I know people already tried rock wool but specificaly the Safe N Sound brand product?
Also who compared the effect on mid frequencies and low-mid frequencies?
Specs here:
https://cdn01.rockwool.com/siteasse...fing-Insulation-Techdata.pdf?f=20190515124445
Seems to be more efficient starting from 250hz according to that sheet, but what about twice the thickness (6'') ?