CR,
First let me say that any of the small form stuff that I do sounds terrible without bass augmentation. While you can obtain a decent "D" going rearward like with the rings pictured above, you give up benefits of the floor and the larger space for the front wave launch. Recently I tried prototyping some similar shapes using styrofoam, but it was a waste of time because styrofoam transmits sound far too well.
I revisited the wooden rings to compare to some newer shapes I'm working on, and they're one of those "sound better than they should" kind of things, but the deficiencies are too pronounced to copy. I remain very sold on the benefits of addressing edge diffraction, and if I was building box speakers a circular front would be easy with the proper edge geometry. The problem with the rings is having a constant "D" all around the baffle that maximizes the peaks and nulls as the front and rear waves interact above the dipole bass cancellation point. That was the point I was missing in the original diffraction ring thread.
