Nonlinear parametric sound enhancement through different fluid layer and its application to noninvasive measurement

This paper presents a theoretical and experimental study on the enhancement of secondary waves generated from a parametric array through different fluid layers in water. A theoretical analysis was carried out using the Burgers equation for nonlinear sound propagation through a fluid layer in water, and the enhancement ratio of secondary waves was studied. The result indicated that among the five types of fluid layers studied, the ethanol layer in water had the highest enhancement ratio for signal amplitudes because of nonlinear fluid properties of ethanol and the fluid/water interface. To confirm the theoretical result, experiments were carried out using an ethanol layer in water. The result indicated that the enhancement ratio of secondary waves increased by more than three times because of the ethanol layer and decreased gradually in the far field in water. The concept of different fluid layers combined with a shadow imaging technique was applied to the noninvasive measurement of a target square cylinder in water. It was found that the accuracy of the size measurement of the target structure in water using an ethanol layer was better than that of conventional nonlinear imaging.