Elliptically curved acoustic lens for emitting strongly focused finite-amplitude beams: Application of the spheroidal beam equation model to the theoretical prediction

Abstract

Strong focusing of a plane progressive ultrasound wave by a plano-concave lens with a widely opening aperture, which is made of acrylic resin and is submerged in water, is investigated theoretically and experimentally. To eliminate spherical aberration, an elliptic surface lens is introduced. The amplitude of the sound emitted from the lens increases abruptly with propagation toward the focus, then nonlinear harmonic generation in the beam becomes active significantly. The spheroidal beam equation (SBE), which has been previously proposed to be more amendable to the analysis of a highly focused nonlinear beam, is used to predict the first three harmonic components in the beam. To make sure of the effectiveness of the present theory, experiment is executed in water using an elliptic surface lens of eccentricity 0.544 which is attached tightly to a 1.7-MHz planar transducer with a circular aperture of 75-mm in diameter. It has been shown that the experimental data are overall in excellent agreement with the theoretical prediction. Especially, remarkable suppression of side-lobe levels in beam patterns of higher harmonics has been demonstrated successfully.