A finite-element model of the aperture method for determining the effective radiating area of physiotherapy treatment heads

This paper describes a theoretical study of the way in which a circular aperture placed in front of a plane-piston modifies the ultrasonic field it generates. Specifically, the radiated acoustic power transmitted by the aperture and the radiation force experienced by an absorbing target placed in the transmitted beam, are evaluated at a distance from the exit-side of the aperture. The calculations used a finite element (FE) method, in conjunction with a surface Helmholtz integral formulation to solve the fluid/structure interaction problem. The PAFEC (Program for Automatic Finite Element Computation) vibroacoustics software was used for the FE calculations and the implementation of the surface Helmholtz integral formulation method. Acoustic pressures and particle velocities were computed at required points, whilst accounting for the presence of the aperture in the medium, together with its dynamic properties when subjected to an incident sound field. This enabled the calculation of the radiation force on the absorber and its variation with the applied aperture diameter was investigated. As part of the validation process for the new FE aperture model, the ratio of radiation force to radiated acoustic power obtained using the FE method in the unapertured case, through the use of the Rayleigh integral, yielded good agreement with results obtained through an analytical solution. The study has been carried out to provide a better understanding of the factors affecting the measurement uncertainty for the aperture method of determining the effective radiating area (AER) of physiotherapy ultrasound treatment heads.