Acoustic Parameter Measurement Of Viscoelastic Material At Intermediate And High Frequency

With the development of underwater acoustics, underwater acoustic material is used more and more extensively in military and civil field. Viscoelastic material is one of important materials used in underwater anechoic equipments. It could convert the receiving sound energy into heat energy and decreases the intensity of reflective sound or even eliminates the reflection of target, so it is necessary to obtain the characteristics of the material before structural design is carried. In order to obtain acoustic parameters of viscoelastic material at intermediate and high frequency, a new method is presented in this paper. Under the condition of free field, dynamic acoustic parameters of viscoelastic material are determined by inversion method from the experimental data of target scattering field at intermediate and high frequency.At first, A physical model of underwater sound scattering of elastic target is built, and the analytic solution of the field is derived. Based on the theoretical derivation, scattering field is simulated and analyzed with different size and frequency, and the corresponding coefficients of Legendre expansion of the directional pattern of scattering field are given, and a cost function for parameters inversion is constructed. In parameters inversion, Genetic Algorithm is introduced.In numerical simulation, a type of aluminous sphere is selected as target. The result shows high precision of the inversion by Genetic Algorithm. In the simulation of absorbing rubber spheres with complex modulus, sensitivity of cost function to all parameters is studied, and its analytical method is presented.For the purpose of improving the precision at inversion, several error factors of the measurement are discussed. In the thesis, one method is put forward to calibrate the distance error between sound source, target and hydrophone. The formula for scattering field had also been modified.In experimental research, aluminous sphere is taken as benchmark target and its scattering field is measured. The experimental data agree with theoretical curve of scattering field. With the experimental data, acoustic parameters are determined, and the precision is improved apparently after error correction. A conclusion could be drawn that the measurement method presented in this paper is feasible and the result is reliable.Base on the above analysis, the method is introduced into the parameters measurement of viscoelastic material. Two rubber spheres with different size are used in experimental measurement. With the experimental data, the parameters of material in mediate and high frequency are obtained. As the real values of the parameters of rubber are unknown, comparison between the inversed and the measured scattering field is carried to verify the reliability of inversed result. The variation of dynamic elastic modulus of rubber spheres agrees with the result of dynamic mechanical thermal analyzer (DMTA).The method introduced in this thesis avoids the phase measurement of pressure which is needed by conventional method, so it reduces the experimental difficulty. By this method all material parameters can be determined simultaneously, and the work efficiency is improved, so makes the engineering prediction of material parameters become more conveniently. An unsolved international problem to measure acoustic parameters of the viscoelastic material at intermediate and high frequency is solved in this thesis.