Measuring the transmission coefficient of a steel plate using an angular spectrum approach

Abstract

Ultrasonic measurements were performed on a thin steel plate submerged in water in order to obtain the plane-wave transmission coefficient as a function of angle and frequency. An angular spectrum approach was utilized wherein the acoustic field parallel to the plate was scanned with sufficient spatial accuracy to obtain a frequency-wavenumber description of the transmitted field, representing an expansion of the field into time-harmonic plane waves propagating within a range of angles. Isotropy of the plate material and symmetry of the measurement configuration allowed for a vast reduction of the number of scan points required in order to obtain the transmitted angular spectrum. Once the transmitted acoustic data is acquired, the specific method of windowing the data in space and time can have a drastic effect on the resulting angular spectrum. With sufficient care, a broadband estimation of the transmission coefficient with a high angular resolution is obtained. Finally, inverse methods were utilized in order to estimate the thickness and material properties of the plate using the measured transmission coefficient data. [Work supported by ONR.]