A hybrid impedance measurement method to inversely determine the impedance of finite porous absorber material samples

Abstract

The determination of complex angle-dependent reflection factors is not common practice. Mostly, because existing methods are complicated and very time-consuming. This work presents a method that only needs a single sound pressure measurement of a finite porous absorber piece, placed in a semi-anechoic chamber, as input data. The complex pressure data obtained of the measurement are compared with that of a finite element method (FEM) simulation, which is based on the same geometric dimensions of the material sample as in the measurement, but with an arbitrary porous material. With a non-linear fitting algorithm, the simulated complex pressure data are adjusted to fit the measured data iteratively by changing the absorber model parameters (porosity, flow resistance, etc.) in the FEM simulation. When the simulated and measured results are identical within a defined threshold, the characteristic absorber properties are found and with that the angle-dependent complex reflection factors are determined. Applying this inverse approach allows also for a correction of the edge effects of the finite material sample, such that the found impedance is the impedance as measured on an infinite extended material sample and it also determines the individual material properties as porosity and flow resistivity of a porous material.