Reeder D Benjamin, Joseph John E, Rago Thomas A, Bullard Jeremy M, Honegger David, Haller Merrick C
Department of Oceanography, Naval Postgraduate School, 1 University Circle, Monterey, California 93943, USA.
College of Engineering, School of Civil and Construction Engineering, Oregon State University, 1491 SW Campus Way, Corvallis, Oregon 97331, USA.
J Acoust Soc Am. 2022 Apr;151(4):2429. doi: 10.1121/10.0009923.
Estuaries constitute a unique waveguide for acoustic propagation. The spatiotemporally varying three-dimensional front between the seawater and the outflowing freshwater during both flood and ebb constitutes an interfacial sound speed gradient capable of supporting significant vertical and horizontal acoustic refraction. The collision of these two water masses often produces breaking waves, injecting air bubbles into the water column; the negative vertical velocities of the denser saltwater often subduct bubbles to the bottom of these shallow waveguides, filling the water column with a bubbly mixture possessing a significantly lower effective sound speed. A field experiment was carried out in the mouth of Mobile Bay, Alabama in June 2021 to characterize estuarine bubble clouds in terms of their depth-dependent plume structure, frequency-dependent sound speed and attenuation, bubble size distribution, bubble number density, and void fraction. Results demonstrate that sound speed in the bubbly liquid consistently falls below the intrinsic sound speed of bubble-free water; specifically, the bubbly liquid 1.3 m below the surface in a front in this environment possesses effective sound speeds, void fractions, and bubble number densities of approximately 750 m/s, 0.001%, and 2 × 10 bubbles/m, respectively.
河口构成了一个独特的声学传播波导。在涨潮和落潮期间，海水与流出的淡水之间时空变化的三维前沿构成了一个界面声速梯度，能够支持显著的垂直和水平声折射。这两种水体的碰撞常常产生破碎波，将气泡注入水柱；密度较大的咸水的负垂直速度常常将气泡带到这些浅波导的底部，使水柱充满有效声速显著较低的气泡混合物。2021年6月在阿拉巴马州莫比尔湾河口进行了一项现场实验，以根据河口气泡云随深度变化的羽状结构、随频率变化的声速和衰减、气泡大小分布、气泡数密度和空隙率对其进行表征。结果表明，气泡液体中的声速始终低于无气泡水的固有声速；具体而言，在该环境中前沿表面以下1.3米处的气泡液体的有效声速、空隙率和气泡数密度分别约为750米/秒、0.001%和2×10个气泡/立方米。
