Something that I have noticed is that for an absorption material, there are different applications which might require a different absorption coeffient curve than another material Example. For critical music/speech listening environment, we have a hearing curve which is not a flat curve, and I've noticed that some materials that have an absorption curve that is closer matched to hearing curves that they sound better than other materials for these types of applications. That's why I stay away from building materials like fiberglass (compressed or non-compressed), or mineral wool. Also, open cell foams have different cell structure and density so there are differences in open cell foams too. Another material that isn't discussed is activated carbon, it's highly porous and very dense material, but it comes in typically pellet forms and there's a wide variety of different types of activated carbon on the market. I did notice one company mfg's their own activated carbon and they use it as a fill material inside a pressure based diaphragmatic absorbing products more for the low frequencies down to 30hz. I read a Salford University study on using Activated Carbon for low frequency absorption.

thank u so much, beautifully explained...

9:00 the milk test - that's all you need to know

why do you get more absorption at certain frequencies than others, (last graphs)