In situ acoustic characterization of a locally reacting porous material by means of PU measurement and model fitting

Reliable data on acoustical properties of materials are crucial for the design of a desired acoustic environment as well as to obtain accurate results from acoustic simulations. Although the acoustical properties of materials can be obtained via laboratory measurements, situations where in situ measurements are needed are often encountered. However, in situ measurement methods presented so far are limited by their poor portability or inaccuracies in the low-frequency range. In this work, we propose a characterization method that combines an in situ pressure-velocity (PU) measurement with a model fitting procedure using the Delany-Bazley-Miki impedance model for porous materials. The method uses an optimization routine to find the best match of measured and modelled reflection coefficient values within a given frequency range for the optimization parameters: flow resistivity, panel thickness, and probe-sample distance. The optimal parameter values allow, in turn, calculating the porous panel's reflection coefficients for a broad frequency range including frequencies below the lower bounds of the optimization frequency range. The sensitivity of the method to panel width, lower bound of fitting frequency range, and to excluding parasitic reflections by time windowing is studied. The study shows that the proposed method provides characterization results in good agreement with reference data for panels of dimensions larger than 1800 mm and that the method is robust for reduction of one dimension of the panel down to 300 mm. It also shows that the model fitting accuracy is best when the frequency range of analysis is restricted to 1000–5000 Hz