Acoustic modems and models in constrained marine spaces

Abstract

Constrained underwater spaces such as harbors and ports present many interesting acoustic features and challenges. Quays, wharves, seawalls, ships, floats, bridge piers, jetties, are often part of the “scenery.” Passive and active acoustic security systems are often used in these settings. Our focus is on how acoustic communications between system modems are affected by these reflectors. The evidence is mainly anecdotal, but there is a concern that the complexity of the environment can be bad for communications, just as a convention center room can have lousy acoustics. One might think that standard ocean acoustic models would allow one to study this. However, ocean acoustic modeling has been shaped by the needs of open ocean applications. There the standard process has been to assume that the sound launched along a specific azimuth, stays in that vertical plane. This is the so-called Nx2D approximation and not appropriate to model the above mentioned features that knock the sound out of its launch plane. We have been developing a 3D beam tracing model for feature-laden applications and testing it in various sites. Here we discuss the modeling and the measurements in terms of both the fundamental echo structure and the effect on acoustic modems.