Inverse acoustic property characterization using impedance tube measurements

Abstract

The ever-increasing demand for improved fuel-efficiency and weight reduction has resulted in increased focus on porous materials. Cellular porous materials offer designers a route towards improving structural properties while improving the structure functionality—such as increase thermal dissipation, fluid transport, and acoustic absorption. The acoustic properties of porous materials, the subject of this thesis, offer an ideal solution for reducing aircraft noise emanated from commercial turbofan engines. However, including such materials within engineering design requires an improved understanding of their acoustic properties and the dependence of these properties on the various transport parameters associated with porous materials. While these properties may be directly measured using lab instrumentation, such measurements are often time consuming and require significant investment. The recently developed inverse characterization technique which allows the calculation of acoustic transport properties from impedance tube measurements offers a cheaper and more efficient alternative to such direct methods. To this end, the focus of this thesis is to clarify the suitability of this method when applied to various porous materials. Here, we compare the direct measurement with predictions obtained using the commercial inverse characterization software FOAMX, which uses impedance tube absorption data to inversely calculate acoustic transport properties using either the equivalent fluid model or the poroelastic model. The study compare the predictions obtained by both models and demonstrate the suitability of each method for various porous material. The results obtained show that the calculated material parameters can vary drastically depending on the method used and material model selected by the user. Further, the effect of the measured material properties on the predicted absorption behavior is compared with experimental results.