Study Of Structural Characteristics And Acoustic Properties Of Fiber Assembly

According to the classical statement, the acoustic absorption property of fiber materials is determined by fiber mophology, mechanical property, or packing structure, and the interaction between fibers. Obviously, it is the saying of shape and structure representation. In the past30years, researchers have achieved major breakthrough, both in fractal structure of soft materials, and in the phononic crystals. These improvements provide new theory and design consideration in the study of acoustical materials. The present thesis focuses mainly on the relationship between structural characteristics and sound absorption property of loose fiber assembly and nonwovens.The double-microphone standing wave tube was adopted to measure the sound absorption property of fiber assemblies. The structural characteristics of loose fiber assembly include packing density, fiber volume fraction, fiber arrangement, and air gap between fibers and background metal plat. The valid pore size of fiber assembly is calculated by theoretical modeling, and also the relationship between sound absorption coefficient and the sound frequency. The results demonstrate that the sound absorption coefficient increases with the fiber volume fraction, while the critical frequency moves toward to lower; the variety of fiber seems not effective to sound absorption; the different fiber arrangement has influence on sound absorption, and the random arrangement makes the highest absorption coefficient; the valid pore size of fiber assembly is the key parameter in sound absorbing; on the frequency range below5kHz, the dominant mechanism of sound absorption is viscous and resonant of air gap, so on these frequency range the air gap can replace the fiber in sound absorbing; while on frequency above that, the vibration effect of fiber structure can not be ignored. In a word, when the fiber volume fraction of fiber assembly increases, the valid pore size decreases, and sound absorption coefficient increases, while the peak of absorption curve moves toward to lower frequency.A high-resolution X-ray micro-computed tomography, Skyscan1172was adopted to scan the loose fiber assembly; the scanning process was nondestructive and in situ, which obtained two dimentional cross-sectional images of fiber assemlies. The box-counting fractal approach has been used to calculate the fractal dimensions of these cross-sectional images. The experimental results indicate that the structure of fiber assembly has fractal characteristic. The correlation of fractal structure and acoustic property has been built up. For acrylic fiber assembly, the maximum absorption coefficient am increases with fractal dimensions, while critical frequency fcri has negative correlation with fractal dimensions. For down fiber assembly am increases with fractal dimensions initially, but after a critical dimension, am decreases instead, the critical frequency fcri has negative correlation with fractal dimensions, just like the acrylic fiber assembly.The relationships between structural parameters of nonwovens and their air permeability and sound absorption are investigated. The structural parameters include thickness, weight per square meter, packing density, porosity, average pore size, and pore size distribution. The measured results show that the air permeability has not simply linear relationship with the porosity, but some parameters even not easy to measure, such as the tortuosity of pore channel, shape factor, etc. The acoustical properties of nonwovens without air gap, with15mm and30mm air gap were measured by standing wave tube. It is proved that the introducing air gap could improve the sound absorption of nonwovens, and the increasing air gap makes the absorption curve rise fast and move toward to lower frequency. Traditional capillary theory can explain the issue of maximum absorption coefficient for nonwovens. About critical frequency issue, the theory of membrane vibration can obtain an anastomotic result. After comparisons of maximum absorption coefficient, resonant frequency, five structural parameters, and air permeability, the pore characteristic has been considered to be the most important factor in sound absorbing, especially the average pore size.The images of nonwovens are obtained by scanning electron microscope. After series of image process such as binaryzation, these images are used to calculate the fractal dimension of nonwovens. The high degree of correlation in fractal calculation reveals the obvious fractal feature of nonwoven. Through comparions between fractal dimension and porosity, average pore size, it is proved that fractal dimension has remarkable negative linear correlation with these two parameters. While the fractal dimensions has also linear correlation with acoustic properties, the maximum absorption coefficient αm increases with increasing fractal dimension, and the critical frequency fcri decreases with it.For the first time, the possibility of the fiber materials applied as phononic crystals is discussed. When the fiber-air composite materials has periodic structure, it is possible to be a phononic crystal. The properties of fiber itself and the structure parameters of composite materials have both influence on band structure of phononic crystals. By changing these parameters, the plain wave expansion method is used to calculate the band gap of different fiber-air two dimensional phononic crystals. In addition, theory and calculating model of several textile examples are built up. Results show that the effect of fiber property is not extremely notable; with the increasing of lattice constant, the frequency of first band gap is dropping down, at the mean time the frequency range with band gap is reducing;with the packing ratio of fibers in composite materials, the width of band gap is increasing too; and the lattice arrangement has also effection on the band structure of phononic crystals.