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Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Ari Poikonen Name of the doctoral dissertation Measurements, analysis and modeling of wind-driven ambient noise in shallow brack-ish water Publisher School of Electrical Engineering Unit Department of Signal Processing and Acoustics Series Aalto University publication series DOCTORAL DISSERTATIONS 18/2012 Field of research Underwater acoustics Manuscript submitted 30 May 2011 Manuscript revised 21 October 2011 Date of the defence 9 March 2012 Language English Monograph Article dissertation (summary + original articles) Abstract Underwater ambient noise measurements were conducted in shallow brackish water environments in the Gulf of Finland over a frequency range from 20 Hz to 70 kHz. The ambient noise levels are in fairly good agreement with average oceanic deep water levels for the highest wind speeds but the wind speed dependences differ markedly from each other. In shallow brackish water the wind speed dependence factor at 200 Hz is ~ 2.4 which is significantly higher than the typical factor of ~ 1.5 for the ocean environment. The high-frequency behavior of the spectra was resolved by modeling dispersion and noise in bubbly water. Absorption in brackish and fresh water, unlike in ocean water, tends to decrease above a frequency of 10 kHz due to the low proportion of small bubbles in bubbly mixtures created by breaking waves. The excess high-frequency attenuation in spectra above 10 kHz cannot therefore be attributed to the effects of absorption in a bubbly mixture. Bubble size distributions fitted to the brackish water spectra exhibit a distinctive maximum in the radius range 0.1 0.3 mm, and a substantial drop in bubble density below a radius of 0.1 mm. The brackish water bubble size distributions were tied to an oceanic spectrum with a spectral slope of 5.7 dB/octave obtained with a -3/2 power law dependence of bubble size density on radius. The measurements were carried out in all four seasons of the year but no significant seasonal effects were found in any parameter calculated from the spectra. This is probably due to the dominance of near field conditions in the measurements, where the role of long-range propagation effects was negligible. One should, however, be careful not to generalize the present brackish water results too much due to the inherent complexity of the coastal situation. Bubble densities are known to have considerable spatial variability depending on seasonal, biological, and even weather conditions.Underwater ambient noise measurements were conducted in shallow brackish water environments in the Gulf of Finland over a frequency range from 20 Hz to 70 kHz. The ambient noise levels are in fairly good agreement with average oceanic deep water levels for the highest wind speeds but the wind speed dependences differ markedly from each other. In shallow brackish water the wind speed dependence factor at 200 Hz is ~ 2.4 which is significantly higher than the typical factor of ~ 1.5 for the ocean environment. The high-frequency behavior of the spectra was resolved by modeling dispersion and noise in bubbly water. Absorption in brackish and fresh water, unlike in ocean water, tends to decrease above a frequency of 10 kHz due to the low proportion of small bubbles in bubbly mixtures created by breaking waves. The excess high-frequency attenuation in spectra above 10 kHz cannot therefore be attributed to the effects of absorption in a bubbly mixture. Bubble size distributions fitted to the brackish water spectra exhibit a distinctive maximum in the radius range 0.1 0.3 mm, and a substantial drop in bubble density below a radius of 0.1 mm. The brackish water bubble size distributions were tied to an oceanic spectrum with a spectral slope of 5.7 dB/octave obtained with a -3/2 power law dependence of bubble size density on radius. The measurements were carried out in all four seasons of the year but no significant seasonal effects were found in any parameter calculated from the spectra. This is probably due to the dominance of near field conditions in the measurements, where the role of long-range propagation effects was negligible. One should, however, be careful not to generalize the present brackish water results too much due to the inherent complexity of the coastal situation. Bubble densities are known to have considerable spatial variability depending on seasonal, biological, and even weather conditions.