Abstract
Porous materials are crucial in a number of applications, from sound absorbers used in room acoustics and noise control to acoustical interactions with outdoor ground surface and ocean floor. A novel porous laminated composite material (PLCM), made of polymer, metal, and polymer fibers of low melting point, is developed. Manufactured by premix, lamination, pre-heating, and molded techniques, the proposed PLCM exhibits very high sound absorption coefficient in the frequency range of 500 – 2000 Hz with a relatively thin and light structure.In order to reduce the time required for developing a novel type of PLCM. Firstly, assumption the most energy loss in PLCM is due to the fibrous surface viscous force only and any motion of the solid frame in the PLCM is ignored. Therefore, this thesis adopts combinative a porous rigid body and a mathematical model of the flow resistivity is derived to predict the sound absorption coefficient of the PLCM. This presentation may be useful for developing a novel PLCM with predictable sound characteristics under adaptive fibrous absorption material for the formulation. However, the prediction model is ignored micro-motion of the solid frame in the PLCM, elastic deformation, and process flow idealism. The phenomenon may invalid in some circumstances and the prediction accuracy is loss.Therefore, this thesis presented two models to predict the sound characteristics for a new PLCM. First, a semi-empirical model is derived utilizing seven parameters: fiber diameter, cross-section of fiber, fibrous density, PLCM density, PLCM thickness, mixing ratio and frequency. The semi-empirical model, in which the flow resistivity is only a function of fibrous surface, intends to predict the flow resistivity of the PLCM providing that the material densities are known and the fibrous diameter fibers are fixed. Secondly, this thesis presents an empirical model deriving the characteristic impedance and propagation constant as a function of the flow resistivity, the fibrous surface of PLCM, and the frequency.Combining feedforward and feedback control systems, an active noise control (ANC) system is constructed. Using the method of recursive Least-Square Parameter Estimation with Self-tuning Forgetting Factor, the system identification of the following cases are executed. Case 1: turn off the noise signal and set the gains of feedforward and feedback controller to be one. Case 2: start on the noise signal and turn off the second signal. Case 3: start on noise signal and set the gains to be one and zero for feedforward and feedback controller, respectively. Based on the three cases above, the controller of the ANC is designed. In order to obtain high performance of noise cancellation in a wide frequency range, this study adopts a hybrid absorption system, which comprises a layer of PLCM in the passive component, and in the ANC the pressure of the reflected wave on the PLCM boundary conditions is set to be zero.Several experiments for prediction and practical applications have been successfully conducted. The experiments included the prediction of the absorption coefficients for single and multiply layer PLCM structure. The practical applications were executed by using the PLCM to composite acoustic board, muffler, room noise cancellation, hybrid noise control, silencer for large space parking lot, etc.Computer simulations were performed to verify the feasibility of the developed model, and experiments were conducted to examine the sound absorption coefficients of the PLCM. Comparisons of the absorption coefficients between the model and the experimental results were also made.