Free air - 42 hz
Baffle with 40% Sd opening - 38 hz
Baffle with no opening - 158 hz

As I expected, when the speaker was mounted face down on a board with no opening, Fts went up because of the small sealed volume between the cone and board. What I didn't expect is Fts went down when the speaker was mounted on a board with a hole smaller than Sd.

I can understand that a sealed box makes the speaker stiffer and raises Fts. I accept Bill Fitzmaurice's explaination that when there is a hole in the baffle, the air mass between the cone and board is allowed to move, so its mass adds to the cone mass, tending to shift Fts down. This means there must be two opposing forces, one stiffness and the other mass. Duh, of course that is the case.

I wonder what size hole strikes a balance between stiffness and mass so Fts isn't changed?