A FINITE-ELEMENT PROPAGATION MODEL FOR EXTRACTING NORMAL INCIDENCE IMPEDANCE IN NONPROGRESSIVE ACOUSTIC-WAVE FIELDS

A propagation model method for extracting the normal incidence impedan ce of an acoustic material installed as a finite length segment in a w all of a duct carrying a nonprogressive wave field is presented. The m ethod recasts the determination of the unknown impedance as the minimi zation of the normalized wall pressure error function. A finite elemen t propagation model is combined with a coarse/fine grid impedance plan e search technique to extract the impedance of the material. Results a re presented for three different materials for which the impedance is known. For each material, the input data required for the prediction s cheme were computed from modal theory and then contaminated by random error. The finite element method reproduces the known impedance of eac h material almost exactly for random errors typical of those found in many measurement environments. Thus, the method developed here provide s a means for determining the impedance of materials in a nonprogressi ve wave environment such as that usually encountered in a commercial a ircraft engine and in most laboratory settings. (C) 1996 Academic Pres s, Inc.