Electrostatic Transducers for Airborne Ultrasonics.

Available from UMI in association with The British Library. Requires signed TDF. This thesis describes work in which both ultrasonic and optical experimental techniques have been used to study ultrasonic electrostatic transducers designed to operate in air. The results were compared to an analytical model presented in chapter 3. The frequency response of a transducer was found to be dependent on both the gross surface features and the microscopic features of a backplate, such as grooves and the surface roughness respectively. The results obtained show that the resonant frequency of a transducer assembled with a backplate that possessed a roughened surface was determined primarily by the mean depth of the air layer formed between the membrane and the backplate, with a shallow air depth producing a high resonant frequency. In addition, they indicated that for smooth optically flat backplates, the depth of the air layer was governed by the surface of the membrane. A Michelson interferometer was used to determine high resonant frequencies which when measured ultrasonically were found to be attenuated by air. It was found that the intensity of the frequency response of transducers constructed with a roughened surface backplates was governed by both the mean depth of the trapped air layer and the average wavelength of the backplates' surface. A detailed optical investigation of a transducer assembled with a grooved backplate showed that the multi -peaked frequency response observed was a consequence of the annular membranes formed by the transducer membrane above the grooves. The annular membranes were shown to be driven by the action of the membrane above adjacent rails. It is the frequency response of the annular membrane that forms the frequency response of a transducer. However, it was found that the resonant frequency of a transducer could correspond to an overtone of the natural frequency and not the natural frequency of the annular membrane. The frequency which forms the resonant frequency of the transducer is influenced by the ratio of the rail width to groove width. In general, the intensity of the frequency response of transducers was found to be determined by the width of a groove, with a narrower annular membrane producing the more intense response.