A Case Study of a Full Inverse Poroelastic Characterization of an Open-Cell Porous Material Using an Impedance Tube: The Need to Properly Prepare the Material and to Control the Measurement 2018-01-1567

This paper presents a case study on the full inverse characterization of the material properties of an open-cell poroelastic foam using impedance tube measurements. It aims to show the importance of controlling the lateral boundary condition in the impedance tube, and selecting an appropriate acoustic model to obtain the most accurate material properties. The case study uses a four-inch thick melamine foam and a 100-mm diameter tube. The foam is mechanically cut to fit within the circular tube. However, the cutting process is not perfect and a tiny lateral air gap exists between the material and the tube (i.e. the foam diameter is 99.5 mm for a 100-mm diameter tube). The typical characterization procedure is to mix direct and indirect measurements to retrieve the material properties of the foam. First, open porosity, bulk density, and static airflow resistivity are directly measured. Second, tortuosity, viscous and thermal characteristic lengths, and elastic properties are identified by inverse characterization using impedance tube measurements. The inverse characterization uses different choices of frame behavior models (rigid, limp, or elastic), and lateral boundary conditions (with and without lateral air gap). The paper discusses the effects of the choice of the frame behavior model and lateral boundary condition. Notably, it shows that the tiny air gap can seriously affect the inversely characterized material properties. Moreover, the choice of the frame model impacts less the quality of the inversion, but allows or not the characterization of the elastic properties. Finally, the paper concludes with recommendations and guidelines to improve the accuracy of the inverse characterization procedure, and discusses its limitations.