The end of the Cold War significantly changed the nature of the deep ocean submarine threat where the need for long detection ranges emphasized development of larger, more powerful transducers and arrays operating at lower frequencies. Naval operations in relatively shallow water nearer coastlines are now expected to be more important; although detection ranges will be shorter, interference from surface and bottom reverberation will be more significant. These conditions will require transducers and arrays operating at higher frequencies with wider bandwidth, allowing frequency agility and operation at optimum sub-bands within the broad system bandwidth. Combined operations with networked acoustic communications involving many small directional transducers will also be required. Projector and hydrophone arrays mounted on unmanned undersea vehicles will play an important part in such operations. We believe that meeting these conditions will require new transduction materials, faster and more thorough numerical modeling techniques, as well as increased efforts in several promising areas that are not presently being pursued.

New transduction materials provide the most likely path to improved transducer performance. This is illustrated by the evolution from quartz to the more highly coupled, higher-power piezoelectric ceramic and single-crystal materials, PZT and PMN-PT, as well as the transition from nickel to the more highly coupled magnetostrictive materials, Terfenol-D and Galfenol. These new materials provide greater energy density and higher coupling coefficients, but there is more to be gained from even higher coupling coefficients. A significantly higher effective coupling coefficient, k _e, makes possible some interesting speculation about transducer parameters...