Acoustic nonlinearity parameter inversion from harmonic interaction

Abstract

This paper is concerned with the measurement of the acoustic nonlinearity parameter of fluids (β=1+B/2A) using harmonic interactions of finite amplitude acoustic waves. The data consist of the on-axis pressure field emanating from a planar circular transducer, measured by a small probe. The source is excited by either a single frequency signal (f) or by a composite signal of two frequencies (f1 and f2). The attenuation of the fluid is measured independently by a standard linear wave propagation technique. The model for predicting the finite amplitude pressures is the Kuznetsov–Zabolotskaya–Khokhlov equation (KZK), which has been used extensively to model nonlinear acoustic beam fields. By taking the pressure measurements just beyond the Rayleigh distance, and providing that the sound pressure level is not too high such that the measurements are taken within one shock formation distance, a quasilinear approximation to the KZK equation is used, which is sufficiently accurate to represent the first-order interactions (2f, f1+f2, f1−f2, etc.) and computationally efficient, thus rendering the one-parameter inversion problem manageable. The accuracy of the inversion technique will be discussed for data consisting of both second-harmonic and difference frequency signals. Results for various fluids will be shown and discussed.