Acoustic Surface Impedance Estimation with a Hybrid Measurement and Wave-Based Simulation Method
Master Thesis of Müller-Giebeler, Mark
Existing and commonly used methods to measure the sound reflection properties of acoustic surfaces have certain restrictions. Either they inherently do not provide enough information (for example phase information for wave- theory based simulations), or they only yield realistic results to a limited extent (for example in a perfectly diffuse sound field, under a plane wave incidence assumption or only for a limited frequency range) . More sophisticated methods to determine the complex angle-dependent reflection factors are often complicated and very time-consuming. This work presents an inverse method that only needs a single sound pressure measurement of a finite porous absorber sample, along with geometric information for simulation, as input data and takes into account the actual incident sound field as well as a potentially non-locally reacting material. With a non-linear fitting algorithm, the simulated complex pressure data is adjusted to match the measured data by changing the absorber model parameters (flow resistivity, porosity, etc.). Several factors that affect the measurement and/or the optimization process are investigated theoretically. Furthermore, an extension to the above mentioned approach is proposed, allowing for a edge effect correction of the finite material probe. Using iteratively refined FEM simulations that are based on the same geometric dimensions of the absorber sample as in the measurement, enables to compensate the introduced error and to determine the impedance as measured on an infinitely extended material probe. The method is validated based on simulations and applied in preliminary measurements.