A novel project-centred micro-perforated panel sound absorber manufacturing technique for building and environmental applications

Abstract

This thesis investigates the properties of micro-perforated panels (MPP) with tapered elliptical perforations, that are created due to different laser punching settings. While the theoretical model for MPP with cylindrical perforation has been widely established and well known, actual production of MPP does not create circular perforations, especially if the panels are made of acrylic for lightweight applications. The conventional circular perforation theory has led to inaccurate prediction of sound absorption. Different manufacturing parameters will create perforations of different sizes on the two sides of the MPP material. A new theoretical model is presented to improve the modelling of tapered elliptical perforation performance. The predicted acoustic impedance is derived and then compared with finite element simulation as a validation. A good agreement of the mathematical model, and the finite element validate the new theoretical model. Impedance tube testing of normal incidence sound absorption is then carried out using actual samples of MPP made by our partner company to further verify the absorption performance. The sound absorption coefficients of fifteen perforations of different configurations and layouts were measured. The predicted absorption coefficients are compared with the measurements. The new model shows better accuracy in predicting the sound absorption of the MPP than the widely established circular perforation theory. With the new model, we are able to realize the different specifications of an MPP that are required for different usage. Working from the noise spectrum that is to be resolved, we can determine the required acoustic performance by looking at the frequency range, and the desired reduction levels. The hole geometry can then be decided upon confirming the other parameters of the MPP, such as the perforation ratio, and thickness of the material. As the relationship of the manufacturing parameters and the dimensions of the perforations are shown to be not linear, an artificial neural network (ANN) model is created to predict the perforation created based on the parameters of laser power, laser duration, focus height, and thickness of the material. The ANN tested is able to accurately predict new perforations that can be created with a new set of laser settings. The fabrication of the MPP can then be done quickly and efficiently without the need to ‘trial-­and-error’. A parametric investigation is undertaken to evaluate the performance of a single side branch absorber incorporating a microperforated panel (MPP) within a rectangular duct subjected to airflow. This study centers on analyzing the absorption coefficient of the side branch absorber, considering various specifications including cavity depth, perforation dimensions, perforation ratio, MPP thickness, and duct flow rate. A test is also conducted with panels that do not have any perforation, to evaluate the effect of the presence of perforations. The attenuation rate of the absorber is not very influenced by the presence of flow inside the duct. The flow inside the main duct does not display pressure loss after passing through the section with the absorber. The results of the testing show the potential of using a sidebranch absorber for broadband damping of sound for building ductwork.