Attenuation of sound in shallow-water areas with gas-saturated bottoms

We investigated the specific features low-frequency (50–300 Hz) sound propagation in shallow-water areas to relatively small distances r ≈ 3 H –50 H from the sound source, where H is the waveguide depth. The bottoms of water areas were assumed to be fluid homogeneous gas-containing media. Situations were compared in which the sound velocity in the bottom is higher and lower than in the water layer (hard and soft bottom). It was confirmed in experiment that the average effective sound velocity in the bottom may have rather low values (≈100 m/s). The mode description of the acoustic field was used in calculations, and both propagating and outgoing modes, including quasi-modes, were taken into account. The averaged dependences of the field intensity decay on distance were obtained for different frequencies and sound velocities in the bottom. The sound damping factors β in the waveguide were found as functions of frequency and sound velocity in the bottom. It is shown that for a soft bottom, the value of β monotonically increases with an increase in the sound velocity in the bottom, while for a hard bottom, β monotonically decreases. The maximum of β depends on the sound frequency and is reached at the approximate equality of the sound velocities in the bottom and water.