Applied Underwater Acoustics

2.6. The “Ideal” Waveguide

In shallow water, the acoustic field is trapped between the sea surface and the sea floor, although partial penetration into the seabed may occur, depending on the nature of the bottom boundary conditions. As a first approximation, the two bounding surfaces may be treated as plane, horizontal boundaries, with the sea surface acting as a pressure-release boundary for sound incident from below. This pressure-release condition is a consequence of the 1000:1 density ratio between seawater and air; and the planar representation of the sea surface is reasonable, at least for frequencies up to several hundred hertz, where acoustic scattering from surface roughness is negligible. The bottom boundary is generally more complicated ...