A Finite Element Propagation Model For Extracting Normal Incidence Impedance In Nonprogressive Acoustic Wave Fields

A propagation model method for extracting the normal incidence impedance of an acoustic material installed as a �nite length segment in a wall of a duct carrying a nonprogressive wave �eld is presented. The method recasts the determination of the unknown impedance as the minimization of the normalized wall pressure error function. A �nite element propagation model is combined with a coarse��ne grid impedance plane search technique to extract the impedance of the material. Results are presented for three di�erent materials for which the impedance is known. For each material � the input data required for the prediction scheme was computed from modal theory and then contaminated by random error. The �nite element method reproduces the known impedance of each material almost exactly for random errors typical of those found is many measurement environments. Thus � the method developed here provides a means for determining the impedance of materials in a nonprogressive wave environment such as that usually encountered in a commercial aircraft engine and most laboratory settings.