Modeling these XBL^2 drivers is kinda difficult. I emailed Chad yesterday about enclosure sizes and Q, and also about using the Rythmik 250 from Kyle. I thought sharing this on the forum might be beneficial since some of this info is not up on his website yet.
"Recommended enclsoure sizes will be up in a few days. (The holiday prevented me from working on the website all weekend!!) To get a Q of .707, the Atlas 15 only needs a 3.7 cubic foot sealed enclosure. To get a Q of .5 you need a 6.5 cubic foot enclosure. These enclosures would be filled with polyfill. (not jam-packed with stuffing, but simply filled up lightly throughout.)
There is also something else worth mentioning about these drivers. (which will also soon be on the website...) XBL^2 does something to the frequency response of these drivers. "Standard" non-XBL^2 drivers with a single gap and a long voice coil will lose strength the more the driver moves. If you look at their BL curve, it is typically parabolic. This translates to less output and less strength at the lower frequencies at higher volumes, where the driver is moving the most.
Take this "standard driver" and put it into a sealed enclosure with a Q of .707 for example. The louder you play it, the more the driver moves. The more the driver moves, the more it loses motor strength. This translates to a
frequency response that starts to "sag" in the lower frequencies. It may have a Q of .707 on paper, but at higher volumes the measured frequency response will
actually LOOK more like a higher Q of .8 etc.
The XBL^2 motor has a very flat BL curve until near its excursion limits. Because of this, the frequency response tends to "hold" its shape and remain a true Q of .707 until you push it to its extremes. For this reason, it will tend
to sound "flatter" or more like a lower Q. People have become use the sound of standard drivers having a higher apparent enclosure Q at higher output.
Our drivers don't do this. Instead, they actually hold their frequency response. As a result, you can typically put them into smaller enclosures than you are used to using. Their frequency response will not change as they get
louder like other drivers, so they sound like they are a lower Q than they actually are.
This characteristic was immediately apparent when I first started testing my prototypes. They sounded "flatter" in relation to the frequency response,and the low end also seemed to stay stronger even at higher output levels.
Ground plane testing confirmed this. Once you think about the motor design, it actually makes sense.
Sorry about a really long answer to a really short question. A smaller enclosure will give you more "punch" and a larger enclosure will have a less punch and more neutrality. It all depends on what type of sound you are trying to achieve. I played classical bass in orchestras for over a decade, but I admit I still prefer a little "punch" to the sound.
I would say that for a Q of .707 that 3.6 cubic feet sealed is a good place to start. As mentioned above, I also stuff my enclosures with enough polyfil to fill the enclosure. I don't stuff it tight or pack in as much as I can, but
rather just fill up the enclosure. If you are not using stuffing, I would recommend just over 4 cubic feet.
I hope that this answers your question. If you need more info, PLEASE let me know and I will be more than happy to provice all of the answers that I can.
P.S. 250 watts is all you need to get these subwoofers to really pound, so that amplifier sound perfect. With direct comparisons to other similar subwoofers, we have found the sensitivity to be reliatvely high on our products".
I hope this helps. Which 'standard driver' do you think he refers to? Perhaps "Tempest."