Enhancement techniques in transmitting and receiving underwater acoustic modes

Modes, or eigenfunction solutions to the scalar wave equation, have been useful in understanding acoustic interference phenomena observed in shallow water. The premise that one can enhance modes compatible with propagation conditions to suit one´s purpose is the basis for current investigation in shallow-water acoustic propagation at the Naval Underwater Systems Center. Mode Enhancement Techniques (METS) for transmission and reception of acoustic signals are described in this paper. Also described is the use of a vertical array of transducers, a novel experimental tool employed in developing METS. During reception, the individual signals can be weighted and summed to reject unwanted modes. During transmission, the signal amplitude and phase distribution over the array can be varied to manipulate and enhance particular modes. In addition,factors affecting systems performance, such as reverberation and ambient noise,are viewed in terms of mode rejection.