Application of micro perforated and impervious membranes for noise barriers

Abstract

Membrane materials have been commonly used for decades in buildings. When acoustic environments are concerned, the acoustic properties of these membrane structures are of special interest. This thesis aims to investigate acoustic properties of micro perforated membranes (MPMs) and impervious membranes and enhance the sound insulation of double layer impervious membranes by combining these with MPMs, thereby increasing the internal loss mechanisms of what is essentially a reactive wall. This thesis firstly develops a new model of an impervious membrane, taking into consideration the tension and the internal damping due to the membrane curvature. The sound absorption of MPMs inserted between the impervious layers has been studied by introducing a new boundary condition where the particle velocity at the hole wall boundary is assumed to be equal to the membrane vibration velocity. The comparison between the predicted and measured results demonstrates that MPM 1 to 3 can be considered impervious due to their sufficiently small perforation radii, and MPM 4 is sound absorbing due to its larger perforations. Non-linear sound absorption of MPM 4 has been observed in the experiments. It was found that the non-linear sound absorption coefficient is strongly dependent on both the magnitude of the SPLs and the waveform of the excitation. Two analytical models were developed for the non-linear acoustic impedance of MPMs. In the first model, the non-linear impedance of MPMs is considered to be the sum of the linear impedance, and the non-linear acoustic impedance dependent on the particle velocity within the perforations. The second analytical model presented is inspired by the air motion equation and the mass continuity equation considering the density variation in the time and spatial domains, and provides the most accurate predicted results among the models considered in this study. The analytical models have been developed to predict the STL of double layer impervious membranes separated by a finite-sized air cavity, taking into consideration the fluid-structure coupling on each membrane surface. Comparing the predicted results to the measured STLs, it is found that considering the sound absorbing boundaries of the cavity can enhance the accuracy of the models. STL measurements of double layer impervious membranes with four types of MPMs have been conducted in a diffuse field to quantify the effectiveness of the MPM insertion. The experimental results indicate that the MPM insertion can enhance the STL of the double layer impervious membranes significantly at frequencies above the first resonance frequency of the air cavity. MPMs 1 to 3 have similar main impacts on the STLs, however, MPM 4 has a different effect because of its larger perforations. The normal incidence and diffuse field models for the double layer impervious membranes with inserted MPMs 1 to 3 were developed and the predicted results were compared with the experimental results. The models with MPM 4 were developed by taking into consideration the acoustic impedance of the MPM 4 due to its perforations. These developed models can be used as tools for design of membrane structures.