Sea surface scattering by wind-generated waves and bubbles is regarded to be the main non-platform-related cause of the time variability of shallow acoustic communication channels. Simulations for predicting the quality of acoustic communication links in such channels thus require adequate modelling of these dynamic sea-surface effects. It is known that, for frequencies in the range 1-4 kHz, the effect of bubbles on sea surface reflection loss is mainly due to refraction, which can be modelled with a modified sound-speed profile accounting for the bubble void fraction in the surface layer (Hall-Novarini model). The upward refraction induced by the bubble cloud then effectively acts as a catalyst for increasing the rough-surface scattering. In the present work, it is shown that, for frequencies in the range 4-8 kHz, bubble extinction also provides a significant contribution to the surface loss, including both the effects of bubble scattering and absorption. As this is the frequency band adopted in the European Defence Agency (EDA) project RACUN (Robust Acoustic Communication in Underwater Networks [1]), in which the reported research has been conducted, both bubble refraction and extinction effects should be modelled for acoustic channel simulations in RACUN. These model-based channel simulations will be performed by applying a Gaussian-beam ray-tracer (BELLHOP), together with a toolbox for generation of realistic rough sea surfaces based on both fully-developed ocean and short-fetch North Sea wave-height spectra and angular spreading functions (WAFO).
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