Old foam was disintegrating in my fingers and had to be replaced. It was thinner than anything I found at a local hardware store, though, and had a dozen holes stamped in it. Its primary job seemed to be to hold the Mylar disc against the tweeter's grill and at the same time press that conductive strip against the disc to maintain electrical contact. The holes probably ensured there were large areas of Mylar that were not directly restricted and could produce sound easily. As I was plotting how to replicate the foam, an idea struck.
The hardware store was selling a very lightweight open cell foam as air conditioner filter replacement. Because it was manufactured to allow air flow through it, the foam's cells are far larger than the original's. There would be so much less Mylar surface area in contact with new foam, it seemed to me there would be plenty of unrestricted Mylar movement even without cutting additional holes. I could cut holes later if the tweeter's performance seemed limited. I simply cut the foam into a disc shape that fit into the tweeter's plastic body.
The old foam is too fragile to reuse
New open cell foam
Foam has very large cell structure
The new foam was thicker than the original. Instead of attempting to slice the foam thinner, I positioned the foam disc on top of the tweeter body and made straight cuts along the top of the foam, to about half of the foam's depth. The slits were cut for each of the shell's ridges. When the foam was flipped over, my slits snuggled around the plastic ridges, which allowed the foam to seat lower and effectively reduced the foam's thickness. Clever if I do say so myself! next->