Seasonal and diel variability of the underwater noise in the Baltic Sea

The results of ambient seanoise measurements performed in strongly stratified shallow basins in the southern Baltic Sea are presented. The experiments were conducted with an autonomous hydroacoustic buoy that was equipped with two omnidirectional hydrophones located inside and outside the seasonal waveguide. Data were collected in two areas with contrastingly diverse sound propagation conditions—at the Bornholm Deep in winter and at the Gdansk Deep in summer. The noise spectrum levels indicate a strong dependence on the sound propagation conditions (season, location) and show significant differences with the observation depth. At frequencies f noise at the same wind speeds is approximately 10–15 dB higher in winter than in summer. The wind-driven component of the noise frequently does not match the patterns of the Knudsen curves. Furthermore, the noise demonstrates the diel variability. The relative differences of the noise spectrum levels inside and outside of the subsurface winter waveguide reveal regular changes by as much as 15 dB, and are the most visible in the frequency range of 1–2 kHz after sunset. The changes are synchronized with the vertical migration of marine organisms and correspond to the resonant frequencies of the fishes' swim bladders.

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