| Summary: | The generation of ambient noise by physical processes dependent on shearing of the boundary layer under sea ice is investigated. Special attention is paid to the identification of individual noise-generating mechanisms and the assessment of their relative importance. Recent studies of Arctic ambient noise are reviewed with specific reference to results showing particularly good or poor correlation between ambient noise levels and ice movement or relative current. Potential noise-generating mechanisms are described and categorized according to their small-scale driving forces and expected noise characteristics. More detailed quasi-objective investigations are then used to establish the relative importance of each mechanism as a contributor to the overall under-ice noise spectrum. Flow/Mechanical mechanisms, involving ice sheet fracture as a result of wind and current-induced bending moments, are found to be unlikely contributors. Conversely, processes in which ice fragments in current-driven motion under the ice interact to cause bumping and grinding noises, appear to be of probable importance. Turbulent pressure fluctuations in the boundary layer under sea ice are shown to be of significance at low frequencies on a local scale. The role of resonant cavities in the under-surface of the ice does not appear, however, to be an important one. http://archive.org/details/ambientnoiseduet1094522869 Lieutenant, Royal Navy Approved for public release; distribution is unlimited.
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