A technique is described for utilizing two-dimensional ultrasonic transducer arrays to accurately measure the spacing (i.e, the gap profile) between large-area mating surfaces, The acoustic measurement theory for pulse-echo and pitch-catch gap profile measurements is derived from a generalized KLM model representation of ultrasonic transducers located within the gap between two mating structures. Expressions for the ultrasonically measured gap spacing are derived in terms of the thicknesses and longitudinal acoustic velocities of the constituent layers of the transducer element(s), adhesive layers(s), and gap medium, Extremely thin PVDF two dimensional arrays were fabricated and adhered to the inside of a precision gap profile device for demonstrating the measurement technique. Two-dimensional gap profile measurements were made to +/- 0.001'' accuracy for a variety of gap profiles in the range of 0 -0.200'' thickness. Gap profile measurements were made with both liquid and air as the gap medium and the results show potential for measuring otherwise inaccessible gap profiles between structures.
