Researches On Acoustic Focusing And Acoustic Covering In Multi-path Channel Of Shallow Water

One of the inevitable tendencies of sonar technology is the tight combination of physics of underwater acoustics, signal processing and ocean environment. The basic principles of the acoustic focusing and the acoustic covering, based on the coherence of the acoustic field, is proposed. The acoustic focusing is a spatial filtering technology, which enhances the array response to the local area of the targets, while the acoustic covering is an opposite concept, which reduces the array response to the local area of the noise, but both of which can be realized with time reversal mirror (TRM).There are two focusing mechanisms in the acoustic field, array focusing and multi-path focusing. Time reversal array focusing and focused beamforming can achieve the same effect, although they are different in the method. The acoustic image measurement technology utilizing focusing mechanisms can achieve high precision positioning for targets in the near field and get the distribution image of ship's radiated noise sources, the latter is helpful for developing quiet submarine, assessment of the noise reduction effect, diagnosing the malfunction of the ship and directing the advanced passive acoustic homing torpedo to attack the vital parts of the targets. The virtual time reversal mirror technology is adopted to suppress the multi-path interference of underwater acoustic channel, which may affect the quality of the source space distribution image measurement. Simulation and sea trial researches show that the acoustic image measurement technology is feasible and valid. The sea trials researches show that the general ship can be seen single highlight in near field in the high-frequency band, but the catamaran has double highlights when its two propellers work at the same time, in other words, the radiation part of highlight in the high-frequency band is corresponding to the propeller.Acoustic covering technology in multi-path channel is proposed for the sake of distinguishing two targets in the same bearing, which is impossible for the traditional technology of hydrophone array processing, and solving the problem that the targets under the heavy interference can't be detected only adopting the acoustic focusing technology. The signals received by every element, which contain the noise of undesirable targets and the signal transmitted by the targets in the same bearing, are weighted in the frequency domain according to their different channels, in this way, the focusing on the target and the covering to the noise can be achieved simultaneously. Coherent interference in a selected covering area can be suppressed, so the detection performance is improved to the targets outside the area. When the target is big in the size and in very near field, it can't be modeled as point source, so two methods for broadening the covering area, matrix extending method and recursive method, are discussed. The former has a brief expression, but the computational complexity of the latter is lower.The effect factors of the acoustic focusing and the acoustic covering technology in the multi-path acoustic channel are analyzed by computer simulation and water-tank experimental researches, which also validate the technology's correctness, feasibility and practicability.