Sound pressure field focused by doublet with acrylic plano-concave lens and silicon-rubber Fresnel lens

Aplanatic lens can correct both spherical and coma aberrations. In this report, an aplanatic doublet with acrylic plano-concave lens as a cover and room temperature vulcanizing (RTV) silicon-rubber Fresnel lens was designed using optical ray theory to obtain light weight and better characteristics than the previous aplanatic lenses. Convergence characteristics of the doublet were evaluated by using a ray tracing diagram and two-dimensional finite difference time domain (FDTD) method, and compared with two Fresnel lenses which are aplanatic Fresnel lens without cover and with cover of uniform thickness. Newly designed aplanatic doublet showed larger power and narrower beam width than the previous Fresnel lens with cover. These predictions of the characteristics for the doublet by the FDTD simulation were confirmed by the experiments in water tank. As a result, experimental convergence characteristics of the doublet agreed well with the results of the FDTD simulation except the focal length. Difference of the focal length between simulation and experiment was discussed in the point of view of the variation of the sound source distance and sound velocity.