PROBING POROUS-MEDIA WITH 1ST AND 2ND SOUND .1. DYNAMIC PERMEABILITY

The acoustic properties of first and second sound in He II are used to probe the dynamic permeability k(omega), of porous media whose pores are large enough that the bulk properties of He-4 are relevant. The measurements of phase velocity and attenuation, considered as functions of frequency and temperature, are in excellent agreement with the predictions based on a model k(omega) which involves four parameters: phi (porosity), alpha(infinity) (tortuosity), k0 (dc permeability), and LAMBDA (dynamically connected pore size). The first three of these are measured by independent means; accurate values of LAMBDA are extracted directly from the measurements in the high-frequency region of the data where the observed temperature and frequency dependences of the modes are precisely those predicted on theoretical grounds. It is shown empirically that A is in fact closely related to the dc permeability k0 for all known data sets, thus supporting an earlier conjecture based on theoretical ideas.