I need to explain a bit more on the excursion plot. It is a plot that when you apply signals of equal magnitude to the subwoofer, the excursion the driver will experience at various frequency points. The above is the excursion plot for 1 port mode with extension filter set to 14hz high damping. The output above 13hz is useful and the output below 13hz is just not so good. All speakers driver has excursion limit. It is equivalent to drawing a horizontal line to this plot. What is means out if there is an alternating signals of 8hz and 18hz, the 8hz signal will drive the subwoofer into over excursion when the excursion at 18hz can only achieve -6db down (which is half excursion or 1/4 of power, or reducing the capability of a 600WRMS power amp to the like of a 150WRMS amp). It is not good because all you hear is over-excursion and with 18hz not even driven to its full potential. Next look at the same 1 port mode, but 14hz low damping.
Again let us draw a horizontal line to this plot. Now if we have an alternating 8hz and 18hz signals, the18hz can now be played at full excursion potential. That is the main difference between low damping and high damping on FV15HP. It is subtle, but very important to understand. I have every concern that a roomEQ program will change our low damping curve into something like the high damping curve. It is very much like you want to put in salt and pepper to the soup and the chef comes out and give you a funny look: "don't change my soup, it is just right".
Now LV12 series does not have this flexibility and therefore, I set the lowest extension of something like the above (14hz low damping) so it is almost fool proof. But FV15HP is a higher end model and I want to keep in as flexible. The14hz high damping setting, even though is more vulnerable to over-excursion when the subsonic signal hits, has better group delay, and therefore better for some application. For instance, if the sound track already use subsonic filter to curtail excursion, a 14hz high damping can sound better because 14hz low damping can cut down subsonic signal even further down. The problem is there is no common agreement where subsonic filtering should be applied (there used to be, but some studios become creative and just started doing things differently).
-Edited by Rythmik - 2/20/13 at 8:40am