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。有两个重要的声学参数：吸声系数和声传播常数。用声管法或混响室法测量前者比较容易。另一方面，人们认识到传播常数后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。通过这种方法，可以不需要任何空气空间，这就造成了普通方法难以将测试材料放置在声管端。该方法基于数字信号处理技术。通过测试材料前后声压之间的传递函数，可以很容易地得到传播常数。本课题的目的是开发新的测量方法。经过考虑，我们制作了可分为三部分的音管测量系统。在声音测量之前，可以很容易地重新连接气密。该测量系统既可以测量声阻抗，也可以测量传播常数。在此基础上，对常用的多孔材料进行了传播常数和声阻抗的测量。此外，还对砂子、软纸等颗粒材料进行了特殊测量。结果表明，单位长度的dB衰减值随频率的增加而增大。研究还发现，多孔材料中的声速比空气中的声速低。最后，对试验材料的共振减小进行了研究。

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.