Transmission of Sound in the Southern Baltic in June

In summer in the southern part of the Baltic a well defined sound channel exists caused by a thermal gradient in fresh run off water overlying relatively warm saline bottom water from the North Sea. We have analyzed acoustic transmission loss at 100 Hz, 3.5 kHz and 8.0 kHz and dispersion over a range of 20 Hz to 4 kHz. Measurements in three different areas indicate channeling of the sound, restricted closer to the channel axis at the two higher frequencies, and the transmission of several normal modes with unusual dispersion curves related to the shape of the sound channel. For source and receiver on the axis of the sound channel, transmission loss at 10 km and 3.5 kHz is generally about 40 db less than that where neither source nor receiver is near the axis. (Author).