Wind-generated ambient noise in a shallow brackish water environment in the archipelago of the Gulf of Finland

Abstract

Underwater ambient noise measurements were carried out in a shallow (15-20 m) brackish water in the archipelago of the Gulf of Finland for period of 1 year. The absence of traffic noise made it possible to study wind driven effects in ambient noise at lower frequencies. The ambient noise comes mostly from local sources and the propagation effects are shown to be negligible. The ambient noise develops bubble type spectral features above 100 Hz as wind speed increases. Sharp spectral declines are observed below 500 Hz, which are most likely due to resonances from oscillating bubble clouds created by breaking waves. The low frequency range of the observed declines may partly be attributed to the larger bubble size in fresh and brackish waters. In the present study the wind speed dependence factor was approximately 2.4 at 200 Hz, which is significantly higher than the typical factor of approximately 1.5 for the ocean environment. The average high-frequency spectral slope was -4.9 dB/octave which is approximately 1 dB/octave less than for typical deep water slopes. No significant seasonal effects were found in any parameter calculated from the ambient noise spectra.