In situ PU-based characterization of sound absorbing materials for room acoustic modeling purposes

Numerical room acoustic algorithms are becoming increasingly efficient with the increase of computational power. With hybrid modeling numerical schemes, it has become feasible to simulate room acoustics with sufficient accuracy over the whole audible frequency range (20-20000 Hz). However, in many cases, the measurement of materials in the laboratory fails to provide the material properties used as input for the simulation methods. This is because the as–installed materials may have different properties than in a laboratory room. Therefore, the acoustic properties of the materials being simulated are one of the main impediments to accurate simulation results. That is, the boundary conditions used as input are problematic. The quality of the boundary condition input relies on two complementary processes: 1) accurate estimation of the acoustic properties of the material involved; 2) incorporation of these properties into a boundary condition that is suitable for the intended algorithm.

This project addresses primarily the first of the above items by proposing a method to perform the broadband in situ characterization of sound absorbing materials efficiently and reliably. This is achieved by combining a pressure–velocity in situ measurement scheme with an impedance model fitting procedure. The applicability and accuracy of the proposed method to the following types of sound absorbing materials have been investigated. 1) In situ characterization of a locally reacting hard-backed porous layer. The accuracy of the characterization procedure was assessed for two samples of materials and a sensitivity study was conducted regarding the size of the patch, the parasitic reflection and the frequency range of analysis. 2) In situ characterization of a non-locally reacting system consisting of a porous layer with a large air cavity backing. 3) In situ characterization of a porous layer covered with a rigid perforated facing. For the three types of sound–absorbing systems investigated, the proposed procedure allowed to provide accurate in situ characterization both in terms of retrieved parameters and complex reflection coefficient values.

Furthermore, the project also investigates the impact of using the results of the proposed characterization method to carry out room acoustic Page i simulations, in the following research studies: 1) Measurements of fully controlled room acoustic scenarios and implementation of their digital counterparts in the form of a wave–based simulation method, where a first draft of the in situ characterization approach is used to implement the relevant boundary conditions in the simulation method. 2) Simulation of a single patch of porous material in a reverberant room and a study of how the flow resistivity and thickness of the simulated material affect the simulated reverberation time of such a configuration. The results showed that the good estimation of the thickness of the material and the accounting of the absorption from the side faces are critical to reach good accuracy, in particular at low frequencies.

The work presented in this dissertation contributes with an accurate method to characterize various types of sound absorbing materials and to derive accurate and relevant broadband boundary conditions for room acoustic simulations. It further contributes with applications where the results of the material characterization are used as input in wave– based room acoustic simulations. Good agreement was found between the measured and simulated room acoustics, evaluated through room acoustic parameters.