Development Research on Measuring Method of sound propagation Constants of Porous Materials.

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Porous materials such as glass-wool and rock -wool, are famous and useful as acoustical materials. There are two important acoustical parameters of sound absorption coefficient and sound propagation constant. It is easy to measure former one by using sound tube method or reverberation room method. On the other hand, it is recognized that it is very difficult to measure after propagation constant.Therefor, head investigator had proposed a new measuring method for propagation constant. By this method, there could not be necessity of any air space, which caused difficulty in ordinary method, back of test material positing at sound tube end. The new method based on digital signal processing technique. Propagation constant could be easily obtained from trabsfer function between sound pressures in front of test material and back of that.The purpose of this project is the development the new measuring method.After consideration, We made the measuring system that sound tube could be divided to tree parts. Before sound measurement, easy re-connection with air-tight could be made. The measuring system is possible for the acoustical impedance measuring as well as propagation constant. In addition on this research, for popular porous materials, several propagation constants and sound impedance had be measured. Special measurements for particle material such as sand and soft paper, which could not be measured by except this technique, were carried out. It was found out that dB attenuation value in unit length increased with the increase in the frequency. It was also found out that sound speed in porous material was lower than the sound velocity in the air. Finally of this project, reducing of resonance of the test material could be investigate.