The Nonlinear Acoustic Wave Propagation in Porous Rubberlike Medium

A high value of nonlinearity has been found in a class of rubberlike medium containing micropores. Under external ultrasonic excitation, the micropores display nonlinear oscillation due to the weak compressibility of the rubberlike medium. An approach based on the concept of equivalent nonlinear parameter is presented to estimate the linear and nonlinear properties of porous rubberlike medium with particular interest on the nonlinearity caused by the nonlinear oscillation of the micropores. The porous rubberlike medium is considered as a homogenous medium with an equivalent nonlinear parameter. The frequency dependence of the equivalent nonlinear parameter is studied and its characteristics are associated with the scattering fields of the individual pore. At low frequency range, the presented equivalent nonlinear parameter can also well predict the nonlinearity mainly caused by the microstructure of the porous medium, which is consistent with L. A. Ostrovsky's static results [1]. The results of fundamental frequency field are compared to G.C.Gaunaurd's work [2], showing good agreement, which provides further justification for our method. The effects of size distribution of pores on the fundamental and second harmonic fields are also examined.