Would a home theater enclosure of 6.8ft3 tuned at 22hz work ok with the Fi Q18" or is the fs too high for that?
You can't pull a tuning frequency out of the air. Fs pretty much sets the low extension within an Hz or two. You can see it tales a huge ported box to get an F3 of 28 Hz.
That driver looks as if it is designed for a sealed box with lots of Eq and power.
It really does look to be aimed at the car market.
Look at Ares thread with the 18" Dayton driver in a ported box. He has it up and running and it shakes the floor.
