz.
  • Rear chamber volume : 70 liters.
  • Rear chamber tuning frequency : 27 Hz.

These are the net volumes. After adding the port volume, the volume displaced by the speaker and the brace, chamber size increased a bit. Front will be 26 liters and rear will be 75 liters. Now that you have the volumes and tuning frequencies you can design the box as you see fit. I did it according to my trunk space, but you can alter the dimensions as you see fit, as long as you keep the above values the same.

Port design

To tune the chamber, we will use 100 mm circular ports, flared at both ends. The ports I used were Jantzen Audio 100 mm bass reflex ports. You can find more info about them here (scroll to the bottom). I know that Precision ports are more popular in the US (like this one (Amazon affiliate paid link)), but I’m sure you can cut them to the same size and get almost identical results. The expansion rate of the flare might be a bit different, but that will have a negligible impact on the overall result.

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The modeling software tells us that the ports need to be 388 mm long for the rear chamber and 371 mm long for the front chamber. These lengths are for a basic, straight pipe. However, the subwoofer is capable of moving large amounts of air, so having a flare at both ends of the pipe is mandatory. This will ensure a smoother air motion through the port, but it alters the port length to some extent. More on that later, when we do the actual build.

Panel dimensions

To make the box, I used 22 mm MDF. Some people, might consider this overkill for a 10″ woofer, but that’s how I like it. Indeed the box is extra heavy, but in audio, that’s a good thing. If you want to use 18 mm MDF you have to adjust the sizes of the panels to keep the volumes the same. If not, just follow my design.

Panel list :

  • 727 x 510 mm -> 2 pieces (top and bottom).
  • 727 x 334 mm -> 2 pieces (front and back).
  • 554 x 334 mm -> 2 pieces (sides).
  • 510 x 294 mm -> 3 pieces (double baffle and internal brace).

Here is a diagram with all the panels for a more comprehensive view. Also, you find here the sizes and the positions of the holes for the speaker and the ports.

panel dimensions

The brace and the speaker baffle panels have an additional 4 mm in width (or should I say height). I’m going to make grooves in the top and bottom panel, so these panels puzzle up together. The speaker baffle and brace have 2 mm extra for the bottom groove and 2 mm extra for the top groove. The grooves are 3 mm deep to allow some space for the adhesive.

Build guide

I’m going to give a step by step process on how to actually make this 6th order bandpass box design for the JL woofer. For the sake of it, I’m going to include a lot of pictures with the process. Good in terms of visual aid, bad if you are on your phone with a limited internet plan.  So let’s begin.

Creating the grooves

We need to make 2 grooves on the top and bottom panel, so we can puzzle in the baffle and the brace. The brace will be 22 mm wide, and the double baffle, 44 mm. To give some wiggle room, make sure the grooves are 1 mm wider, so 23 and 45 mm. The depth of the grooves should be 3 mm.

Align the top and bottom panel, and use 2 large clamps to secure them in place.

top and bottom panel stacked

seem between panels

Now, use an Emerson tool (Amazon affiliate paid link) (straight edge clamp) and a router to make the grooves. Set the router at 3 mm depth and calculate the exact spot you need to place the Emerson tool in order to make the groove in the right place. The router will usually have a 6 mm bit, so you need to make multiple cuts to make the 23 and 45 mm groove. First, cut the beginning and the end of the groove, and then cut in between. Check the panel schematics again to see where the grooves go.

router before cut

first groove cut

extremities cut

finished grooves

baffle groove

brace groove

stacked grooves

final check

Tedious part over. Let’s get to some circle cutting.

Baffle, brace and front panel

We need to cut some circles in the baffle, the brace, and the front panel (to accommodate the ports). For the baffle and brace, the center of the circle will be in the center of the panel. So, to not make any measurements, I like to draw two lines  from the corners. Where they intersect, it marks the center of the panel.

center panel

Now, to cut the circles, you have the dimensions in the schematic at the beginning of the article. But most likely you will use the Jasper circle jig (Amazon affiliate paid link). Here are the dimensions for the jig :

  • Outer baffle : 10″ + 10/16″
  • Inner baffle : 9″ + 2/16″
  • Brace : 9″ + 2/16″
  • Port hole : 5″ + 14/16″

After the circles are cut, we need to stick the 2 boards together to make a double baffle.

baffle cuts

baffle glue

baffle clamps

baffle finished

Now it’s time to focus our attention to the front panel. The holes need to be cut halfway height-wise, and 90 mm away from the edge (I’m talking about the position of the center of the circle).

front panel

Assembling the box

Now that we have all the pieces cut to the correct size and shape, the next step in our 6th order bandpass box design, is to start puzzling the pieces together. Get the wood glue, some clamps and a friends to help you out.

panels prepped

glue on panels

panels in place

glue for top panel

top panel in place

clamps in position

glue waiting to dry off

glue waiting to dry off 2

partially finished box

partially finished box 2

partially finished box 3

As you can see, all the panels are in place except for the sides. Problem with the 6th order bandpass box design is that the speaker is inside the cabinet. Once the box is finished you don’t have access to it. I managed a solution. Check it out in the next section.

The side panels

I didn’t fit the side panels to the box yet, because I want to adopt a different solution on how to integrate them in the box. If I were to use wood glue, the only way to take the speaker out, is to destroy the whole box. While this might not be an issue, if you opt to cut the box open, the sawdust might reach the voice coil and do permanent damage.

For this reason, I will use screws and abundant silicone sealant. If you want to take the speaker out, simply unscrew all the screws and pry the side panel open using a crowbar.

First, position the side panels in place using large clamps and the pre-drill the holes for the screws . Don’t make a hole right next to the port cutout!

side panels holes

holes for sides

Before you place the side panel, let’s drill-in the binding posts and connect the cables that will go to the speaker.

binding posts

cable connected

Now all you have to do is place a lot of silicone and screw the panel in place.

silicone applied

Do the same for both panels. Make sure to let it rest for a whole day, so the silicone cures.

assembled box

Above you have the box with all the panels in place. I also used silicone on every inside joint,  before placing the side panels and after.

Port adjustment

The ports come with a 400 mm long pipe and they need to be adjusted to size. I’m going to give you both the dimensions of the bare pipe, and with the flares attaches.

  • Front chamber (small one) : the pipe is 343 mm long, and with the flares attached – 415 mm.
  • Rear chamber (large one) : the pipe is 351 mm long, and with the flares attached – 423 mm.

Here are some pics, so it’s more clear :

bass reflex port

flared bass reflex port

After you cut the ports to size, apply abundant silicone sealant. I always like to go overboard here, but I don’t want to risk any port distortion.

silicone on port

ported box

bass reflex port

I know the quantity of silicone looks like overkill, but you can remove the excess after it has cured. Screw the port screws and let the silicone completely dry out for a whole day.

6th order bandpass box design

Here is the finished box. I made this 6th order bandpass box design just out of curiosity. You can see the sealed box next to it, which is empty. It will go back there, soon enough.

Results

Let’s do some measurements and see how this 6th order bandpass box design stacks up.

frequency response chart

Above you can see the responses of both ports and the combined response. What’s interesting to note here, is the 2 dips. Each port has a dip in its response. That dip marks the tuning frequency of the other port. As you can see, the port in the small chamber has a dip at 27 Hz. In conclusion, 27 Hz is the tuning frequency of the larger chamber, and vice-versa.

frequency response 6th order bandpass

Above is just the combined response of the 2 ports. Or the response of the box as a whole. We have an almost ruler flat response on the pass band :

  • Frequency response : 24 Hz – 63 Hz (+/- 1 dB).
  • Frequency response : 23 Hz – 72 Hz (+/- 3 dB).

Very pleased with the frequency response.

Conclusion

The 6th order bandpass box design is not for everyone. Indeed it has its drawbacks. But for a 10″ woofer the output is absolutely insane, it drowns my mids completely. Listening to it full blast for 30 seconds, hurts my ears for an hour. If you got the mids and highs to warrant this type of enclosure, and you don’t mind the space issue, you might as well give it a try.