Sound-Absorption Properties of Composite Materials Containing Waste-Wool / Polyamide Fibers and Their Relationship with Fractal Dimensions

To solve the problems related to the recycling of waste fibers, composite materials were prepared by the hot-pressing method using waste-wool fibers and waste low-melting-point polyamide fibers combined into a net as the raw materials. The effects of the volume density, mass fraction of waste-wool fibers, and thickness on the sound-absorption properties of the resulting composite materials were studied by the controlling-variable method. The sound-absorption properties of the composite materials were studied by the transfer-function method, and under optimized technological conditions, the sound-absorption coefficients were above 0.8 and the sound-absorption bands were wide. According to the box-counting-dimension method, which is based on the fractal theory, the fractal dimensions of the composite materials were calculated using the Matlab program. The relationships between the fractal dimensions and the volume densities, mass fractions of waste-wool fibers, and thicknesses of the composite materials were also analyzed. Then, quantitative relationships between the fractal dimension and the maximum sound-absorption coefficient, and between the fractal dimension and the resonant sound-absorption frequency, which play a major role in the sound-absorption design of composite materials, were deduced.