A finite element approach to predicting free-field diffusion coefficients according to ISO 17497-2

The experimental measurement of the diffusion coefficient according to ISO 17497-2 is time-consuming. It requires several sample periods of a diffusor, an anechoic or large reflection-free volume, and far-field conditions. Wave-based methods have accurately predicted the diffusion coefficient without the need to fabricate 3-D print scale models or full-scale samples for testing. This work describes a Finite Element approach with a Perfectly Matched Layer that predicts the free-field diffusion coefficient directly from a 3-D CAD file, addressing limitations found with the Boundary Element Method, such as the thin-shape breakdown, while also allowing frequency and region-dependent complex fluid properties to predict scattering from equivalent-fluid models. The results are validated with scale model boundary-plane goniometer measurements and numerical references, verifying that it is possible to predict diffusion coefficients following ISO 17497-2 and overcome the limitations of the BEM method.