Apparatus and method for remote, noninvasive characterization of structures and fluids inside containers

摘要

An apparatus and method for the remote, noncontact evaluation of structures and containers at large distances (on the order of several meters) in air is described. The invention utilizes an air-coupled, parametric acoustic array to excite resonance vibrations of elastic, fluid-filled vessels and structural members. A nonlinear mixing process in the air medium transforms highly directional, narrow beamwidth higher acoustic frequencies into lower acoustic frequencies suitable for vibrational excitation of common structures. The parametric array also has an advantage for nondestructive evaluation applications in that it is capable of producing a broader bandwidth than typical linear devices, such as speakers and ultrasonic transducers. Typical carrier frequencies are in the 200 kHz range, and the sound field 3 m from the array has been demonstrated to have a bandwidth of greater than 25 kHz at a center frequency of 15 kHz. Vibrations have been excited in a fluid-filled, steel container at distances greater than 3 m from the array which are readily detected using a laser vibrometer in a fixed position relative to the acoustic array. It is demonstrated that the fluid contained within the steel vessel may be classified by analyzing the change in the response of the generated lowest-order, antisymmetric Lamb wave to changes in the interior fluid loading.