This is in relation to the sensitivity of the subs. Notice that the sensitivity is over 97dB. This balances the need for high excursion, but also does point to the fact that control is needed for these subs, and high efficiency is needed as well. Metering, and audible performance are two VERY different measurable instances. An SA8 can do 50+, but the audible passband will suffer greatly, UNLESS your preference is for a peaky response. 4th orders can actually give a wonderful passband if it is designed around the transfer function and resonance sensitivity of the environment and STILL get great number results, but you have to be careful of the limitations. Blocking a sub from broadband output with a high sensitivity rating is generally frowned upon and direct propagation is nominally a better choice.
That is why I would revert from a 4th in this case.....not due to TS parameters. You will get more use out of it with the design you have shown us beyond the quote I am referring to now. Much better choice than a 4th for this particular driver for those reasons.
XMAX is merely a make-up parameter for low sensitivity. I can take a pro audio driver and extend its output to that of a comp sub any day with the right enclosure even though xmax is a major limitation in that driver selection, and do it with half+ the power, as long as we have the right room available. That is one of the huge limitations we have in car audio, is the room. By the forced low efficiency driven from dimensional limits, we are forced to use higher excursion drivers to obtain the same output in a smaller space. Look at the 8" velodyne SPL-8 sub for example..the power this thing actually produces relies on resonance control, due to the fact that it is sealed and has the ability to create enough pressure due to the high excursion driver used. Put any other driver in there with less xmax and higher sensitivity, and it would not perform as well....due to the nature of the design, hence the name.
In car audio, we deal with the same limitations of size. SO, for this particular sub, the apxx you are referring to, if you have the room for it to operate properly and DO NOT constrain it, you will get great results regardless of its excursion limits, seeing that it still has a nominal excursion capability of most 10s, it will suffice as a decent diver in the right box, and will operate similar to 3-4 10s in the same space virtually, giving more room for the box to do its job. Just giving you my two cents on the situation.
