Measurements of dynamic compressibility of air-filled porous sound-absorbing materials are compared with predictions involving two parameters, the static thermal permeability k 0 Ј and the thermal characteristic dimension ⌳Ј. Emphasis on the notion of dynamic and static thermal permeability-the latter being a geometrical parameter equal to the inverse trapping constant of the solid frame-is apparently new. The static thermal permeability plays, in the description of the thermal exchanges between frame and saturating fluid, a role similar to the viscous permeability in the description of the viscous forces. Using both parameters, a simple model is constructed for the dynamic thermal permeability kЈ(), which is completely analogous to the Johnson et al. ͓J. Fluid Mech. 176, 379 ͑1987͔͒ model of dynamic viscous permeability k(). The resultant modeling of dynamic compressibility provides predictions which are closer to the experimental results than the previously used simpler model where the compressibility is the same as in identical circular cross-sectional shaped pores, or distributions of slits, related to a given ⌳Ј.
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 American Institute of Physics.
A model for the prediction of the frequency dependence of the dynamic bulk modulus of air in a porous medium with one parameter, the thermal characteristic dimension Λ′, was proposed previously by one of the authors. This parameter depends only on the porosity and the area of the surface of porous frame in contact with the air in the material. This area can be evaluated with the standard Brunauer, Emmett, and Teller (BET) method, from krypton adsorption by the frame at liquid–nitrogen temperature. Comparisons between acoustically measured bulk moduli and predictions obtained with values of Λ′ evaluated with the BET method are presented. The BET results provide reasonable agreement with acoustic measurements of thermal dimensions and dynamic bulk moduli.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.