Study Of Theory And Experiment Of Underwater Acoustic Field Excited By A Helicopter

It is an exigent task to study the acoustic field excited by an airborne source, for Anti-submarine Helicopter is treated as the fatal threats to submarine in shallow water. Ray and normal mode theory are mainly employed to research the characteristics of acoustic field excited by an airborne moving source which dates to 1950s.Ray theory is primarily adopted to explain the detecting range that is nearly 300-500m in published articles. But boundary effects (surface and bottom) shouldn't be neglected. The shallow water should be treated as waveguide. And wave theory is more adorable since low line spectrum and continuous spectrum are the main components of the noise radiated by a helicopter. It is barely published to treat the air, water and seabed as three layer medium and study the influence of air sound speed profile to the underwater acoustic field excited by an airborne source. The vector field has seldom been paid attention to, while researchers always focus on pressure field.According to research status of theory and experiment to underwater detecting helicopter, the following points are mainly studied in this paper:Firstly sea experiment of detecting helicopter in shallow water was carried out and the receiver was a suspending vector hydrophone. The results shows that the detecting range at least reaches to 7km which is obviously different from the 300-500m detecting range in published articles and the performance of vertical vector is better than pressure. For anther point, two kinds of Doppler frequency wave can be received which relate to the air sound speed and water sound speed due to the helicopter's moving.In the theory research domain, the atmosphere, shallow water and seabed are modeled as three layer medium that the sound speed of air layer is assumed as Epstein distribution while the water column and seabed layer are both assumed as uniform soud speed distribution. Pressure and vector field solution are both derived under the assumed model. And "water wave" and "water surface wave" which can travel in long range exist in the underwater field excited by the airborne moving source. Water wave is normal mode wave whose eigenvalue is real while absorption of sea and seabed are negalected. The of amplitude Water surface wave decays in exponential versus depth and the sound speed of water surface wave is between atmosphere speed above the sea surface and at high altitude. The water surface wave can travel in long range which is different from lateral wave which only exists near the source. Water surface wave is affected by atmosphere sound speed distribution badly while the water wave not. A helicopter can be monitored at shallower depth in long range and the performance of vertical vector is better than pressure when the atmosphere column is Epstein waveguide.In numerical calculation domain, fast field program (FFP) and normal mode are both given to compute the field excited by a static airborne source. The effects of sound speed profile and parameters of the airborne source are analyzed and the calculation results are presented in the paper. For a moving airborne source, the effects to the number of normal mode and effective depth are firstly analyzed and then one dimensional wave number integral are employed to calculate the waveform in time domain excited by the airborne moving source. And the LOFAR maps are presented in the paper at different depth which verify the conclusion that two different Doppler frequency wave exist, water wave and water surface wave. The helicopter can be monitored at shallower depth in long range and the performance of vertical vector is better than pressure when the atmosphere column is Epstein waveguide.The range of underwater detecting a helicopter is much further than several hundred meters which is proved by the result of sea experiment data analyse, theory under three layer medium model and numerical value computation. These contains of above, in the thesis, added up to improve the ability for the submarine and anti-submaine warefare.