Numerical and experimental analysis of acoustic impedance and sound absorption measurements using the in situ technique with two microphones

Abstract

Acoustic impedance and sound absorption are important parameters for several areas of Acoustic Engineering, such as: Room Acoustics, Noise Control, Auralization and Electroacoustics. Thus, the measurement with precision and accuracy of the acoustic impedance of materials is necessary. In this work, the in situ measurement method of sound absorption and acoustic impedance using two microphones (PP System) is described. Then, two models for deduction of surface impedance are presented, they are: PWA model and q-term algorithm. A model using the Finite Element Method (FEM) is proposed and an analysis based on it is performed. Through the FEM model it was possible to demonstrate the convergence of the q-term algorithm for locally reactive samples and the divergence of the PWA model for low frequencies. Also through the FEM model an analysis of finite samples of circular surfaces was performed. The error due to small sample sizes was identified, a literature-based strategy was applied and a significant improvement was obtained. An experimental bench was built and tested. The experimental results from the bench for a locally reactive sample had good agreement with the Delany and Bazley model and with the results obtained in Impedance Tube, agreeing with the predictions made from the FEM model. Errors found in the numerical results were also found in the experimental results, showing that the FEM model is a valid tool to make predictions about the in situ measurement method using the PP System. At the end, the measurement of a sample in a common environment is presented, showing the good applicability of the in situ technique using the PP System.