SOUND-PROPAGATION IN CAPILLARY-TUBE-TYPE POROUS-MEDIA WITH SMALL PORES IN THE CAPILLARY WALLS

Sound propagation in gas-filled capillary-tube-type porous media was investigated. The capillary tubes were taken to be nominally straight with very small pores in the walls of the capillary tubes. The complex wave number and the characteristic impedance of such media were evaluated. Application to ceramic samples having capillary pores with square cross sections and porous walls is developed as an explanation for the anomalous tortuosity factor previously inferred for this material. Specific acoustic impedance (SAI) measurements were performed for rigid-backed square pore ceramic media having finite wall porosity. It is shown that phase velocity is decreased, attenuation is increased, and characteristic impedance is decreased by finite wall porosity. SAI measurements were also performed after the wall pores were filled with water. These measurements agree favorably with the porous wall and nonporous wall theories, respectively. This work provides a model for the acoustical properties of gas-filled monolithic catalyst supports of which the square pore ceramic media is an example.