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Sensors 2017, 17(12), 2960; doi:10.3390/s17122960

Remotely Exploring Deeper-Into-Matter by Non-Contact Detection of Audible Transients Excited by Laser Radiation

UMALaserlab, Departamento de Química Analítica, Facultad de Ciencias, Universidad de Málaga, Jiménez Fraud 4th, 29010 Málaga, Spain
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Received: 8 November 2017 / Revised: 13 December 2017 / Accepted: 16 December 2017 / Published: 20 December 2017
(This article belongs to the Special Issue Spectroscopy Based Sensors)
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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. View Full-Text
Keywords: laser-based sensor; acoustic emission; subsurface spectroscopy; concealed materials laser-based sensor; acoustic emission; subsurface spectroscopy; concealed materials
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Moros, J.; Gaona, I.; Laserna, J.J. Remotely Exploring Deeper-Into-Matter by Non-Contact Detection of Audible Transients Excited by Laser Radiation. Sensors 2017, 17, 2960.

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