ABSTRACT The thesis focuses on aluminum porous board used in noise reduction. The experiment evaluates propagation of sound wave and examines acoustic characteristics of aluminum porous board when sound wave impinging on the aluminum porous board. Also, the material properties and acoustic properties of aluminum porous board are analyzed to help further understanding of aluminum porous board. Biot`s poroelastic theory is essentially used to obtain the equation of motion of the elastic porous material, following the calculation of sound transmission loss via application of appropriate boundary condition. Supported by the theoretical analysis and measured data, the result in the thesis shows that aluminum porous board can provide well noise reduction throughout all frequency ranges. In addition, aluminum porous board lined with multi-panel structure, through analysis on sound transmission loss, proved better effect than single-paneled aluminum porous board in noise reducing. The difference is about 5dB in low frequency and more than 10dB in high frequency. The studying of material mechanical properties of aluminum porous board is also included in the thesis, According to the database obtained in this thesis, acoustic properties and material properties of aluminum porous board can be worked out to estimate suitable aluminum porous board applied in noise reduction. The analysis suggests if higher noise reduction is required, one needs to thicken the aluminum porous board, or to increase density and Young`s modulus of the material, which results improvement in high frequency; but no effect in low frequency. Moreover, increasing material thickness and density will shift the resonance frequency to lower value; in addition, increasing Young`s modulus will move the resonance frequency to higher value.

This thesis uses aluminum porous board (AP board) to study the noise reduction for cooling tower. It sets the barrier to isolate the sound propagation. But it will happen diffracted phenomenon when sound wave impinging at the edge of barrier. So the mathematical model of acoustic diffraction on the barriers, which is set up by Hayek, is applied in this thesis. Base on this theorem, the AP board and the paths of sound propagation are analyzed. In experiment, it uses the sound intensity method to measure the cooling tower for determining the major source first. The major source is determined by ranking the sound power. And the suitable insulation material is selected by analysis the frequency band of major source. After analyzing, the major source of cooling tower is the region of fans by motor driven that is located upper the cooling tower. And its frequency range is between 25 Hz to 2500 Hz. So the AP board is a candidate since it has broadband characteristic on noise insulation. After aim of the major source, the noise barrier is studied for noise reduction. In this thesis, the U profile of barrier (looking down from above), which considers the situation in the field, is designed to surround the cooling tower. This barrier is made of aluminum board, and the aluminum porous board is applied to add on the upper barrier for noise reduction. To study the acoustic diffraction on the boundary of barrier, the thick of porous board is added on the upper barrier. The insulation effect is compared in the different condition after measuring the transmission loss. The T shape barrier is also designed for noise reduction evaluating in this thesis. Finally, the best-insulated effect is obtained when the complex board is added on the upper barrier. And the noise level is down to 59 dB around the environment. This result is matched the EPA noise standards.