Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
Figure 4.8. Geometry for Air-to-Water Sound Transmission.
surface, by non-acoustic (hydrostatic)" pressure transmission from the surface
and from bottom reflections in shallow water.
As a result, most of the acoustic energy transmitted into the water from
a source in air arrives through a cone with a 26 degree apex angle which
intersects the surface and traces a "footprint" directly beneath the path of
the source.
For underwater observation points in shallow water within this cone the
directly transmitted sound energy is generally greater than the energy
contribution from bottom reflected paths. At horizontal distances greater
than 1 water depth from the boundary of the acoustic intercept cone on the
surface, the energy transmitted by reflected paths becomes dominant and is an
important feature of air-to-water transmission in shallow water. Thus two
terms become necessary in the air - water transmission model to predict
underwater received levels for the full range of expected source - receiver
*This has been called "evanescent wave" transmission by Urick and others. It
is important for transmission at low frequencies to receiver locations near
the surface.