One isotope of Unobtainium I've wanted to get my hands on for awhile is rock wool. Joelpearce
mentioned it many months ago as a cup dampener and said he had it leftover from a construction project. It turns out that it's not just sold in bulk for construction but also commonly used in the plant nursery and hydroponics industry as an artificial "soil" for germinating seeds.
I went to a hydroponics shop in town (I now know where marijuana grow-ops get their equipment) and picked up some 3-inch and 4-inch cubes of rock wool. Cheap. It's made by Grodan with a 70 kg/m^3 density (vs the Roxul Safe'n'Sound fiber Joel used with its 40 kg/m^3).
I cut 1-cm-thick slabs and cut them to fit into the bottom of the cups and around the hanger "hump," thusly:
I tried the rock wool with various materials on the back of the driver (on top of the stock white material). The only effect of these over-the-driver materials was to enhance or decrease the mids and treble, from about 1 kHz up (no effect on lower mids or bass). Plush microfiber (from a dusting cloth) gave the brightest sound, then, in descending order, thin/smooth microfiber (from a lens cleaner), dense cotton (from a nursing pad) and stock material alone.
The thin microfiber with the rock wool gave the flattest frequency response and, to my ears, the most pleasing sound.
(Note the apparent "ridge" at 60 Hz in the heat graph. I'm not sure what that is but it's consistent across all my mods involving rock wool. I can't hear it, though. Maybe that's why Purrin truncates his amazing waterfall charts to ignore the lower end of the spectrum?)
This mod measures similarly to my mods involving absorbent cotton in the cup and plush microfiber over the stock white driver material. However, I think this sounds a little more open (although I'm not sure what I'd need to measure to convince myself that it is truly so!
) I'll stick with this mod until I find some fault with its sound and start mod'ing again.