Propagation characteristics of airborne ultrasonic wave in porous materials

Abstract

The attenuation coefficient and propagation speed of airborne ultrasound are measured for highly porous open-cell polyurethane foams and fibers at frequencies of 1 kHz to 1.7 MHz. A theoretical model is proposed to explain the frequency responses of the transmission loss and speed of an air coupled wave physically in porous materials. The model is derived from Biot’s flow resistance and density [M. A. Biot, J. Acoust. Soc. Am. 28, 179–191 (1956)], Lambert’s bulk modulus of fluids in pores [R. F. Lambert, J. Acoust. Soc. Am. 72, 879–887 (1982)], and Zwikker and Kosten’s concept for compliance of the side holes with entrance resistance [Sound Absorbing Materials (Elsevier, Amsterdam, 1949)]. Using measured data of static flow resistance to determine the mean pore size and the proposed model, theoretical prediction is made for the transmission losses and sound speeds in porous materials. Good agreement of theory and results of experiments in the whole frequency range supports the usefulness of the present model. Additionally, the model provides some findings for the extra attenuation coefficient of a slow wave in cemented glass bead specimens and sandstone for high-frequency ranges [P. B. Nagy, J. Acoust. Soc. Am. 93, 3224–3234 (1993)].