Research On Measurement Technology Of Acoustic Velocity In Seawater Using Acoustic Resonance Spectroscopy

The acoustic resonance spectroscopy (ARS), which is a fundamental, promising and not very mature measurement, is used to measure acoustic velocity of sea water. In this paper, several key issues on measurement technology of acoustic velocity in seawater using ARS are discussed. Aiming at the key issues on measurement technology of acoustic velocity in seawater using ARS, the finite element method (FEM) is introduced as the numerical simulation method for the first time. And the influence of the Fluid Structure Interaction (FSI) effect on the speed of sound measurement accuracy is considered for the first time, too. Firstly, in order to get the equation of the cylindrical cavity's resonance frequency, a mathematical model of a cylindrical resonator in seawater is established, and based on fluctuation theory, the analytical expression of the sound pressure is established with the method of variable separation. Secondly, the cylindrical cavity's resonant frequency is got by using the numerical simulation, which is through establishing the FEM fluid model of the resonator and analyzing its acoustic modes using SYSNOISE. And the results show that the emulational resonant frequency and the theoretical resonant frequency are nearly equal. Then the resonator's FEM structural model is established using MSC.NASTRAN, and its structural mode is analyzed at the same time. And also Fluid Structure Interaction (FSI) model is established and its coupling modes are analyzed to get the coupled resonant frequency. Then the effect returned by the coupling effect to the cylindrical cavity's resonant frequency is analyzed. After doing these, the internal sound field in the resonator excited by a sound source is calculated, and several points'acoustic pressure spectrograms are received. This paper also studies different-size-resonators'coupled modes, and researches the influence on the accuracy of acoustic velocity measurement using ARS which is caused by the different-size-resonators, and the results of numerical simulation and discussion which can be used to provide guidance to choose the actual cavity are given. At last, an experimental system to measure acoustic velocity of seawater with the method of ARS is designed. In addition, the paper also establishes a FEM model of the cylindrical resonator which includes a biggish suspended particle, and the result shows that the suspended particles'existence almost has no impact on the resonant frequency.To be concluding, when ARS is used to measure acoustic velocity of sea water, the main error source was the resonator's FSI effect, which is closely related to the resonator's structure, size and material properties. And this study is of great significance to sound velocity measurement.