Triply resonant broadband transducers

The Navy\´s current emphasis on broadband sonar signal processing and frequency agility has prompted the need for new classes of broadband sonar transducers that can transmit these complex signals. The design approaches used are based on the longitudinal vibrator tonpilz type double resonant sonar transducer development that began with the "Rodrigo" type design (named after the pioneer of such design) also commonly referred to as "double head mass" transducer by the British naval community. This design is a series mechanical arrangement of an inactive compliant material such as aluminum or composite polymer material that is sandwiched between a central mass and head mass (radiating piston) and active driver material such as piezoelectric ceramic stack or magnetostrictive rod sandwiched between the central mass and a tail mass. This method creates a double resonant three-degree-of-freedom (i.e. mass-spring-mass-spring-mass system) transducer, which offers greater bandwidth than conventional tonpilz transducer designs. These designs can be expanded by the addition of other series resonators (mass-spring) to produce triple resonant transducer (TRT) or higher order multiple resonant transducer (MRT) devices. Several prototype transducer elements designs were fabricated at the Naval Undersea Warfare Center that demonstrated proof of concept by generating three resonances at 15, 25 and 35 kHz. The theory of operation, modeling approaches, fabrication technique and test results will be presented.