A new underwater transducer design technique is described that provides for ultrabroad-bandwidth directive arrays that are lightweight and thin (for easy towing). An example of this new technique is a planar array that is 1 m high, 0.75 m wide, and 0.15 m thick. This array has greater than 60% radiation efficiency throughout the frequency band extending from 400 Hz to 4 kHz, and a nominal power output within this band of 10 kW. The array is neutrally buoyant, and uses Navy type-III PZT actuators. The broadband goal is met by designing the transducers in such a way that their own stiffness reactances are mostly canceled by their masslike radiation reactances over the entire operating band. This feat is accomplished both by properly adjusting the ratio of actuator cross-sectional area to radiation area, and by using low dynamic transducer mass. Since the principle of operation requires for each transducer that its motion be influenced by its own radiation load, it is necessary to control each transducer's motion so that the desired radiation pattern is achieved. This is done by a filtered-X feedforward LMS adaptive controller.