An efficient way to propagate energy from one medium to another is with sound. Acoustic transducers provide a means for redirecting energy over long distances. This energy redirection results in instrumentation that provides information for many everyday needs. The operating frequency ranges (and corresponding wavelength sizes) of the various applications provided by acoustic transducers serve to dictate the preferred transduction means. To couple transformations of energy from electrical to mechanical and then to acoustical (and vice versa), it is necessary to use active materials capable of transferring energy from one type to another. A common excitation mechanism is based on piezoelectric ceramics. Acoustic transmitters (projectors) operate by applying an electric field to the active materials within the transducer device to produce mechanical movement within the device such that an acoustic pressure (sound) is output from the transducer. The converse is also true; that is, a sound impinging upon an acoustic receiver (a microphone for in-air detection or a hydrophone for underwater use) mechanically excites the active material in the transducer so that an electrical signal is generated and may be analyzed in terms of a local sound field. The active materials that convert electrical signals to mechanical vibrations and then to sound are the focus of most transducer research, which centers on developing such materials to achieve specific performance gains. Thus, the design of many transducers begins with the tailoring of the active material component with regard to its intended applications. Among the active materials are piezoceramics, piezocomposites, electrostrictive relaxors, magnetostrictive alloys, and single crystals.