Design for an Aspherical Acoustic Fresnel Lens with Phase Continuity

A convex acoustic lens using room temperature vlucanizing (RTV) silicone rubber, whose acoustic impedance is similar to that of water, is a typical acoustic lens. A phase-continuous Fresnel lens was proposed to thin the shape of the lens because a convex acoustic lens has large attenuation due to its thickness. However, a Fresnel lens based on a convex spherical acoustic lens could not concentrate sound pressure completely on the focal point because of a spherical abberation. We designed an aspherical Fresnel lens by ray theory to remove the spherical abberration. A two dimensional finite difference time domain (2-D FDTD) method was used to survey the sound pressure field focused by the acoustic lens under condition in which the angle of incidence and the aperture were varied. Results showed that the aspherical Fresnel lens can concentrate greater sound pressure than the spherical Fresnel lens at normal and small angles of incidence. [DOI: 10.1143/JJAP.47.4354]