Sound fields focused by a biconcave acoustic lens for normal and oblique incidence

In this report, a sound pressure field focused by a biconcave lens was investigated using the wave theory and the numerical methods of the parabolic equation (PE) method and finite difference time domain (FDTD) method for normal incidence. For oblique incidence, the hybrid method which combines ray theory and wave theory was used in the calculation. The experimental data were compared with the theoretical results shown above. The experimental on-axis characteristics around the focal point agreed well with the calculated results but disagreed with the results for in front and behind the focal region. It was found that the second peak existed behind the focal point for normal incidence, though there is no second peak in the theoretical results by the analytical method and the FDTD method. After the numerical analysis by the hybrid method and the PE method, the second peak was found at the same position as the experiment. This was thought to be caused by the influence of the edge of the lens, which has a finite size. The dependence of convergent characteristics for oblique incidence was also compared with the results by the hybrid method.