<title>Near Field Acoustic Pattern Of Fresnel Zone Plate Transducers</title>

Abstract

Piezoelectric transducers with an electrode pattern in the form of a Fresnel zone plate (FZP) on the air side and a full, planar electrode on the water side have been used in many acoustical imaging systems to focus the acoustic beam. The goal is to get an acoustic pressure pattern in the shape of an FZP. The acoustic pressure pattern, however, is never an exact replica of the FZP electrode pattern because of mechanical aberrations introduced by the piezo-electric plate. In a previous paper we derived the focal-plane acoustic field distribution of such a transducer. The distribution just in front of the transducer, however, was not given in that paper. In this paper we use a linear spatial piezo-electric model to construct a spatial transfer function for the piezoelectric plate. We show that elastic-wave generation and propagation inside the piezoelectric plate cause the acoustic field pattern just in front of the transducer to differ substantially from the electrode pattern. A plot of amplitude versus position on the piezoelectric plate shows that the sharp edges associated with the electrode pattern are severely rounded off by these effects and the amplitude is greatly reduced in the region of the outer zones. The aberrations also cause a vari-ation with the position of the phase of the out-put field.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.