Abstract:
Many designs for constant beamwidth transducers have appeared over the years, but these usually involve complicated array shading techniques or ``lenses'' applied to ultrasonic devices. With locally reacting materials, such as PVDF for example, it is possible to manufacture a composite material which automatically produces the appropriate frequency-dependent shading of the response in a single, continuous transducer. The material makes use of the capacitive (dielectric) nature of the active element in conjunction with a specially designed, electrically resistive layer, to which a single point connection is made. The resulting CR circuit acts as an integrator, and since the resistance to the remote parts of the transducer is largest, the response of those parts progressively reduces as frequency is increased. In other words, the effective size of the transducer reduces (i.e., ``shrinks'') as frequency is increased. Correct design of the composite provides a constant directivity characteristic. Several applications of this ``shrinker technology'' are discussed, including microphones, loudspeakers, and hydrophones.