3D acoustic propagation through an estuarine salt wedge at low-to-mid-frequencies: Modeling and measurement

Three-dimensional anthropogenic underwater noise modeling in an Arctic fjord for acoustic risk assessment

Sea-ice covering is drastically declining in the Arctic, opening new maritime routes and thus introducing underwater noise pollution in nearly pristine acoustic environments. Evaluating underwater noise pollution requires good acoustic propagation modeling to predict sound exposure levels. However, underwater noise modeling for acoustic risk assessments has often been carried out using simplistic propagation models, which approximate a 3D propagation in several planes (Nx2D), instead of using full 3D propagation models. However, Nx2D propagation models are impractical for winding geography and steep bathymetry as found in Arctic fjords. The purpose of this study is to estimate disturbance and masking effects on Arctic animal species from shipping noises, modeled through a traditional Nx2D BELLHOP model and a full 3D BELLHOP model. Classical Nx2D propagation modeling largely underestimates the anthropogenic noise footprint in Arctic fjords compared to using a full 3D propagation model.

Acoustic spectrometry of bubbles in an estuarine front: Sound speed dispersion, void fraction, and bubble density

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 × 10⁶ bubbles/m³, respectively.