Propagation characteristics of airborne ultrasonic waves in porous materials

Abstract

The attenuation coefficient and propagation speed of an airborne ultrasound wave are measured for highly porous open-cell polyurethane foams and fibers at frequencies from 1 kHz to 1.7 MHz. A theoretical model is proposed to explain physically the frequency responses of the insertion loss and the speed of an air-coupled wave in porous materials. The model is derived from Biot’s flow resistance and density, Lambert’s bulk modulus for fluids in pores, and Zwikker and Kosten’s concept for the compliance of the side holes with entrance resistance. Using measured data of static flow resistance to determine the mean pore size and the proposed model, theoretical prediction is performed for the transmission losses and sound speeds. Good agreement between theory and experiment over the entire frequency range confirms the usefulness of the present model. In addition, the model provides findings for Nagy’s extra attenuation coefficient for a slow wave measured in cemented glass bead specimens and in sandstone for high-frequency ranges.