Series tuned 6th order bandpass experiment

[Triticum Agricolam]

So I've been wanting to experiment with a series tuned 6th order bandpass design for quite a while. A conversation I've been having with another SMD member has finally motivated me to proceed with this experiment. First a little background.

You don't see a lot of series tuned BP6 boxes out there and there is even less information on how to design them. What I have been able to find has mostly been conflicting information as to if and how the tunings for the different chambers interact with each other. Some info said they don't interact all all, some said the rear chamber will be tuned lower than if it was vented to the outside, and other info said both chambers will affect each other's tuning. I really want to know what happens so I built a test box to see what's going on.

Here are some pictures of the box construction. Its mostly made out of 3/4" OSB. I used OSB because I don't care how the box looks, it actually stiffer and holds screws a little better than MDF, and its $14 a sheet. The box has a 3 cube rear chamber that I was shooting for a 24 hz tuning, the front chamber is 2.4 cubes with a target 54 Hz tuning. The sub is a Dayton Titanic 15" I picked up a while back for a song.

Here is the basic layout of the box. The shorter port panel in the rear chamber is removable so that I can adjust the tuning.

This is the box, ready to go. The front chamber ports are completely external so I can change them without affecting chamber volume. The three 4" ports are just press fit into the panel so I can pull them out and cut them down easily. There is a small window looking into the front chamber so I can see what the cone excursion looks like. With a little bit of sawdust on the cone it's very easy to see what frequency it stops moving and thus is the tuning frequency.

Here I put a bunch of foam board insulation in the rear chamber to decrease its volume and further raise the tuning. You can see my Behringer NU3000DSP amp off the left that I was powering the thing with.

The question:

The question I'm trying to answer with this project is "how does the tuning of one chamber affect the other?"

The experiment:

To answer the question I individually changed the tuning of each chamber and measured if/how it affected the other chamber.

The results:

The results were pretty conclusive, the tunings of the two chambers are both linked together. This was not the result I was hoping for or expected. I expected that changes in the front chamber tuning would affect the rear chamber, but not vice-versa. However what I found was when I lowered the rear chamber tuning it lowered the front as well. Conversely, when I raised the rear chamber the front chamber's tuning was raised as well. When I sealed off the rear chamber, making it a 4th order bandpass, I then measured the front chamber and its tuning frequency was significantly lower than when the rear chamber was allowed to vent into it like normal. As I expected, changes to the front chamber's tuning frequency also affected the rear chamber in a similar fashion. This means anyone who tries to design one of these boxes using the regular methods for tuning a ported box or 4th order bandpass is in for a big and unfortunate surprise.

Conclusion:

This was definitely a worthwhile project for me. While I did not get the answer I was hoping for, at least I did get an answer. Unfortunately this means that designing a series tuned 6th order bandpass is a total pain in the ass. Everything is fine until you want move the two chambers tuning frequencies closer together or farther apart. Then it becomes like a dog chasing its own tail, you may be moving, but you aren't getting anywhere.

Based on what I've learned I would venture a guess that the vast majority of series tuned 6th order boxes other people have built are not functioning like the builder's think they are. I know boxes don't always have to work as intended for people to be happy with them. I just wonder what kind of performance they are leaving on the table.

When I was done playing around with the thing I threw it in my Jeep to see how it sounds. It actually sounded really good, quite punchy. I still need to drag the box outside to measure the frequency response and efficiency. When I do I'll be sure to post the results to this thread.

For anyone that is interested, the Hornresp software can model series tuned 6th orders and it turns out it models them correctly. That's where I got the data indicating that the front and rear chamber will both affect each other, so I guess whoever wrote that piece of software knew what they were doing.

[diegoclass2010]

awesome testing! good to hear is sounded great, im in for more frequency response and efficiency... overall good read ... crazy how 6th order work..

[Joe X]

I got curious to see if some modeling would support the experiment results, it seems so, the sub shouldn't be great for 6th order although cabin gain may even out things a bit:

Red: series tuned 6th BP, rear 3 net @ 24Hz, Front 2.4net@54Hz,

Yellow 4th order, rear 3net, front 2.4 net@ 54Hz

Magnitude (anechoic - no cabin gain):

Excursion:

It looks like the 4th order port did load at lower frequency than the 6th despite tuning being the same.

The second set is the same 6th order but with the rear chamber retuned from 34 Hz to 24 Hz

Red: series tuned 6th BP, rear 3 net @ 24Hz, Front 2.4net@54Hz,

Green: series tuned 6th BP, rear 3 net @ 34Hz, Front 2.4net@54Hz,

Magnitude (anechoic - no cabin gain):

Excursion:

So uptuning the rear chamber resulted in the port on the front chamber loading at higher frequency (despite tuning unchanged) as well.

[Triticum Agricolam]

That's good to see that BBP is in agreement with the results of my experimenting. I sure wish I knew why changing the rear chamber tuning affects the front chamber tuning.

Here is modeled response from Hornresp:

Its not the best looking frequency response, like you said the sub is probably better in a 4th order BP. Its what I had on hand that looked the best in a BP6 though.

[reedal]

The front chamber being ported basically makes it an extension of the rear chamber maybe? So shrinking the volume of the rear chamber essentially shrinks the volume of the front chamber too? Does that make sense? That's what I think is the reason for the frequency change.

[db_speakerbox]

Good stuff dude.

My conclusion is this is why I stay away from these designs lol.

[Joe X]

That's good to see that BBP is in agreement with the results of my experimenting. I sure wish I knew why changing the rear chamber tuning affects the front chamber tuning.

Here is modeled response from Hornresp:

Its not the best looking frequency response, like you said the sub is probably better in a 4th order BP. Its what I had on hand that looked the best in a BP6 though.

The result you were expecting you might have gotten doing a parallel tuned box, using your same box specs, and retuning the rear chamber from 24 to 34Hz results in no effect on the port loading frequency of the front chamber:

Red 24Hz

Yellow 34Hz

[Triticum Agricolam]

Joe X, in that last graph is that just comparing a 24 Hz tuned rear chamber to a 34 Hz tuned rear chamber? If so I'll have to try modeling that in Hornresp and see if it shows something similar. I find it odd that there is a spot where changing the rear chamber tuning doesn't affect the front chamber. The power went out in my house and I'm stuck on my laptop and can't check it out.

Before I built the box I modeled what the parallel tuned version in Hornresp as well, I'll post what it looked like when I get a chance. I really need to measure the output from the box just to see what I actually ended up with.

[Joe X]

I should have posted the specs like this:

Red: parallel tuned 6th BP, Rear 3 net @ 24Hz, Front 2.4net@54Hz,

Yellow: parallel tuned 6th BP, Rear 3 net @ 34Hz, Front 2.4net@54Hz,

[Triticum Agricolam]

Thanks for the clarification Joe X, that makes a lot more sense to me now.

Here is a comparison between the series tuned (in green) and parallel tuned (in red) versions of the box.

The parallel tuned version appears to be a good bit more efficient, but over a more limited frequency range.