Sonorock Eco datasheet suggests > 5 KPa · s/m2 https://www.diaterm.com/wp-content/uploads/2019/07/FT_Sonorock-Eco_ES.pdf
We also have Alpharock 225 which is > 20 KPa · s/m2 https://www.diaterm.com/wp-content/uploads/2019/07/FT_Alpharock-Premium_ES.pdf
I can probably find something in-between...
I am planning to build some acoustic absorption panels to treat the room where I will be recording my speaking voice. Using the Porous Absorber Calculator (http://www.acousticmodelling.com/porous.php) suggests that to achieve an absorption coefficient of 0.7 at 100 Hz, I need a 12-inch panel...