I'm not totally sure what you're referring to. Fs is just where the impedance of the speaker will peak at in a free air enviroment. As soon as you put it in an enclosure, it's going to change the impedance and allow you to play below Fs. A speaker can always play below Fs, even in free air, but it is not safe to do so - the impedance will be very low and cause the driver to overexert.
ok if thats the case kancko what about using the abc boxes excursion control to use a tune much lower than the fs? From what I think I remember it reduces excursion by 50 percent at fs.
