I am considering lining the inner walls of a bass-midrange enclosure with 10mm hard wool felt (solidity H3 = density 0.6). How should this material be considered in volume calculation - is it porous, solid or something in between?
Don’t. This felt (mostly used under carpets?) doesn’t have the acoustic properties you likely need. I know lots of people promote it. But there’s hardly any need. Use real acoustic absorbing material.
That list is pretty long, but relatively thin and dense fabric won’t be found on it. Mineral wools from 40kg/m3, acoustic foams (PU, melamine and others), acoustic cotton wool, aramid fibers come to mind.
Yes, what Markbakk said.
The lowest cost materials that work well are fiberglass and mineral wool insulation. Shredded denim (recycled cotton) insulation is also very good. Open cell polyurethane foam of a middle density works well, and it can be fixed to the walls so it does not shift around. The consensus is that the two best materials are acoustic grade melamine foam (Basotect or equivalent), and natural wool fiber stuffing or wadding. These are more expensive options.
The lowest cost materials that work well are fiberglass and mineral wool insulation. Shredded denim (recycled cotton) insulation is also very good. Open cell polyurethane foam of a middle density works well, and it can be fixed to the walls so it does not shift around. The consensus is that the two best materials are acoustic grade melamine foam (Basotect or equivalent), and natural wool fiber stuffing or wadding. These are more expensive options.