The concept of characteristic thermal dimension was recently introduced to predict the behavior at high frequencies of the bulk modulus of fluids saturating a porous frame. This thermal dimension is closely related to the specific surface of the solid porous frame. It is shown that the standard Brunauer, Emmett, and Teller method (BET) of measuring the specific surface and acoustical measurement of the bulk modulus are in good agreement, and that the viscous dimension can be evaluated unambiguously from the BET method plus an acoustical measurement. ©1995 American Institute of Physics.