On Wednesday, June 11, 2014 8:49:22 PM UTC-7, GS wrote:

Its not foam or rubber. It's treated cloth. Some kind of substance.
I've seen the treatment seep down, but the stuff might be different
on different drivers.

Greg
On reflection, I would guess that the cloth is impregnated with silicone rubber. I do not remember any of it oozing down; however, my sample size is small. Still, foam does not make sense for acoustic suspension woofers.