Long-range sound propagation in the central region of the Baltic Sea

Abstract

The data obtained from a set of experiments on the long-range, low-frequency (<5 kHz) sound propagation in the central region of the Baltic Sea are analyzed. The experiments were carried out in the summer season, with a fully developed underwater sound channel. Experimental data on the sound attenuation are presented. A significant excess of the attenuation coefficients over the predicted absorption coefficients is obtained. The quantitative estimates indicate that the sound scattering by internal waves is the most probable mechanism responsible for the observed excessive sound attenuation. The frequency dependence of the attenuation coefficient exhibits a minimum whose position on the frequency axis at the beginning of the summer season noticeably differs from that at the end of summer. The analysis of the propagation conditions allows one to relate the position of this minimum to the critical frequency of the water modes. In addition to the intensity parameters of the sound field, the formation of the time structure of explosion-generated signals propagating in the Baltic underwater sound channel is considered for the case of the sound propagation along the 360-km path crossing the Gotland Hollow. The specific role of the bottom waves in the time structure formation at short distances from the sound source is demonstrated.