Extended reacting impedance characteristics of polyurethane foam in a duct with flow

Abstract

Extensive use at the Boeing Company of Scottfelt, Grade 900, polyurethane foam as a relatively inexpensive test material for jet-engine nacelle acoustic studies prompted the development of a polyurethane-foam impedance math mode. A math model for an extended reacting foam liner in a duct with flow has been formulated, based on results of previous normal incidence impedance tube tests and math model studies reported at the 86th Meeting of the Acoustical Society of America. Using this model, predictions of the complex axial wavenumber are compared to measurements of firmness 4 Scottfelt in the Boeing/Wichita Grazing Flow Impedance Tube. Results for thicknesses of 1/2 and 1 in. at duct Mach numbers of 0, 0.35, and 0.5 are reported. The extended reacting impedance model has also been incorporated into a duct attenuation prediction program. Predicted attenuations are compared with polyurethane-foam flow duct test results as a further check on the math model.