Adaptive Control Method For The Low Frequency Radiated Sound Field Of Elastic Structure Near Water Surface

The low frequency radiated noise of underwater vehicle is the key feature to expose itself,so the control of its low frequency radiated sound field is very necessary.Noise control technology is divided into passive noise control technology and active noise control technology.Passive noise control technology has been able to reduce the radiation noise of underwater structures to a certain level,but it is still difficult to suppress the specific low frequency line spectrum noise.In order to solve this problem,the active control technology of target noise suppression based on the principle of acoustic interference cancellation has been paid more and more attention.General active noise control methods choose the sum of squares of sound pressure at one or more points in the sound field as the control target,but for complex underwater elastic structures,the radiated sound power of the whole system will not necessarily decrease,and sometimes may increase.For underwater moving targets,secondary sound sources and error sensors need to be arranged according to the structure of the target itself.How to use the control system which has been placed on the surface to realize sound pressure control in the far field is a major problem.In addition,when the operating condition of underwater vehicle changes,its mechanical noise frequency will also change.If the underwater vehicle approaches the water surface,the water surface will also have an impact on the characteristics of its radiated sound field.Therefore,when the underwater vehicle changes the working condition and depth,whether the placement position of secondary sound source and error sensor needs to be optimized again is a problem that this thesis focuses on and needs to solve.Aiming at the control problem of low-frequency radiated noise of underwater structures near-surface,this thesis carries out theoretical research,numerical analysis and experimental research on the adaptive control method for underwater low-frequency noise based on sound pressure and normal acoustic energy flow.Firstly,a large-scale single-layer shell structure is equivalent to a monopole sound source with directivity under the condition of full space accoding to its far-field sound pressuredistribution characteristics.The optimal configuration of secondary sound sources and error sensors is carried out based on the adaptive active noise control method of sound pressure method and sound energy flow method respectively.Secondly,the equivalent monopole sound source is placed in half space with different depths from the water surface to observe its far-field sound pressure level distribution.Meanwhile,the secondary sound sources and error sensors are optimized by using adaptive noise active control method based on normal sound energy flow.Thirdly,the finite element numerical calculation of the surface acoustic and vibration characteristics and radiation sound fields both in free field and near water have been carried out for the large scale single-layer shell structure model.And the control effects of different secondary sound sources and error points on the radiation noise are explored.The simulation results show that both sound pressure method and energy flow method can achieve certain noise reduction effects in full space or in half space.From the far field noise reduction,the energy flow method is better than the sound pressure method.When the same secondary sources and error sensors configuration is adopted,the water surface has a slight influence on the noise reduction,but the overall noise reduction law is roughly the same as the whole space.Therefore,it is not necessary to adjust the positions of secondary sources and error sensors when the underwater structure changes the water depth.The results show that the adaptive control method based on normal acoustic energy flow has better noise reduction effect at the error point and far field observation surface Finally,the large-scale monolayer shell structure is used to carry out the original rationality of the active control method.The experimental results show that the active control method based on normal acoustic energy flow has better noise reduction at error points and far field observation planes,which provides a new idea for the far field radiation control of underwater moving targets.