In recent years, noise pollution is one of the serious social problems, and many efforts have been paid to the development of efficient sound absorbing material. Normal sound absorbing materials are made of inorganic fibers, and inorganic fiber has difficulty with processing after the disposal. From the viewpoint of environmental protection, therefore, sound absorbing material made of natural fiber is expected. Furthermore, normal sound absorbing materials consist of single-layered structure and the sound absorbing properties have a limit level of frequency. Now, it is expected that the multi-layered structure improve the sound absorbing properties. Under these circumstances, eco-friendly and also high performance sound absorbing material with density graded structure was developed by using wool fibers in this study. As a result, it was cleared here that the density graded structure is good for the sound absorbing performance.
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