Are the knudsen curves acceptable in the Baltic Sea?

Results of ambient noise measurements performed in the southern part of the Baltic Sea are presented. An autonomous measurement buoy was deployed in two Baltic Deeps under dissimilar sound propagation conditions - at the Bornholm Deep in winter and at the Gdansk Deep in summer. The sound was registered and analyzed in the frequency range from 350 Hz up to 35 kHz. The parameters of dependence of the ambient noise sound pressure level versus wind speed were determined. It was found that the measured ambient noise spectrum level depends not only on wind speed and location but also on a depth of observation. The results show that the wind dependent ambient noise component in the Baltic Sea frequently does not match the Knudsen curves, and we have found that at some specific location sound level at low frequencies does not depend on wind. The collected data on the noise levels are discussed with other historical and contemporary data on the noise level in the Baltic Sea.