The specific surface (in m²/kg) of an open-cell porous material is the internal pore area (~wet surface) per unit mass of solid.
This specific surface can be obtained by the classical BET technique  generally used for powders in chemistry.
The specific surface (So) can be estimated from the following relation :
 S. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. Soc. 60, 309 (1938).
 P. Leclaire, M. J. Swift, and K. V. Horoshenkov, “Determining the specific area of porous acoustic materials from water extraction data,” J. Appl. Phys. 84(12), 6886-6890 (1998).