| Low-frequency sound transmission of a point source moving underwater is investigated in this thesis, where theoretical formula is derived, and the effects of source speed, liquid’s stratification and the reflective bottom on sound transmission through a gas-liquid interface are discussed based on numerical simulations.The thesis firstly introduces the progress in studies of low-frequency sound transmission, and assorts the researches on sound radiation considering the factors of the layered medium and the source speed. Then the sound transmission from water to air at low frequencies is investigated based on the analytical formula and numerical simulations. Contrary to the conventional viewpoint based on the ray-theoretical predictions, infrasonic energy from localized waterborne sources can be effectively transmitted into air. For shallow sources, an increased power flux into the air due to inhomogeneous waves is accompanied by a decrease in downward acoustic power flux due to the destructive interference of direct and surface-reflected waves under water.Based on the wave-number integration and the Doppler frequency shift, a theoretical approach is then proposed for analyzing the enhanced transmission of a point source moving underwater. It is found that the moving sound source can radiate more acoustic energy into the air than a stationary one while the energy radiated to water remains almost the same. As the source moves faster, the total energy variation becomes sharper and the relative contribution of inhomogeneous waves to sound transmission through the interface becomes larger.Finally a model of low-frequency sound transmission through a stratified liquid is built. Based on the sound radiation of spherical waves in layered medium, factors of the liquid stratification and the reflective bottom are considered. For a monopole source in liquid, almost all emitted energy is radiated into the gas rather than into the liquid, as long as the source depth is sufficiently small. |
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