Estimation of acoustic absorption in porous materials based on visco-thermal boundary layers modeled as boundary conditions.

A method for estimating acoustic absorption in porous materials is presented in which the thermal and viscous boundary layers are modeled through boundary conditions to the Helmholtz equation for the acoustic pressure. The method is proposed for rigid-framed porous materials in which vibration of the frame is negligible compared to pressure fluctuations in air. The method reduces computation times by 2 orders of magnitude compared to a full thermoviscous acoustic solver.

Sound absorption by metallic foam after triaxial hydrostatic compression.

An engineering method for triaxial hydrostatic compression of metallic foam is presented to preferentially alter the foam's microstructure. The method is demonstrated on an assortment of open-cell aluminum foams with varying pore size and porosity. Measurements of acoustic absorption indicate that the compressed samples absorb significantly more sound than the conventional samples of equal thickness in the test range from 0.

Analysis of thermal and viscous boundary layers in acoustic absorption by metallic foam.

A method for estimating acoustic absorption in foams is presented using a combination of micro-computed tomography, finite element analysis, and boundary layer loss theory. In the method, the foam is assumed to be rigid framed and the viscous and thermal boundary layers at the fluid and frame interface are assumed to be small compared to foam dimensions. The boundary layer losses are approximated using an infinite planar model.