Underwater Sound Characteristic Analysis And Design Of The Multi-mechanism Synergistic Composite Acoustic Structure

The underwater anechoic coating is a key technology for submarine stealth,which can simultaneously suppress sound echo and vibration response of the hull.With the development of underwater sonar detection technology,the detection capability has been continuously enhanced,and the frequency band corresponding to sonar detection has also expanded to low frequencies.Traditional underwater acoustic structures are limited by the law of mass density and it is difficult to solve the problem of low-frequency sound absorption.Compared with traditional underwater acoustic structures,small-scale controlling low-frequency sound waves can be achieved by acoustic metamaterials.Due to the inherent properties of the internal local resonance mechanism of acoustic metamaterials,the narrow sound absorption frequency band is a key factor restricting its underwater sound absorption applications.To this end,in order to provide design methods and technical support for the development of underwater low-frequency broadband sound absorption anechoic coating,this paper aims at the shortcomings of traditional acoustic structures and new acoustic structures in underwater sound absorption applications,and the research on underwater sound absorption characteristics and design of multi-mechanism synergistic composite acoustic structures are carries out on the basis of the research on the acoustic characteristics of the existing underwater composite acoustic structure.The research work carried out is as follows:Based on the finite element method(FEM),the acoustic-structure coupling finite element equation of underwater composite acoustic structure is derived,and the multiple coupling boundaries existing in the physical model of the composite acoustic structure are equivalently simplified.The finite element calculation program is compiled to establish the acoustic calculation model of underwater composite acoustic structure.In the numerical calculation,the sound absorbing structure of homogeneous medium,the sound absorbing structure with cavities and the sound absorbing structure with local resonance unit are used as the analysis objects,respectively,and the correctness of the established calculation model is verified.Aiming at the problem that the sound absorption frequency band of the local resonance acoustic structure is too narrow,a novel underwater multi-scale composite acoustic structure is proposed by introducing the multi-layered resonators inside the acoustic structure.Firstly,based on the coupling resonance effect between the multi-layered resonators,the calculation model for the band gap characteristics of the multi-scale composite acoustic structure is established,and the investigation of the influence of the coupling resonance effect of the vibrators on the band gap characteristics of the composite acoustic structure has been carried out.Based on the established acoustical calculation model of underwater composite acoustic structures,the sound absorption characteristics of underwater multi-scale composite acoustic structures has been investigated.The relationship between the band gap of the composite acoustic structure and its sound absorption frequency band is established,and the sound absorption mechanism of the multi-scale composite acoustic structure is revealed.Finally,the effects of the material parameters,structural parameters and the backing form of the multi-scale composite acoustic structure on its sound absorption performance are analyzed.Numerical results show that the coupling resonance effect between the resonators in the multi-scale composite acoustic structure can generate more vibration modes,which can effectively improve the sound absorption performance of the acoustic structure in the high frequency range and broaden the sound absorption frequency band of the acoustic structure.According to the characteristics of functionally graded materials,an underwater functionally graded acoustic anechoic coating with cavities is designed.Through introducing gradient element grids,the acoustic calculation model of the underwater functionally graded acoustic anechoic coating is established by graded finite element method(GFEM),and the feasibility of the method and the correctness of the established acoustic calculation model are verified.Considering the hydrostatic pressure,the acoustic calculation model of functionally graded acoustic structure under pressure is established.Based on the established acoustic calculation model of the functionally graded acoustic anechoic coating,the sound absorption behaviors of material graded acoustic structure,geometrically gradient composite acoustic structure and composite gradient acoustic structure are studied,respectively.In addition,numerical calculations have been conducted to study the influence of the gradient change form,material parameters,structural geometric parameters,and cavity shape on the sound absorption performance of the composite gradient acoustic structure,and explore the influence rules of sound absorption characteristics of the underwater composite gradient acoustic structure.Numerical results show that,the convergence and accuracy of the underwater acoustic calculation model based on the gradient finite element method in this paper is better than that of the traditional finite element method,and the modeling method for hydrostatic pressure in this paper effectively simplifies the modeling process.Besides,the composite gradient acoustic structure designed in this paper can effectively improve the sound absorption performance of the acoustic structure in the high frequency range while maintaining the low-frequency sound absorption performance.A multi-mechanism synergistic composite acoustic structure is designed by combining traditional acoustic structures and acoustic metamaterials.The acoustic calculation model of the underwater multi-mechanism synergistic composite structure is established by using the gradient finite element method,and the sound absorption characteristics of cavity-resonator coupled resonance acoustic structure,functionally gradient-local resonance composite acoustic structure and multi-mechanism synergistic composite structure have been investigated,respectively.In addition,the effects of sound absorption characteristics of multi-mechanism synergistic composite structure are explored from the perspective of material parameters and geometric parameters.Then,the optimization algorithm is employed to optimize the sound absorption performance of multi-mechanism synergistic composite structure.Lastly,the multi-mechanism synergistic composite acoustic structure with multiple lateral plates is designed based on the idea of acoustic metamaterials with lateral plates.The underwater sound absorption characteristics of the multi-mechanism synergistic composite acoustic structure with multiple lateral plates are studied.The influence of each substructure on the acoustic performance of the multi-mechanism synergistic composite acoustic structure with multiple lateral plates is studied.Numerical results show that the synergy between the mechanisms increases the acoustic dissipation modes,which can effectively broaden the sound absorption frequency band of the acoustic structure,thereby achieving underwater low-frequency broadband sound absorption performance.