Because the driver does not have enough displacement to make the sounds it is being asked to.

It is possible to have a 6" driver produce audible sounds in the 25 Hz region, BUT ONLY if it is a very high excursion driver and it is very efficiently acoustically coupled (ie: a larg horn loaded sub )

Since the driver does not have huge excursion and it is not efficiently coupled to the air, all it can do a flap.

Fred

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Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!