here is one implementation for how it could work with two td15m woofers for the seosr
the black line is one td15m 5 cubic feet, tuned to 50hz. max spl is excursion limited to about 122db (not shown in this plot).
now, we want to add in a second td15m, but preserve the frequency response of the single driver.
step 1: add a -6db shelf filter which is shown toward the bottom in yellow. this will transform the frequency response of the black line into the red line.
step 2: add a second td15m. this is the blue line. we use the same -6db shelf filter as the first woofer, but we also add to that a 2nd order low pass filter at 100hz so that it rolls off as the frequency goes up.
so from about 200hz and up, almost all of the response is on one woofer, the red line.
down at 50hz and below the two woofers are operating fully together which produces +6db.
the NET response of the red line and the blue line is the black line!
since the black line was where we started with the unadjusted response of a single woofer, we have achieved the objective of maximizing headroom while preserving the original frequency response of the single driver.
at 50hz for example, each driver will only be called upon to produce ~112db spl, but together they will be outputting 118db!
now max spl is no longer excursion limited to 122db, but is instead increased to 128db (again, not shown).Edited by LTD02 - 9/26/13 at 4:15am