以注水彈性阻抗管測量材料之水中聲學特性之研究

Abstract

本論文內容主要是以阻抗管法量測吸音材之特性為基礎，自行設計駐波管，用來量測水下吸音材之吸音特性。理論部份，首先探討彈性波導管內波傳特性之行為，以了解在彈性波導管內，傳統的平面波理論是否合理，文中並提出一個線性彈簧數學模式以模擬吸音材的阻抗，利用回歸法，將材料聲學阻抗值，歸納成三個物理常數，即材料的「質量」，「彈性係數」及「阻尼」，最後重新反算吸音材料之反射係數，並和實測數據相比較，以作為此數學模式之可行性驗證。實驗過程中以兩種不同長度及厚度之阻抗管，針對「自由液面」及多種吸音材料進行量測。其中「自由液面」之量測結果顯示與理論全反射(R=-1)已相當允合。當阻抗管長度及厚度增加，測量數據更具一致性，實驗誤差也相對減少。

The purpose of this study is to investigate the acoustic reflection coefficient of the underwater materials measured in a water-filled impedance tube. In the theory, some modifications are proposed to discuss the elastic waveguide inside the tube, assessing the feasibility of plane wave theory. Then, linear spring mathematic model is proposed. It is expected that the measured complex material impedance can be equivalent to three physical parameters, say, the equivalent “mass”, “spring stiffness”, and “damping coefficient.” Next, the three physical parameters can be recalculated to obtain the reflection coefficient, which further can be compared with the measured result to verify the feasibility of the suggested model. In the experimental process, free surface and materials are measured with impedance tube of different length and thickness. The result of free surface proves consistent with the theory. However, with longer and thicker tube, the measured result shows even more consistencies, with fewer errors.