Low frequency sound propagation across a land–water boundary

Abstract

Army noise assessment capability has recently been improved to account for the effects of land–water transitions on a received sound level. Sound waves propagating beyond a shoreline experience changes in surface impedance, surface roughness, and refraction profile. Measurements of the temperature and wind with tethersonde balloons indicate significantly weaker sound speed gradients over water than over land. When a temperature inversion was observed over land, the inversion was always smaller over the water. When the solar heating produced strong upward refraction near the land surface, the temperature profile over water was neutral (lapse). Simulations were run with a parabolic equation model incorporating range-dependent boundary refraction profiles and range-dependent boundary conditions. The signal levels exhibit changes with distance due to strong refraction over land, but the changes nearly cease in places where the propagation extends over water. Measured spectrum levels from explosion sound propagating across land–water boundaries agree well with the simulations. [Work supported by the USACERL, USACHPPM, and NAVFAC.]