Modeling And Analysis Of The Underwater Acoustic Field Excited By Airborne Source

The research on air-to-water sound transmission is essential for designing underwater acoustic system to detect airborne target. This dissertation studies mainly the underwater acoustic modeling methods for airborne source. Then, these models are used to study the underwater acoustic field characteristics. The main contributions are as follows:(1) Ray method is used to model the refracted sound in deep water; in addition the lateral wave introduced by wave method is discussed. Then, the direct wavenumber integration model is deduced. It is much precise, and can be used to obtain a benchmark result.(2) A fast field program (FFP) is developed to calculate the shallow water acoustic field excited by airborne source. And it is extended to handle the elastic bottom environment. The fast field program is also extended to include moving media, and used to analyze the influence of the wind and sound speed profile in air. With these models, the average transmission loss in shallow water, and the influence of the source height together with the lateral wave are studied.(3) A two-dimensional wavenumber integration model is developed for the underwater acoustic field of airborne rapidly moving source. At far field, its one dimensional approximation is deduced. For the case in which a source track is passing over the receiver, a fast field program is developed. The acoustic filed in deep water and shallow water are studied with these models. In deep water, both the amplitude and the frequency of the received signal change with time. The change rate is affected by the receiver depth, the source height and the minimum horizontal range between the source and the receiver. In shallow water, the amplitude of the received signal fluctuates quickly, the Doppler shift is evident, and the spectrum broadening is significant. Furthermore, the different paths may be separated in the time-frequency plane. The Doppler frequency shift of the lateral wave is much larger than the one of the refracted wave, and is consistent with the signal in the air.(4) Two experiments are performed to study the acoustic field in water for the airborne source. The first one is the air-to-water sound transmission experiment, which is carded out in anechoic tank. This experiment confirms the underwater sound characteristic predicted by the theory. The second experiment is performed in Yangling Water Sports Center to simulate the shallow water acoustic field exited by an airborne source. The signal excited by an airborne source is observed at 230m underwater. The measured average transmission loss agree with the theoretical prediction well.