Quantitative Air‐Coupled Ultrasonic Materials Characterization with Highly Focussed Acoustic Beams
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Plate‐wave dispersion spectra have been inferred directly in a single air‐coupled coordinate scan using mirror‐focussed acoustic beams on both transmitting and receiving transducers. The spectra of the leaky Lamb modes are extracted through a two‐dimensional DFT of the measured broadband focused air‐coupled signal, as has been demonstrated previously in water. To enhance the typically weak air‐coupled signal, a novel signal coding scheme using a random‐phase constant‐amplitude analog burst 200 μs in length has been employed. Subsequent correlation yields the system impulse response, from which the processed wavenumber‐versus‐frequency plot is obtained by the two‐dimensional DFT. Comparison of measured dispersion spectra with calculated dispersion curves based on nominal stiffness values shows agreement between the measured and calculated data.
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This article appeared in AIP Conference Proceedings, 700 (2004): 1376–1381 and may be found at http://dx.doi.org/10.1063/1.1711776.