# Nico F. Declercq

Nico Felicien Declercq (born 27 December 1975) is a Belgian physicist, mechanical engineer, poet, historian and philosopher. He is a professor at the Georgia Institute of Technology in Atlanta and Georgia Tech Europe in France. He specializes in ultrasonic nondestructive evaluation of materials, propagation of ultrasonic waves in highly complex materials, in acoustics, in theoretical and experimental linear and nonlinear ultrasonics, acousto-optics, medical physics and acoustic microscopy. He has investigated the acoustics of Chichen Itza and Epidaurus. As a Ph.D. student, Declercq published 30 peer-reviewed articles in reputed scientific journals, including Annalen der Physik, and made 42 presentations (with papers in proceedings) at international congresses in his field. His work has been covered in Nature News, New Scientist, USA Today, The Economist, The Washington Post, Die Zeit, and Acoustics Today.

## Education, career and awards

Declercq received his BSc and MSc in physics (with a major in astrophysics) from the Katholieke Universiteit Leuven in 1996 and 2000, respectively, and received a PhD in engineering physics from Ghent University in 2005. He worked as a Belgian National Science Foundation (FWO Vlaanderen) postdoctoral fellow with Ghent University and has been a visiting scientist, supported by NATO, at the National Center for Physical Acoustics at the University of Mississippi, before joining Georgia Tech in 2006.
Declercq received the International Dennis Gabor Award from the NOVOFER Foundation of the Hungarian Academy of Sciences on December 21, 2006. He received the ICA Early Career Award "For outstanding contributions to ultrasonics, particularly for studies of propagation and diffraction of acoustic waves" from the International Commission for Acoustics in 2007. In 2024, he received the Raman-Charpak Award from the Indo-French Centre for the Promotion of Advanced Research, alongside collaborators from the University of Allahabad.
Declercq served as president (2013-2015) of the steering board of the International Congress on Ultrasonics, as well as president of their 2015 congress. He is an associate editor of the journal Acta Acustica united with Acustica, associate editor of the Journal of Nondestructive Evaluation, and founding editor-in-chief of Elsevier's Physics in Medicine. He serves on technical committees of the French Acoustical Society and is the Chair of the Ultrasonics Technical Committee of the European Acoustics Association. Declercq has received several Teaching Excellence Awards. In addition to his tenure as a full professor at Georgia Tech, he serves in a courtesy role as a full professor in the Doctoral School of the University of Lorraine in France.

## Professional Connections with France

Declercq is a Full Professor at the Georgia Institute of Technology in Atlanta and serves on doctoral committees worldwide, including France. Beyond his primary academic responsibilities, he advises PhD students at the Doctoral Schools of École Nationale Supérieure des Arts et Métiers and Université de Lorraine in France. Moreover, in France, Declercq has mentored PhD students even in cases where administrative constraints prevented his formal recognition as primary advisor. His academic involvement in France includes serving on Habilitation and Doctoral Committees at prestigious institutions such as Sorbonne Université, the Université de Lille, the Université de Franche-Comté, the Université du Maine, and the Université Polytechnique Hauts-de-France. He has also held various positions at multiple levels within the Société Française d'Acoustique. In addition, Declercq leads an ultrasonics laboratory at Georgia Tech Europe, which operates within the French National Centre for Scientific Research (CNRS).

## Connections with the Indian sub-continent

Declercq holds the title Mahāchārya (महाचार्य in Hindi, මහාචාර්ය in Sinhala) in both India and Sri Lanka, a title that recognizes his distinguished scholarly stature. Given his background in astrophysics, acoustics, and acousto-optics, in 2019, he inaugurated the Aryabhatt Auditorium. The auditorium is named after Indias mathematician-astronomer Aryabhatt. It belongs to the "Prof. Rajendra Singh (Rajju Bhaiya) Institute of Physical Sciences for Study and Research" at the VBSPU University at Jaunpur, which is named after Rajju Bhaiya, a pupil of Nobel Prize laureate C.V. Raman. In 2008 Declercq wrote a book on ultrasonics, together with his Indian colleagues P.C. Mishra, Rajendra Kumar Singh, and Sri Singh. In 2022, he was named Physics Professor Honoris Causa at the University of Allahabad, India. Declercq was the chief guest at Bundelkhand University in Jhansi in 2023 for the celebration of the 50th anniversary of the Ultrasonics Society of India.
Declercq has conducted scholarly research on the history of India and Sri Lanka. He conducted a study and documented the history of the Portuguese along the Kalu Ganga river in Sri Lanka, specifically focusing on the village of Uduwara and the town of Kalutara.
In Sri Lanka, he is a frequently invited speaker at the nation’s institutes of higher education. On July 20, 2024, during the esteemed Esala Maha Perahera religious festival, Declercq was honored as a guest by the chief-custodian (බස්නායක නිලමේ in Sinhala) of Ruhunu Kataragama Maha Dewala, Dishan Gunasekara, the highest authority of the pilgrimage site at Kataragama in Sri Lanka, and attended the Perahera together with Shavendra Silva. The following morning, he was invited to the inauguration of the renovated steps on the Menik Ganga, attended by prominent officials, including President Mahinda Rajapaksa and M.P. Shasheendra Rajapaksa. Declercq was among the first to walk the new sacred stairs of the Golden Gate, a recognition for his research on acoustic effects. This recognition was bestowed upon him for his investigations into acoustic effects at this sacred site, including his research on the echoing stairs two decades earlier.

## Scientific Discoveries

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.
His theoretical insights have informed underwater communication systems and seafloor mapping technologies, where precise control of surface waves is essential. For instance, research on Scholte wave excitation at sinusoidal seafloor interfaces builds on Declercq's work, modeling wave interactions with periodic underwater structures to enhance signal clarity.

### 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

Declercq, who publishes as Ernesto de Montisalbi, is a philosopher whose work explores the profound intersections of human existence, ethics, and the natural world. He often blends scientific insights with poetic reflections on morality and society. His philosophy characterizes inquiry into the ethical implications of technological and societal advancements.
* Demitasse Chastity (2023, 454 pages) focuses on the concept of haecquidessence, the personal essence transformed through actions. The book delves into themes of identity, authenticity, and morality, encouraging readers to reflect on the human condition through a mix of narrative and philosophical exploration.
* Grievous Reminiscence (2024, 493 pages) blends autobiographical fiction with historical reflection, addressing the ethical and existential challenges faced by individuals during societal conflicts. The book offers philosophical inquiries into themes of authority, responsibility, and the long-lasting effects of historical events on personal identity.

## Poetry

Declercq has published three volumes of poetry, collections written under the pseudonym Ernesto de Montisalbi.
* Heart Embers, 97 poems.
* Light Whispers, 94 poems.
* Soul Sparks, 93 poems.

## Historical Studies

Declercq has published several books exploring history through the lens of his ancestors' lives, with the collection compiled in an Omnibus Edition. His historical research encompassed several pivotal periods and events across different regions, with a particular emphasis on Europe. His work explored the transition from the Roman Empire to the early medieval period, focusing on the Visigothic resurgence and the Islamic conquests in Iberia. A notable contribution of his research was the identification of the Desclergues family's role as local elites during the Byzantine and Visigothic eras. He demonstrated how the family, through their resilience and adaptability, maintained influence through various political shifts, especially during the Byzantine reconquest. His analysis highlighted the diplomatic complexities between Visigothic rulers and the Eastern Roman Empire. Furthermore, he revealed how the Desclergues family integrated into Islamic society, maintaining their prominence through administrative and scholarly roles during early Muslim rule in Spain.
His research also delved into the Reconquista, particularly the Christian efforts to reclaim the Iberian Peninsula from Muslim rule. His findings brought to light the Desclergues family's active participation in significant military campaigns and their shrewd alliances with Christian military orders, such as the Knights Templar. These alliances, a testament to the family's strategic acumen, bolstered their rise to power. Additionally, he uncovered previously unknown details about the family's land acquisitions following the Fall of Granada and their contributions to regional urban development and repopulation efforts.
In his exploration of the Burgundian and Habsburg dynasties, he shed light on the influence of figures like Philip the Bold and Charles the Bold in shaping European politics. His research uncovered new connections between the Desclergues family and the Burgundian court, underscoring their role as military and financial supporters. His analysis of the Habsburg dynasty, particularly under Charles V, provided new insights into the Desclergues family's rise within the European aristocracy through diplomatic marriages. Their involvement in the Spanish Netherlands during the 16th and 17th centuries suggested a critical role in sustaining Habsburg rule.
Furthering his examination of military history, his work on the Eighty Years' War uncovered new details about the Spanish Army's efforts to retain control over the Low Countries. Through the study of previously unpublished military records, he highlighted the significant roles that members of the Desclergues family played in critical battles, such as the 1604 Siege of Ostend, as commanders and logistical organizers. His research emphasized the importance of familial networks in supplying troops and resources, particularly in the war's later stages. He also explored the Desclergues family's involvement in the War of Devolution (1667–1668) and the War of the Spanish Succession (1701–1714), uncovering the family's previously undocumented role in Catalonian resistance against French incursions. He examined their military strategies and alliances, particularly in defending local territories. Through analysis of property records and military dispatches, he demonstrated how the family's fortunes were reshaped by the redistribution of noble lands following the Bourbon victory.
Declercq also conducted in-depth research into military records from the Napoleonic Wars, World War I, and World War II, focusing on reconstructing the lives of individual soldiers. Through this work, he uncovered how their personal experiences, struggles, and decisions were intricately woven into the larger European conflicts. His findings shed light on how these soldiers' lives intersected with the broader historical forces at play, offering a human perspective on the vast and complex wars that shaped Europe.
Declercq applied scientific methods, such as ultraviolet fluorescence and infrared spectroscopy, in his historical research to analyze artifacts and materials.

## Coat of arms and lineage

Declercq's coat of arms consists of the following elements:
Motto: Érudit et sage à pied levé (English: educated and wise on a raised foot)
Blazon: on the bordure a compone of 14 pieces gold and azure of which five at the top, an azur shield, having a silver crane with raised foot, membered and beaked in red, holding in the right foot an upright feather in gold, accompanied in chief by two six-pointed stars and at the bottom a sun in gold (original blazon in French: A la bordure componnée de quatorze pièces d’or et d’azur dont cinq au chef, un écusson d’azur, a une grue d’argent sur pied, membrée et becquée de gueules, tenant de la patte dextre une plume d’oie taillée d’or posée en pal, accompagnée en chef de deux étoiles radiée de 6 pointes d’or et en ponte un soleil d'or.)
Crest: A rising silver swan with red beak (original crest in French: un cygne éployé d'argent et becquée de gueules)
Origin: The coat of arms contains colours and elements referring to that of Declercq’s ancestors de Patin (a Flemish family originating in the 14th century in Cambrai, France), with shield placed in front of the original shield (in catalan: Cortado Jaquelado de oro y azur, 2º, de azur, dos palos de oro) of his Spanish paternal ancestors Desclergues (originating in the 16th century in Catalunya, Spain).
Declercq is a patrilineal descendant of Don Pere Desclergue (I), who was born early in the 15th century AD
at Montblanc in the Conca de Barberà in Catalunya, Spain. Pere Desclergue is matrilineally descended from Francesc Desclergue, juror and magistrate of Majorca, who crowned Jaume II as King of Majorca in the Church of Santa Eulàlia on September 12, 1276. Francesc was the son of Knight Templar Ramon Clergue from Albi in France, who joined King James I of Aragon in the Conquest of Mallorca in 1229. Patrilineally, Pere Desclergue is descended from a Torró lineage that included Beltrán Torró, son of Bertrand Du Guesclin, equally with ancient Clergue roots in Languedoc-Roussillon. Pere Desclergue's grandson Bonaventura Desclergue (II) i Cortés, a notary at Montblanc, had four sons, one of which would stay as heir at Montblanc, while the three others (Jeroni Joan, Antoni (I) and Enric) would go to Flanders as military during the Eighty-year war. Don Jeroni Joan Desclergue had been a military since 1587. He stayed in Flanders from 1587-1598 and 1601-1604. In 1587, he became a commander in the infantry company of Lluís de Queralt (a Catalan); while in Flanders he joined the company of Diego de Durango, in the tercio of Luis del Villar. It has been reported that he was involved in military interventions in Holland, Flanders and France, including the Siege of Cambrai of 1595. On March 10 of 1597 he captured, with his company, the city of Amiens and helped further defending it against the French. On January 14 of 1598 he was recognized for his military career by his captain Diego de Durango. Once returned to Barcelona, he went to the service of the Viceroy Duke of Feria. His stay in Barcelona allowed him to attend, with his father Bonaventura Desclergue (II) and his elder brother Francesc Desclergue, in the Cortes of Barcelona of 1599. In 1602 he returned to Flanders in August, where he commanded as Tercio captain of a company of Iñigo de Borja, with which he participated in the Siege of Ostend (August 1601 - September 27 of 1604), where he died, with half of his company, while leading an assault, as a result of the explosion of a mine, on 4 September of 1604, which also wounded his brother Enric Desclergue. Enric was awarded for his military achievements and died in 1631. Antoni, Jeroni's son, appeared in contemporary literature which shows that he went back and forth to Spain, while Jeroni’s grandson, Antoni, was the continuator of the Desclergue bloodline in Flanders through his marriage, in Flanders, with Martina de la Cruz in 1656. Bonaventura Desclergues (II) is the one who established the monumental 'Casal dels Desclergue’, or ’la Casa Desclergues’ on the Plaza Mayor of Montblanc; Nico Declercq stems from Jeroni Desclergue twelve generations later. The Desclergues of Montblanc originate from Bertrand du Guesclin,$^{:97–130}$ who was a military commander from Brittany and who supported Henry of Trastámara, the first King of Castile and León in the period 1365-1370.
Ernest Constant Declercq (1872-1947),$^{:463–526}$ a poet, is the great-grandfather of Nico F. Declercq. On his mother's side, Declercq is blood-related to archaeologist Marc Waelkens (1948-2021)$^{:981}$ and astronomer Tom Van Flandern (1940-2009),$^{:989–1009}$ while, on his paternal grandmother's side, to legal scholar Charles-Philippe de Patin (1687-1773)$^{:1047–1063}$ and, through both paternal grandparents, to musician Peter Benoit (1834-1901).$^{:929–943}$