when you say "flex", are you talking about the excursion of the woofer, or the metal of your car flexing? if you're talking about the woofer, it depends on the "xmax" or the "throw" of the woofer. the longer the throw, the more air can be moved. if you're talking about your car, it's the sound pressure your subwoofer creates. i'm sure some more experienced people can help ya out with this one.