Abstract
Owing to the rapid industrial development and the rising of environmental consciousness in Taiwan, the researches on noise control become more important. Perforated plates and porous materials (such as glass fiber materials and foams) are usually applied as the acoustic absorbing materials for noise control. However, since single kind of acoustic absorbing material has better acoustic absorption only at certain frequency bands, the multi-layer absorbers composed of perforated plates, porous materials and airspaces are usually applied to absorb broadband noise. The acoustic absorption of these multi-layer absorbers is very dependent on the shape and arrangement of their components. The main objective of this work is thus to respectively discuss the influence of the shape and arrangement of those components on the acoustic absorption of the multi-layer absorbers thoroughly.In this work, the multi-layer transmission analysis, which can be used to successfully analyze the acoustic absorption of the multi-layer absorbers composed of arbitrary number of compartments, is first derived. Each compartment is composed of one layer of perforated plates and multi-layer porous materials and/or airspaces. Such a multi-layer transmission analysis can also compensate the drawback of the equivalent electrical circuit approach in analyzing multi-layer absorbers, which cannot deal with multi-layer porous materials and airspaces between two adjacent perforated plates.Due to the geometric constraint of components, the application of analytical approach is limited. Hence, a rigorous finite element analysis model which can accurately tackle the acoustic effects of perforated plates, porous materials and airspaces with arbitrary shapes is also developed in this work. Four types of basic inner structure compartments adopted in the multi-layer absorbers are selected to study the influence of the inner structures on the acoustic absorption of the multi-layer absorbers. These compartments are composed of porous materials inlaid with perforated plates of various shapes, say, triangle, semicircle, convex rectangle and plate shapes. As is different from the conventional finite element analysis for acoustics, the perforated plates are simulated by appropriate equivalent boundary conditions, depending on their thickness, hole radius, hole pitch and porosity and the air contained in the holes. A large number of total degrees of freedoms generated from meshing the air in the holes of perforated plates are thus avoided.Based on the results obtained, the multi-layer absorber composed of a novel inner structure and proper perforated plates and porous materials is designed and manufactured successfully. Both the finite element and experimental results show that its acoustic absorption is distinctly higher than the conventional acoustic absorbers.Finally, some acoustic applications are also discussed in this work, such as the insertion loss of multi-layer acoustic bafflers. It is found that the insertion loss of the acoustic baffler composed of perforated plates, porous materials and airspaces is higher than the acoustic baffler composed of concrete. In addition, the sound pressure distribution of this system also reveals that the sound diffraction has significant influence on the sound attenuation caused by the acoustic bafflers.