Research On Mechanical Self-noise Of Underwater Acoustic Field In Sonar

Sonar is a kind of equipment for target detection, tracking, positioning, navigation and communication in the water. It plays an important role in the ship. If sonar detection ability and the concealment of the ship are improved the risk of accidents will effectively avoid. The ship’s survive ability will be improved. All over the world many scholars have research on sonar detection ability and concealment. Reduce sonar self noise is the key method to improve the performance of sonar. Based on the analysis of factors affecting the characteristics of self noise, some favorable method which reduces the self noise would be put forward. And the performance of sonar will be improved.The self-noise of sonar consist of the hydrodynamic noise, propeller noise, mechanical noise, and ambient noise in the ocean. When the speed of the ship is above 10 knots the main self noise of sonar chamber is dynamic noise. Propeller noise is the main noise sources of ship under water. It influence on sonar self noise intensity, with the speed of 5 ～6 times. In the low frequency and low speed the vibration and noise of marine mechanical equipment is an important component of sonar chamber noise.The main work of this paper is combining sonar chamber structural characteristics with the theory of self-noise prediction then establishing a model of sonar chamber mechanical noise characteristics. Mechanical noise of sonar chamber is analyzed by using the method of modal analysis. By the qualitative simulation, the causes of mechanical noise which influence on sonar self noise are summarized.Firstly, this paper established the analysis model of sonar chamber according to coupling theory. By using the modal method analysis the sound field which in the sonar chamber. The fluid solid coupling equation is established. The solid coupling modal equations is solved which is without absorbing boundary flow. The sound pressure distribution solution is solved when the elastic boundary under external excitation.Secondly, the elastic plate simulates the sonar platform which effected by mechanical vibration. And rectangular cavity simulates sonar chamber. External excitation in the elastic plate, the mechanical noise transmitted to the sonar array through the vibration of elastic plate. By using MATLAB software analyze the noise in the rectangular cavity. The rule of sound field in the sonar chamber is studied by changing the system parameters. For example, internal damping, the cavity size and material, acoustic window size and so on. In order to check the correctness of the theoretical model, using LMS Virtual Lab soft ware study the sound in the rectangular cavity. The basis is provided for reducing and controlling mechanical noise of sonar chamber.Lastly, in order to find a method to reduce the noise of sonar chamber, the sandwich board rectangular cavity model is put forward. Establishing and solving the differential equations of motion of sandwich plate structure. The MATLAB simulation results show that sandwich plate and the rectangular plate with the same thickness the sandwich plate’s noise reduction is better than the rectangular plate’s. And square rectangular sandwich plate is better than rectangular plate. When the plate is a rectangle with the size of the plate becomes larger, the noise reduction effect is better. The damping and the loss factor of sandwich plate also have an effect on the amount of noise reduction. Improving the damping or the loss factor can reduce the noise in the cavity near the resonance point mutation. This study provides a reference to the method of reducing mechanical noise. It has important application value.