Good morning Alan and Kip,
I want to add a couple thoughts here. I don't have any modeling software nor am I in any way an expert, but I do understand that there are a lot of complexities that go into something as simple as a reflex loaded sub cabinet.
To suggest that the problem is that something is what B&C suggested seems a bit odd to me. I'm not saying that their recommended enclosure is the best enclosure for every application using this driver. However I think wisdom would be to consider that a company that produces some of the highest quality prosound drivers available has substantially knowledgeable people who come up with their suggested designs and further that they would not publish a suggested enclosure that would result in poor driver performance nor result in conditions likely to result in driver failure. I wouldn't just lightly assume that their design is crippled or poor. I also wouldn't assume that Alan's design is crippled or poor.
Alan, you say look at post 73 to demonstrate your concerns about the B&C design. You list
1)vent velocity is up there.
How high is "up there"? That is why I was suggesting to Kip that he consider an even larger vent area than the B&C design calls for just to help with keeping port behavior reasonable. Both of the proposed designs have compromised ports that will have problems at high output levels.
2)Power handling is down right where you want it not to be down, Xmax is over 12mm most of the time right in the sweet spot.
I assume you mean power handling is down due to exceeding Xmax from 45 to 56. While Xmax is 12mm, as discussed earlier in the thread Xvar is 14mm and is a more accurate reflection of the driver's performance envelope. Using Xvar and looking at your models in post 73, the B&C design stays under Xvar at all frequencies down to 32hz. Your design exceeds Xvar at more like 38hz. I think you were running that simulation at 3000 watts.
3)Dip in SPL right in the most important part.
The B&C design does have less output in the 45 to 55 range than your design does. This is with a cold voice coil. The design will become less damped as the voice coil heats up and there will be a ripple or increase in the output likely in that portion of the spectrum.
The question is whether you go with the design with somewhat higher Fb or one with lower Fb. To me, as long as the driver is within excursion limits (I would use Xvar here) in the 50 hz range then a lower Fb makes sense. Why leave that capability on the table if you don't have to. Then it comes to port behavior. If the port can't keep up at high output levels then either design will be compromised by that. Port modeling is way beyond my knowledge/skill level. Phil's article again provides good insight. Look at the pic of the Clair Bowtie sub. Huge port area on a sub that is universally respected as being very good.
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I would just suggest using the largest port yu can reasonably fit in your box regardless of where you choose to put Fb.
kidding, of course.