Abstract
A novel technique for classifying fluids in sealed, metal containers at large stand-off distances has been developed. It utilizes a recently constructed air-coupled acoustic array made from inexpensive, commercial-off-the-shelf components to excite the resonance vibrations of fluid-filled vessels. The sound field from the array is constructed by transmitting a high-frequency modulated carrier wave which utilizes the nonlinearity in the air medium to demodulate the carrier frequency along its propagation path in air. The array has a narrow beamwidth and an operating bandwidth of greater than 25 kHz. The vibrations are detected using a laser vibrometer in a monostatic configuration with the acoustic source. It is shown that the propagation characteristics of the ao Lamb wave are highly affected by different interior loading conditions on the interior wall of a cylindrical container. Classification of the interior fluid is obtained by analyzing the change of this response as a function of frequency. Experiments demonstrate that classification of the fluid-filler inside closed, steel vessels is possible with incident sound pressure levels of the demodulated wave as low as 80 dB at the container location. Preliminary experiments demonstrate that stand-off distances greater than 3 m are achievable for classification purposes.
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