Another problem is though, that these scales don´t seem to translate linearly which makes it a whole lot harder to actually compare the hardness.
THIS is the right and perfect answer: it's impossible to compare 2 sponges based solely on their hardness:
- how companies measure hardness ? system used ? units ? devices ? methods ?
- my tennis ball may be as hard as ... a given plank of wood, but that plank of wood isn't full of air, and isn't capable of the bounces the tennis ball can do. Oh yeah you may say it's an idiocy to compare such 2 different things, but science doesn't care about what's "something" looks like and is used for, that's why scientists always search for other measuring methods, tools, units, systems.
Hardness measurement is based on 3 parameters:
1- scratch
2- indentation
3- ... rebound !
and in each parameters there are even different scales, units so ! to measure each of those parameters; Vickers, Shore, Brinell, Leeb, Bennett, etc. So... I guess it's impossible to give a valid and correct comparison to most of us mere TT players mortals who don't have enough clue and education about hardness science. Here such a word as "hardness" is only used for advertising purposes IMHO.