The ef?cacy of sound to penetrate the seas made acoustic systems in the past c- tury the leading tools for sensing objects in and measuring properties of the seas. For over sixty years The United States Of?ce of Naval Research (ONR) has been a major sponsor of undersea research and development at universities, national laboratories, and industrial organizations. Appropriately ONR is the sponsor of this monograph series. The intent of the series is to summarize recent accomplishments in, and to outline perspectives for, underwater acoustics in speci?c ?elds of research. The general ?eld has escalated in importance and spread broadly with richness and depth of understanding. It has also, quite naturally, become more specialized. The goal of this series is to present monographs that critically review both past and recent accomplishments in order to address the shortcomings in present und- standing. In this way, these works will bridge the gaps in understanding among the specialists and favorably color the direction of new research and development. Each monograph is intended to be a stand-alone advanced contribution to the ?eld. We trust that the reader will also ?nd that each is a critical introduction to related specialized topics of interest as well.