Abstract
An acoustic spectroscopic approach to detect contents within different packaging, with substantially wider applicability than other currently available subsurface spectroscopies, is presented. A frequency-doubled Nd:YAG (neodymium-doped yttrium aluminum garnet) pulsed laser (13 ns pulse length) operated at 1 Hz was used to generate the sound field of a two-component system at a distance of 50 cm. The acoustic emission was captured using a unidirectional microphone and analyzed in the frequency domain. The focused laser pulse hitting the system, with intensity above that necessary to ablate the irradiated surface, transferred an impulsive force which led the structure to vibrate. Acoustic airborne transients were directly radiated by the vibrating elastic structure of the outer component that excited the surrounding air in contact with. However, under boundary conditions, sound field is modulated by the inner component that modified the dynamical integrity of the system. Thus, the resulting frequency spectra are useful indicators of the concealed content that influences the contributions originating from the wall of the container. High-quality acoustic spectra could be recorded from a gas (air), liquid (water), and solid (sand) placed inside opaque chemical-resistant polypropylene and stainless steel sample containers. Discussion about effects of laser excitation energy and sampling position on the acoustic emission events is reported. Acoustic spectroscopy may complement the other subsurface alternative spectroscopies, severely limited by their inherent optical requirements for numerous detection scenarios.
Highlights
The inspection of suspicious objects poses specific, and often, difficult analytical challenges.The use of sensors to operate in real time, in situ, and contactless, but rapidly and with appropriate sensitivity is often necessary in an IED scenario due to the boundary conditions that characterize it
Optical emission spectra were used in a preliminary step for labeling the target scenario into a particular class so to collate the acoustic output from the unknown against a discrete set of acoustic spectra rather than with all the sound data stored in a spectral library
This is a short burst of sinusoidal components that drop exponentially to become depleted after about 3 ms
Summary
The inspection of suspicious objects poses specific, and often, difficult analytical challenges. Obtained from a pulsed laser in the focus of a lens, when the solid is subjected to thermal and/or mechanical changes, the pattern of sound generation may become complex since nonlinear acoustic emissions arise due to the sudden release of thermal energy [37,38,39,40] In any case, these laser-generated acoustic transients have become a promising tool for monitoring and control the ablation process from fundamental properties like the stress power (laser energy coupling to the solid target) [41], the plasma formation mechanism and its evolution dynamics [42,43] and the ablation rate [44,45,46], as well as for standardizing the retrieved analytical signal via normalization of optical emissions [47,48]. The acoustic measurements of target content can aid in the detection, diagnosis, and positive characterization of a threat towards effective making decisions
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