Glue is drying on my 6th order band pass sub prototype. If it works I will make 4 more. Designed for extending the sub in use with my 1850s. 

My goals were economy 30hz with a decentish efficiency, and the driver needs to be reasonable and obviously sim well for me to take a chance on it. My driver for the design was the Eminence Definimax 4018LF. There are better drivers, but it simmed decently and did not cost scary amounts, and recone kits are readily available. In the past I have mostly used european drivers, and I have not blown any (yet) but recone supplies are much more difficult to obtain and the cost include a hefty import price tag so this time I tried to go domestic instead.

Secondarily, I wanted to make the access panel removable to allow the rear chamber to be used as a standard reflex box, with a slightly awkward waveguide where the front chamber would have been. I feel like it can work though, so here's me trying.

Internal view.

Rear. The large handle on the back is also the recess for the speakons.

In Reflex mode, the lower f3 is 35 hz. 

Front chamber is 70 liters tuned to 66 hz. Rear chamber is 190 Liters tuned to 31hz.

External dimensions are 63.6 cm (to match the 1850s for stacking purposes) by 71.2 cm by 78 cm deep. It is big. Not as big as the 1850s though. Unloaded, I would guess it weighs about 100 pounds.

Drivers are here. Glue can't dry fast enough.

Looking good hope it works for you!

Whether it is a success or failure I will be sure to post my observations.

Do you have the equipement to measure the cabinet and adjust vent lengths if needed?

Really though, reducing the volume of the enclosure can also raise the tuning, and since I am not right at the smallest chambers the speaker can deal with, I have some room there. So if I need to raise the tuning, I will probably just subtract volume by adding more bracing. At least I will try that before I start butchering the vents.

It's very useful for telling you whether your ports will be noise-free. Those seem quite narrow to me - as you've said you haven't been able to test it at full power yet, I would be interested to know if the ports can cope with the driver's full output.

As for the vents, I did flare them slightly on both sides, but my simulations have these at very low vent velocities because there are 12 per chamber. The area of vent per chamber is 204cm^2. I get mach 0.02 from the front calculation and 0.01 from the rear so I think it will work ok even at high power. We shall see I suppose. I have a gig to test it at next week. I believe a single sub will be totally underwhelming with the rest of the system, but I can run it to full power and make sure it does not whistle or self destruct under pressure. I was surprised that I could work with a vent that small, but it is not a particularly high xmax driver.

I can say for sure that when I listen to it, it does not seem to lobe between chambers at low power. A sim curve for frequency response for a BP6 kind of looks like a tapped horn with an upper lobe and a lower lobe and hopefully you can try to get the middle part within a db or so. The first one I designed, I made the upper lobe even with the middle and the lower lobe up about 1.5dbs. It worked, but you definitely can tell which vent you are listening to at any given moment. That lower one comes on and just crushes. For this one I tried to get a flatter sub response, and the result sounds more natural I think. 

totally subjective ear observations: For casual listening the reflex mode sounds best. For a great need of efficient sub, the BP6 has all the advantages.

So this is my unscientific measurement with only a 30 hz hpf

This is the bit I intend to use as a bottom with my 1850s HPF at 30hz and LPF at 56hz.

My sims indicated that it could take 800 watts before xmax, but in reality it seems that 500 watts is plenty enough to drive it pretty hard. Then again, I am in a confined and rattley basement, so I may find that to not be the case when I take it into battle.

I am happy enough with how it sounds, but if I don't have four, I am unlikely to have a great result trying to see if one sub will keep up with four 1850s. I know it won't and I might just blow it trying to do so. So I need 4. Shucks. Guess you gotta take some risks in life.

As for pics, I should have some good build pics over the next couple days. I will share when I have some banked.

Fin. Oh thank the lord. These took some effort.

one man lift. Just lift that one man right up off the ground and take him to the morgue. He is dead for sure.

They weigh just a tiny bit less than my 1850s, but it is a dense weight, where the 1850s are top heavy so with leverage, they seem easier to move.

Seriously though, it is hard to really sound check in bursts because your ears have kind oa panic reaction in a closed space with sudden loudness.

I have tested with the band pass sub running straight to my mid horns (they are good to go down to 100 hz before there are xmax issues but they really have an F3 up around 170hz). It sort of works ok, but I miss the kicks without my 1850s horns. there is no use running these subs higher than 100hz. You are just making the driver work hard before the box eats everything but the standing waves. I like it 80hz or lower for the upper crossover. 60hz with the 1850s. The wedding 10*1s were just some yamaha bass reflex I usually use for DJ monitors. They run down to 65 according to spec. I used 75hz for the crossover.

I have a small main room gig coming up and have been working on stacking configs with the 1850s and my mid horns. This is what I have come up with for each side. Sure the 1850s would be subbier with all 4 together and ground loaded, but the BP6s are better than the 1850s for sub anyhow and it seems that in this config there is a natural drop off at 70hz for the 1850s, which sounds kinda nice instead of processing it out to transition to the subs. They are stacked upside down so that the solid panel on the front of the Band pass sub will act slightly like a horn extension following the path of expansion. ( <- That part is just a hypothesis).

I normally keep the treble on yokes but I think this works. The third level of the stack, where the mids and treble are, is just at my eyeball level, so I don't think I would angle it down much anyhow. The treble being up on top of the mids would allow the stack to have no ovehang, but it is too tall for the ceiling in some places.

Anyhow, the stack looks mean and the band passes fit pretty well as planned.