DYNAMIC COMPRESSIBILITY OF AIR IN POROUS STRUCTURES AT AUDIBLE FREQUENCIES

Measurements of dynamic compressibility of air-filled porous sound-abs orbing materials are compared with predictions involving two parameter s, the static thermal permeability k(0)' and the thermal characteristi c dimension Lambda'. Emphasis on the notion of dynamic and static ther mal permeability-the latter being a geometrical parameter equal to the inverse trapping constant of the solid frame-is apparently new. The s tatic thermal permeability plays, in the description of the thermal ex changes between frame and saturating fluid, a role similar to the Visc ous permeability in the description of the viscous forces. Using both parameters, a simple model is constructed for the dynamic thermal perm eability k'(omega), which is completely analogous to the Johnson et al . [J. Fluid Mech. 176, 379 (1987)] model of dynamic viscous permeabili ty k(omega). The resultant modeling of dynamic compressibility provide s predictions which are closer to the experimental results than the pr eviously used simpler model where the compressibility is the same as i n identical circular cross-sectional shaped pores, or distributions of slits, related to a given Lambda'. (C) 1997 Acoustical Society of Ame rica. [S0001-4966(97)00310-X] PACS numbers: 43.20.Gp, 43.20.Jr, 43.55. Ev [JEG].