Declercq's research delves into the fundamental
physics of wave interactions with various materials, specifically within
engineering. His work explores these interactions and their applications in
technological advancements.
Acoustics in Archaeology His research in
archaeoacoustics has contributed to new insights into how ancient civilizations engineered architectural spaces to manipulate sound for religious and ceremonial purposes. His interdisciplinary approach, merging
acoustics and
archaeology, provides a fresh perspective on acoustics in ancient societies. Declercq's investigations revised earlier understandings about the
Greek theater of Epidaurus, renowned for its exceptional acoustics. He demonstrated that the
limestone seats act as an acoustic filter, enhancing high-frequency sounds, such as speech, while suppressing low-frequency noise. His findings, published in
Nature, revealed that the theater's design amplifies critical sound frequencies, enabling clear audibility even at distant seating. This research improved our understanding of
Ancient Greek acoustic engineering, showing how seat curvature and arrangement optimized sound reflection and focus. His investigations at
Epidaurus were followed by further studies on ancient stepped architecture, including
Indian step wells exhibiting analogous acoustic properties. The retroreflective acoustics of these structures have become a key focus, with methodologies directly derived from Declercq's work. At
Chichen Itza, Declercq's research revealed the chirped echo phenomenon at the
Kukulkan Pyramid. A clap at its base produces an echo resembling the
quetzal bird's call, sacred to the
Maya, due to acoustic diffraction from the stepped architecture. Declercq's theoretical models, published in the
Journal of the Acoustical Society of America and
Nature, discuss the pyramid's design suggested a potential influence on cognition. This discovery contributed to the field of
archaeoacoustics and influenced interpretations interpretations of ancient acoustical design. Bilsen (2006) expanded on Declercq's findings, proposing new theoretical frameworks for acoustic phenomena in ancient monuments, suggesting intentional sound manipulation in
Mesoamerican and
Mediterranean structures. Valenzuela et al. (2020), for instance, applied Declercq's principles to study how sacred spaces may have been acoustically engineered to influence cognition during rituals. Pentcheva's (2018) study on
Byzantine aural architecture integrates Declercq's acoustics research to demonstrate how sacred Byzantine spaces were acoustically engineered to amplify religious experiences through sound. Declercq's research on ancient acoustics has also contributed to understanding of
architectural acoustics and its application to modern architectural design, contemporary
soundscapes, and
urban design. For instance, Wang et al. (2018) applied acoustic modeling techniques inspired by Declercq's methods to investigate how sound scattering from periodically corrugated surfaces could control noise in modern environments, optimizing sound distribution and enhancing desired acoustics in urban spaces. Declercq's investigation of the acoustic raindrop effect, reported in
New Scientist, revealed similarities between audible acoustics and
SAW devices used in
microelectronics.
Wave Propagation in Anisotropic and Piezoelectric Materials His investigations in
wave propagation in
anisotropic and
piezoelectric materials have contributed to understanding of
electric field and
mechanical stress effects on inhomogeneous waves. His focus on piezoelectric crystals, which generate electric charges under mechanical deformation, has broad applications in
acousto-optic devices,
non-destructive evaluation (NDE), and
sonar systems. His studies, published in
Annalen der Physik and
Ultrasonics, revealed that inhomogeneous waves in piezoelectric materials are far more sensitive to electric fields and mechanical stress than homogeneous waves. These waves, with complex wave vectors and exponential decay, show altered velocities and
polarization due to
piezoelectric stiffening, a critical insight for improving acousto-optic technology, such as
Bragg cells for
laser modulation. Prior to Declercq's work, studies on sound in stressed
piezoelectric materials were limited. An essential application of this research is in
sonar technology under high
hydrostatic pressures, where pressure amplifies piezoelectric effects, ensuring accurate performance in deep-sea military operations or extreme environments, such as subsurface ocean exploration on
icy moons. His research on
piezoelectric materials has advanced
Surface Acoustic Wave (SAW) sensor development across applications such as
torque measurement, mass sensitivity, and
biosensing, including optimizing biomolecule detection in biological applications. His research extends beyond
SAW sensors, influencing
acousto-optic devices and
phononic crystals. His models have optimized acousto-optic modulators, improving frequency stability and stress performance. His work on anisotropic and piezoelectric materials has been significant in advancing phononic crystals for sound wave control in periodic structures, with applications in
signal processing, wave filtering, and next-generation
metamaterials. His work on inhomogeneous waves obliquely incident on periodic structures has advanced our understanding of wave propagation in
phononic crystals and
metamaterials. His theoretical and experimental contributions have enabled more precise control of sound waves, with significant applications in
structural health monitoring and advanced
signal processing.
Backward Beam Displacement in Ultrasonic Waves Declercq is recognized for providing a theoretical explanation for the backward beam displacement of
ultrasonic waves reflected from periodically corrugated surfaces, a phenomenon observed experimentally by
Mack A. Breazeale in 1976 but lacking a clear theoretical basis for nearly three decades. His research elucidated the role of
Scholte-Stoneley and
leaky Rayleigh waves in producing this lateral shift of reflected ultrasonic beams, contributing to the understanding of wave interactions with structured surfaces. His work, published in
Applied Physics Letters and the
Journal of Applied Physics, extended classical diffraction theory and inhomogeneous wave theory to reveal that the effect arises under specific angles and beam configurations, addressing the gaps left by earlier studies. His contributions have had implications for
acoustics and related fields. His findings are now integral to
non-destructive testing (NDT), particularly in detecting defects on corrugated surfaces and assessing material integrity. His work on backward beam displacement has contributed to advances in
acoustics and influenced fields like
optics,
quantum mechanics, and
metamaterials. His findings have drawn parallels with lateral optical beam shifts, notably the
Goos–Hänchen effect. These insights have prompted renewed studies in optics, particularly on
surface plasmon resonance and wave reflection in quantum mechanics. Studies by Chen et al. have further demonstrated the applicability of his theories in wave localization and beam displacement within
phononic crystals. He was the first to observe backward displacement in transmission and in pulsed ultrasonic applications.
Non-Destructive Evaluation (NDE) Techniques He has made advancements in
non-destructive evaluation (NDE) techniques, particularly for detecting internal damage in
fiber-reinforced polymers (FRPs) and
composites used in the aerospace and automotive industries. These lightweight, high-strength materials are prone to
delamination,
micro-cracking, and
fiber breakage, which traditional methods often fail to detect. His research in
Lamb waves and
terahertz-based NDE has bridged this gap. His work has enhanced detection capabilities, identifying internal defects in
polyamide-based composites susceptible to
fatigue and
impact damage. He has enabled real-time damage detection by employing techniques like Lamb waves,
infrared thermography, and
X-ray tomography, offering solutions for operational monitoring in industries reliant on these materials. His ultrasonic-guided wave techniques have demonstrated efficient in detecting
delamination, of importance in
aerospace engineering, where real-time, noninvasive integrity monitoring of components under extreme stress is essential. His advancements in
Scanning Acoustic Microscopy (SAM) have enabled subsurface analysis of composite and biological materials, extending its utility to evaluating microscale periodic structures, notably in
semiconductors and
microelectronics. SAM's precision in measuring surface and subsurface properties is now pivotal in determining the
viscoelastic characteristics of materials, a key aspect for both scientific inquiry and industrial applications. In the
biomedical domain, his study utilizes
Scanning Acoustic Microscopy (SAM) to investigate the
elastic properties of
biological tissues. A key study on
Descemet's membrane in the
human eye demonstrated SAM's ability to noninvasively detect elasticity changes due to
Fuchs' endothelial dystrophy, highlighting its potential for early disease detection and monitoring. His work in
Structural Health Monitoring (SHM) has advanced high-pressure environments like
hydrogen storage systems for
automotive and
aerospace applications. He used embedded
piezoelectric sensors in composite pressure vessels, utilizing
ultrasonic guided waves for early-stage damage detection, useful for enhancing the safety and reliability of
hydrogen-powered vehicles, where timely damage detection mitigates catastrophic failure risks. His incorporation of
machine learning with ultrasonic
NDE techniques improves damage detection in noisy environments, important for industries like
aerospace, where real-time accuracy is essential. Declercq has worked on
terahertz (THz) technology for non-destructive evaluation, integrating polarization-resolved THz imaging with ultrasonic techniques to detect subsurface damage in
fiber-reinforced composites. His work on THz wave interactions with
carbon fiber orientation enables precise differentiation between intra-laminar and inter-laminar damage, an essential advancement for aerospace applications. Building on his expertise with polarized ultrasound in non-destructive testing (NDT), he successfully extended these principles to terahertz (THz) technology. Its integration with advanced imaging methods improves the precision and efficacy of
non-destructive testing in complex material systems.
Biomedical Research He has contributed to the biological field, enhancing our understanding of
blood storage, the
Descemet's membrane in
corneal disease diagnostics, and ultrasonic wave interactions with
biological tissues. In
transfusion medicine, his research on ultrasound-based evaluation of
stored blood addresses the
storage lesion phenomenon, where
red blood cells deteriorate during storage, affecting transfusion efficacy. He has demonstrated that ultrasound can noninvasively monitor blood quality in real-time without compromising the storage environment. This method enables clinicians to detect when blood begins to degrade, optimizing storage, minimizing waste, and enhancing transfusion safety. His research has made strides in
ophthalmology, employing GHz
Scanning Acoustic Microscopy (GHz-SAM) to examine the biomechanical properties of
corneal tissues, particularly the
Descemet's membrane. His contributions are important for advancing the diagnosis of
Fuchs' Endothelial Dystrophy (FECD), which leads to membrane thickening, corneal edema, and vision impairment. By leveraging GHz-SAM for high-resolution, noninvasive imaging, He has enabled early detection of corneal diseases through precise elasticity mapping at the microscopic level.
Solar Panel Inspection and Lightweight Automotive Solutions for a Sustainable Economy Declercq's research in the
renewable energy sector, particularly in the ultrasonic inspection of
solar photovoltaic modules, has been instrumental in detecting cracks and defects in
thin-film solar panels' front glass. He has enabled faster, more efficient damage detection by utilizing high-order
Lamb waves, for ensuring long-term solar system reliability. Published in several influential journals, his contributions support the widespread adoption of
solar energy by addressing critical challenges in maintaining solar infrastructure. His contributions to the
automotive industry, particularly in ultrasonic damage detection for composites, have played a pivotal role in reducing vehicle mass and
CO2 emissions. His collaboration with Fodil Meraghni from
ENSAM Metz, funded by
Peugeot-PSA, led to the development of a novel acoustic damage indicator for
fiber-reinforced composites. This indicator offers reliable damage estimation that aligns with
X-ray analysis, providing essential
non-destructive evaluation (NDE) techniques applicable in both production and maintenance phases. The importance of this work is highlighted in the 2020 report by the French Committee of Automobile Constructors (CCFA), which cites the successful reduction of 100 kg in average car mass as one of the developments contributing to reduced
carbon dioxide emissions in France. Prior to his contributions to the automotive sector, his work involved the development of Polar Scan systems for the inspection of lightweight composite structures composed of carbon fibers embedded in
polypropylene thermoplastic (PPT), as seen in applications such as the
Airbus A380. He developed a comprehensive simulation model for multilayered anisotropic media with
triclinic symmetry.
Phononic Crystals and Metamaterials His research in
phononic crystals and
acoustic metamaterials has contributed to understanding of wave propagation in periodic structures, mainly through his pioneering work on
acoustic bandgaps—regions where sound cannot propagate. Prior to his research, the effects caused by acoustic or ultrasonic
evanescent waves or the finite dimensions of phononic crystals were not investigated. His findings have implications for
noise reduction,
sonar systems, and
signal processing applications. An essential contribution is his demonstration of how phononic crystals can be engineered to control sound waves at specific frequencies, analogous to
photonic crystals for light. By fine-tuning these structures, materials can be designed to redirect, focus, or block sound, benefiting industries like automotive noise control and defense stealth technologies. His research has contributed to innovations in next-generation sensors and
acoustic lenses, providing enhanced control over sound wave propagation and focusing. His studies on the
Goos-Hänchen effect in acoustic waves have informed the design of
metamaterials for
sonar and
ultrasound imaging, enabling more precise sound control in these technologies.
Sonar Applications and NDT in the Naval Sector Throughout his research career, he has maintained strong affiliations with the
maritime industry, particularly through his contributions to
sonar technologies and the nondestructive testing of
ship hulls. His theoretical and experimental work on detecting
navigable mud depths via
bounded ultrasonic beams emitted by sonars marked innovation in the field. Prior to his research, this approach had not been considered within the scope of sonar applications, representing important advancement for both
military and
civilian maritime operations. His research on autonomous inspection systems using
ultrasonic guided waves (UGWs) has improved industrial inspections, particularly for large metallic structures like
ship hulls and
storage tanks. His work integrates
Lamb waves-based sensors into robotic platforms in collaboration with other teams, enabling long-range, non-destructive evaluations (
NDE) with minimal human intervention, improving safety and efficiency in hazardous or labor-intensive environments. An essential contribution is his integration of
SLAM (Simultaneous Localization and Mapping) algorithms into robotic inspection systems, enabling autonomous navigation in complex, noisy, or visually obstructed environments like underwater or industrial settings. His research has strengthened SLAM algorithms by improving their robustness against heavy-tailed noise distributions, common in industrial conditions where signal interference or environmental noise can distort sensor data. His enhancements in
SLAM systems enable autonomous inspection robots to more effectively navigate and map large-scale metal structures, even in traditionally challenging environments. His integration of multi-order ultrasonic echoes with advanced mapping techniques has further expanded robotic inspection capabilities. By advancing pose-graph SLAM models, his work allows for more accurate mapping, even in the structural complexities of
ship hulls and
natural gas storage tanks. == Philosophical Works ==