Mapeamento de erro em métodos de medição de resistividade ao fluxo de ar de materiais porosos

Abstract

The air flow resistivity is one of the most important macroscopic parameters in the characterization of absorption materials since this parameter is used to describe the absorption in all classical calculation methods. In this paper two experimental methods will be addressed, namely, the one proposed by Dragonetti, Ianniello and Romano and the method standardized by ISO 9053-2:2020. Both methods deal with measurements from an alternating airflow and assume that the measurement frequency takes on values tending to zero, such as 2 Hz. This very low frequency condition can lead to errors due to the low sensitivity of the transducers (microphones and accelerometers) required for the measurement. For this reason, this work aims to map and quantify the errors intrinsic to each of the methods by generating regions in frequency, sample thickness and airflow resistivity that describe the percentage error. The possibility of performing measurements at frequencies at which the transducers have better sensitivity, such as 25 Hz, will also be evaluated. The aforementioned experimental methods were computationally simulated via Finite Element Method (FEM) and the simulations validated from bench measurements. The mapping of the errors of both methods allows identifying which of them is more suitable to perform the flow resistivity measurement according to different types of materials (porous or perforated), observing the limitations of thickness and flow resistivity of each method, besides allowing the evaluation of the possibility of measurements at higher frequencies. The experimental bench to evaluate the methods was built by the author in order to allow the measurement of both methods simultaneously. The results of the bench experiments corroborate the numerical simulations performed computationally indicating errors below 5% for both methods at frequencies below 10 Hz. At 25 Hz the standardized method shows lower error as thickness and airflow resistivity increase while Dragonetti’s method performs better as thickness and airflow resistivity decrease. Dragonetti’s method allows measurements with error below 5% for all simulated thicknesses at frequencies up to 50 Hz, while the standardized method is limited to 10 Hz.