Capacitance devices for the generation of airborne ultrasonic fields

The beam profiles of conventional capacitive air transducers are presented at frequencies up to 1.5 MHz and are shown to be highly non-uniform, as is the case in water. In attempting to remedy this problem, a new capacitive air transducer is fabricated using a micromachined silicon backplate and a commercially available thin metallized Kapton film. The results of a characterization of this new device are presented. They indicate a transducer which is wideband, reversible, and possibly more sensitive in generation and detection than a typical lead zirconate titanate (PZT) air-coupled transducer. The usefulness of this new device as an effective noncontact transducer is demonstrated through the successful detection of airborne ultrasonic waves generated by a Lamb mode propagating in a carbon fiber reinforced polymer plate, and by surface breaking bulk waves propagating in brass