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

多孔材料，如玻璃棉和岩棉，是著名的和有用的声学材料。吸声系数和声传播常数是两个重要的声学参数。前一种方法易于测量，可用声管法或混响室法。另一方面，人们认识到传播常数的测量是非常困难的，为此，首席研究员提出了一种新的传播常数测量方法。这种方法不需要一般方法中所需的空气空间，而空气空间使测试材料的背面位于声管的端部。该方法基于数字信号处理技术。传播常数可由声管前后声压的传递函数求得，本课题的目的是发展一种新的测量方法，经过考虑，我们研制了一种可将声管分成三部分的测量系统。在进行声音测量之前，可以进行简单的气密重新连接。该测量系统可用于声阻抗测量和传播常数测量。此外，本研究亦针对目前常用的多孔材料，量测了其传播常数与声阻抗。对砂、软纸等颗粒材料进行了特殊的测量，这些颗粒材料是除此之外无法测量的。发现单位长度上的dB衰减值随频率的增加而增加。研究还发现，多孔材料中的声速低于空气中的声速。最后，对试验材料的共振衰减进行了研究。

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.