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Declercq and his team are specialized in experimental work and theoretical/numerical work.
The laboratory promotes and enhances environmental consciousness through interdisciplinary research with immediate industrial applications for Nondestructive Evaluation (NDE) of new materials. It is well known for NDE characterization using a wide range of frequencies from 0.5 MHz up to 2 GHz, for linear and nonlinear acoustics, contact, air-coupled, and immersion investigations, and for performing scans at normal incidence or multiple angles. NDE is performed on traditional materials, new composites, and periodic structures such as phononic crystals (PCs), which are human-made periodic structures comparable to atomic lattices but macroscopic, also known as metamaterials.
The global goal is to understand the physics of the interaction of sound and ultrasound with materials aiming at the nondestructive evaluation of materials. Studies involve the diffraction of sound at periodical structures (phononic crystals, staircases, periodically corrugated surfaces and interfaces, multi-layered materials, acoustic barriers …) and the interaction of sound with fibre-reinforced composites and other anisotropic materials. In addition, medical applications are examined, such as blood, the Descemet membrane of the human eye and other biological materials. Investigations done with partners licenced for bio-medical experiments cover both linear and nonlinear interactions.
One of the objectives is to understand complex situations where different phenomena coincide. Casually we tend to call our work 'opening Pandora's box.'

Our lab accommodates undergraduate (BS), graduate (MS) and PhD students.

This research is fundamental in the automotive and aerospace industry (e.g. NDE of parts and composite structures), telecommunication (e.g. the use of phononic crystals for filtering of signals), the acoustics of architectural masterpieces (e.g. diffraction in Chichen Itza – Mexico, Epidaurus - Greece), medicine, renewable energy etcetera.

The work produced by Declercq and his team has been widely covered in the media worldwide (Nature, Washington Post, New York Times, The Economist, and many local newspapers in most countries). The team has been a pioneer in the globalization of research. It is an American research group embedded in the French CNRS organism, protecting its identity while adapting to the local requirements. The team has housed numerous undergraduate and graduate students desiring research experience while studying in Metz. Students are either American studying abroad, embedded in a European experience, or French and other nationalities studying in Metz in partial fulfilment of their academic path in Atlanta. They are the global engineers of the future.

Below are a couple of topics studied or currently under study by the lab.

Solar Panel Quality Control

Solar Panels at Georgia Tech

The energy sector is one of the main components in developing a sustainable system on an urban scale and a global framework.
Per the commitment to cut CO2 emissions and suppress the use of fossil fuels, many countries have been turning their attention to renewable energies. As of 2019, 19.5% of the European Union's energy consumption was drawn from renewable sources. The U.S. Energy Information Administration reported that renewables took a 21% share in national electricity production in 2020 with a projected figure of 42% by 2050, dominating the electricity generation profile by then. The same report also stated that solar power generation would be the predominant renewable supplier of the U.S. electricity grid in the future.
Solar power generation, explicitly utilizing solar photovoltaics (PV), converts virtually unlimited solar energy directly into electricity. The absence of fuel and greenhouse gas emissions and negligible impact on the water cycle during its operation are some of the excellent qualities of solar PV that can be exploited to combat global challenges of the energy crisis, pollution, and climate change. Solar cell technology also evolved to attain a higher efficiency with a declining price level as its production capacity increases. Another advantage of solar PV is its installation versatility, allowing for integration to a building design, supplying power to the property, and even feeding into the grid. Buildings equipped with solar PV combined with other carbon-neutral power sources can form a decentralized energy generation system and promote sustainable urban development.
Our research interest is directed towards nondestructive inspection techniques of solar PV modules, an indispensable part of quality assurance, particularly in manufacturing. The research evaluates defective solar PV modules using ultrasonic techniques at GT-Lorraine, which also operates a semiconductor-focused innovation center named Institut Lafayette, which has strengthened its semiconductor research profile, including for PV applications. GT's international campus is a favorable institution for this area of research. It has a track record in PV-related areas and a strong position for intensive engagement with industries.

Ship Hull investigations

Autonomous inspection of a ship in BugWright2

BUGWRIGHT2 is a collaborative project co-funded by the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No. 871260. The project was initiated by Cedric Pradalier and Nico F. Declercq and is led by Pradalier, with Declercq, being a US faculty, in an external partner role. Within GT Lorraine, the labs of Pradalier and Declercq have joined forces and deliver ultrasonics investigations through robotics. A consortium has been formed with several European partners, each playing significant roles in this project's success.
The objective of BUGWRIGHT2 is to bridge the gap between the current and desired capabilities of ship inspection and service robots by developing and demonstrating an adaptable autonomous robotic solution for servicing outer ship hulls.

The project consists of a large consortium bringing together not only the technological knowledge from academia but the complete value chain of the robotic inspection market:
-two SMEs
-one class society to evaluate the use of these technologies in the certification processes
-a marine service provider
-two harbours to provide access to ships
-one shipyard to deploy the system within a maintenance framework
-two ship-owners
-In addition, specialists in maritime laws and workplace psychologists will ensure that the digitalisation of this market sector is designed around user acceptance. Finally, a specialist in innovation will lead the dissemination and exploitation activities.

The BugWright2 Consortium consists of highly qualified partners covering the entire spectrum of know-how necessary to carry out the project.
The Consortium is composed of 9 academics partners (CNRS, UPORTO, UIB, INSA, RWTH, UNI-KLU, NTNU, UT, WMU) and 11 industrials partners (CETIM, LSL, RBP, BEYE, RINA, GLM, APDL, AASA, TRH, IEIC, DANAOS, SBK).

Website: Bugwright2

Water Purification - BioSensUS Project

BioSensus Bioluminescence of a bio-reporter bacteria exposed to temperature increase or ultrasound during 20 to 120 s and resilience 1h after treatment stopped.

It is well-known that high-intensity ultrasound kills bacteria, but heat, ultraviolet and chemicals kill too. In this project, we were interested in how low-intensity ultrasound may influence the behaviour of bacteria, in particular their growth and reproduction. If the acoustic waves disturb them so that their reproduction is inhibited, the outcome would be a low energy, therefore inexpensive, technique to keep the water clean. The endeavour should be seen in the light of drinking water, heat exchangers, and other systems in which bacterial growth may reduce efficiency over time.
This work was primarily done within the BioSensUS Project "Inactivation of bacteria during exposure to ultrasonic waves: What are bacteria sensing?", a collaboration between the teams of Nico F. Declercq and Laurence Mathieu.
In the quest for microbial inactivation of widely-consumed products such as drinking water, physical methods could be valuable and environment-friendly approaches alternatives to biocide treatments. Amongst these methods, the use of ultrasonic (US) waves is an attractive option as its bactericidal action has already been proven by our teams. Ultrasound-mediated microbial inactivation could also respond to societal challenges as a fight against antimicrobial resistance acquisition and the development of eco-aware low environmental footprint technologies. The BioSensUS project, supported by Lorraine Université d'Excellence, was a pilot investigation that explored the pleiotropic mechanisms of the inactivation of bacterial cells triggered by ultrasound waves. It resulted in an important paper in a reputable journal, producing new essential questions. They have led the two partners P1-UMR 7564 LCPME and P2-UMI 5826 GeorgiaTech, to amplify their interdisciplinary dynamics in a novel scientific investigation scheme that revolved around the following fundamental question: What do bacteria experience when exposed to ultrasound and what bacterial damage and cellular disorder do take place?
BiosensUS aimed to deepen the previous approach and produced significant scientific results to enlarge the interdisciplinary scientific knowledge. The project was composed of three experimental tasks. First, we define relationships between ultrasound wave doses and the effect on model bacteria at the structural and functional scales using multi-target approaches, i.e., fluorescent molecules for cellular damage and bioreporter bacteria. Second, we discriminated between the effects of ultrasound waves from the heat effect to define the doses from only ultrasonic vibrations. Third, we identified the resilience of the bacteria (i.e., permanent or transient effects) due to bacterial tolerance or recovery.BiosensUS enriched the knowledge on bacteria reactivity to ultrasound. It formed the starting point for further research on the effect of ultrasonic waves on planktonic bacteria and in the form of biofilm. It addressed industrial and sanitary concerns about the risk management of water. The collaboration efforts have been reduced due to priority to another project in the lab of Laurence Mathieu.
In the final experiments, we observed the behaviour of an E.Coli (Gram+) bacterial population after exposure with 1 MHz Ultrasound Waves for 15min, 1h and 24h and observed that there was no measurable decrease in the total number of cells at high (12V) and low amplitude (5V), there was no DNA damage, and only minor membrane damage (which did not lead to cell inactivation) was found. We equally observed the behaviour of an S. Aureus (Gram-) bacterial population after exposure with 1 MHz Ultrasound Waves for 15min, 1h and 24h. We found the same behaviour between the control and treated samples and discovered that a more prolonged treatment at 1MHz did not fundamentally change the results.
Furthermore, there was no measurable difference between continuous and pulsed ultrasound modes. Also, the post-exposure dynamic showed no growth in the treated sample of S.Aureus while there was average growth in the treated sample of E.Coli, which was a critical observation.
The National Academy of Engineering defined 14 Grand Challenges for Engineering in the 21st Century, such as providing access to clean water. Specifically, in its ‘Objectifs de développement durable 2030’ (sustainable objectives for 2030), France established several objectives, the sixth being “sustainable use of drinking water, to guarantee access for all.” We provided a proof of concept of an ultrasound-based technique with a low environmental footprint to keep water free of bacteria. It has been the first collaboration between GT-Lorraine and LCPME in Nancy, France. LCPME is well-known for its biological investigations in water and on surfaces.

BioSensUS was a Mirabelle+ Project

SAM and BioMedical Research

Donated Blood (courtesy of the French Red Cross)

With PVA-Tepla, the team developed a new Scanning Acoustic Microscope (SAM) to investigate materials at very high resolution, up to 2 GHz frequencies. Imaging and V(Z) curve investigations are capabilities useful in materials investigation and areas such as biomedical applications.
In 2018, the team expanded research in the medical field. It was the first to provide experimental evidence, through (SAM), of local stiffness decrease, at a microscopic level, of the Descemet membrane of the human eye, caused by Fuchs endothelial dystrophy (FECD). The pioneering work opened the path for diagnosis and treatment and was the first collaboration between GT-L and the hospital of Metz. A consortium was recently formed, seeking funding, between 'GT-Lorraine', the 'Ophthalmology Department Regional Hospital Center of Metz-Thionville, Mercy (CHR Metz-Thionville)', 'Institut Jean Lamour (CNRS-Université de Lorraine, UMR 7198 CNRS)', 'Personalized Biomedical Engineering Lab (Frankfurt UAS)', and 'Diagnostic of Semiconductor Technologies, Center for Applied Microstructure Diagnostics CAM, Fraunhofer Institute for Microstructure of Materials and Systems IMWS'.
The lab has also established a consortium, equally seeking funding, with the French Blood Bank to investigate blood in blood bags for donation. Preliminary results were auspicious.
The ongoing and envisaged work is an answer to the World Health Organization (WHO) to optimize blood supply management worldwide as preparedness for big disasters, such as the recent Covid-19 pandemic. In particular, an easy to apply non-invasive quality control technique for sealed blood bags does not exist and results in storage management determined by date and facilities, not by measurement.

Scanning Acoustic Microscopy images of cells and industrial materials, obtained in our laboratory.

Non-Linear Acoustics and Coda waves

Non-linear acoustics exploits frequency changes due to non-linear elasticity and defects. Several labs worldwide are highly specialized in the deep understanding of the involved phenomena, while our lab primarily exploits the effects for specific purposes. This goal-oriented approach provides us with a technique supporting nondestructive evaluation and offers a complete investigation capacity compared to merely linear acoustic research. Work is done on different liquids, composites and other materials.
Anyone interested in music knows that it usually ends with a coda. Ultrasound can be considered high-frequency music reverberating through the material and equally causing a coda at the end of a measurement. Inasmuch as the end of a signal was traditionally ignored, this coda, currently examined, exposes information about material properties. Even though it stems from seismology, the applications in nondestructive evaluation are promising but require skilful signal analysis. Significant results have recently been obtained for fiber reinforced composites

Joint Ultrasonics-THz research

There has always been mutual fertilization of optics and acoustics throughout scientific history. Recently, new technology allowed the fabrication of tools to produce and apply electromagnetic THz waves. Experience in acoustics is beneficial for collaboration between the ultrasonics and the THz labs at the IRL. In particular, both techniques can be compared, and expertise exchanged. However, there are also cases where they are complementary and may be very beneficial for science and the industry as a combined approach. As in work in robotics, this research occurred in collaboration of the Declercq team and another IRL team, namely the TeraHertz team of Citrin and Locquet. It exposed the interdisciplinary and collaborative nature and intentions of our teams. Many promising materials have been hampered by poor strength at specific locations. Specifically, injection-mould lines are often associated with poor mechanical strength. We applied the technique of SAW (surface acoustic waves, used in scanning acoustic microscopy) to explore the acoustic properties near injection-weld lines by comparing the velocity in locations close to and locations distant from the weld lines.
Scanning acoustic microscopy (SAM) was mainly used to characterize welds in plastic in a trade thermoplastic polymer (ABS) manufactured by injection moulding, particularly at the locations of weld-lines known to form as unavoidable significant defects. The objective was to investigate the complementarity of SAM and THz investigations.
Similarly, a study was applied for polarization-sensitive THz imaging to characterize subsurface damage in woven carbon fibre-reinforced composite laminates.

Anisotropic Media - composites

New requirements in terms of CO2 emissions have pushed the automotive industry to make a new generation of vehicles by building lightweight vehicles and replacing the metallic structural elements with lighter materials with low density and high mechanical properties. The interest in polymer-based composite materials has continuously evolved over the past decade to achieve this goal. We can mention the improvement of specific mechanical properties such as the thermal conductivity, the resistance to weight ratio, and corrosion resistance. Composites can be designed with properties not provided by the existing mineral materials-based. Additionally, as far as large-scale production is concerned, flexibility and customization have further encouraged the growth of composite materials in the automotive industry. With the advent of composite materials with a higher strength to weight ratio, the less load-bearing components in vehicles have been gradually replaced by these new materials.
Composite elements used in the automotive industry are generally divided into two categories: (i) composites that withstand high temperature and humidity, and (ii) composites, destined to improve the vehicle’s structural performance, highly stressed, are operational near their mechanical strength limit and should respect the requirements of security and operational safety. Therefore, identifying the potential loads that the composite structure can undergo in service and studying its behavior under these loads remains essential.
Despite their excellent strength to weight ratio, composites are more susceptible to delamination in different stages of the structure’s life. Additionally, composites exhibit damage mechanisms different from those occurring in metallic materials regarding damage initiation and accumulation. Indeed, microscopic damage mechanisms gradually develop during service and accumulate throughout the volume of the material. Combining the concomitant mechanisms brings about the deterioration of the local properties leading to the final mesoscopic failure.
This investigation was done in a tight collaboration between our lab and Fodil Meraghni and, therefore, formed an alliance between GT Lorraine and Arts et Métiers Sciences et Technology (LEM3 - ENSAM Metz), with financial and technological support from Stellantis (Peugeot PSA, through Stephane Delalande) for several years. Several MS students worked on the project, and two joint PhD students (Pascal Pomarede and Nada Miqoi, who successfully obtained their PhD degrees).

Anisotropic Media - Wood

One of the most common biological composites is wood. This natural orthotropic material has a high anisotropy, determined by the unique disposition of the microstructure elements. The anisotropy of wood can be described in various ways using the values of ultrasonic velocities of bulk waves (longitudinal and shear) observed on the velocity surface deduced from the theoretical relationships given by Christofel's equation. The simultaneous view into the three symmetry planes of the anisotropic behavior of wood is presented on the velocity surface. The spatial filtering action of the wood structure is easily connected with longitudinal and shear velocities. The first step in examining the anisotropy of wood is to relate the velocities to the symmetry axes. The simplest way to describe the anisotropy of wood is to express the ratios of velocities. These ratios can be calculated separately for longitudinal or shear waves or a combination of both. The birefringence of shear waves is particularly interested in the refined definition of anisotropy. A more global appreciation of wood anisotropy than the values of individual velocities is given with acoustic invariants. The stability of the calculation of acoustic invariants versus different propagation angles confirms the validity of the chosen model for the tested material. Wood species have high density, and any essential organized structure in the millimeter scale exhibits a high invariant ratio. The acoustic behavior of tropical wood species is less anisotropic than that of a temperate zone with a low density. This work was done in collaboration with renowned wood expert Voichita Bucur.

Anisotropic Media - Minimum Variance

Ultrasonic guided waves can rapidly interrogate large, plate-like structures for both nondestructive evaluation (NDE) and structural health monitoring (SHM) applications. Distributed sparse arrays of inexpensive piezoelectric transducers offer a cost-effective way to automate the interrogation process. However, the sparse nature of the array limits the amount of information available to perform damage detection and localization. Minimum variance techniques have been incorporated into guided wave imaging to reduce the magnitude of imaging artefacts and improve imaging performance for sparse array SHM applications. The ability of these techniques to improve imaging performance is related to the accuracy of a priori model assumptions, such as scattering characteristics and dispersion. We applied minimum variance imaging under slightly inaccurate model assumptions, such as are expected in realistic environments. Expressly, the imaging algorithm assumes an isotropic, non-dispersive, single-mode propagating environment with a scattering field independent of incident angle and frequency. In actuality, the composite material considered here is slightly anisotropic and dispersive and supports multiple propagating modes.
Additionally, the scattering field is dependent on incident angle, scattered angle, and frequency. Anisotropic propagation velocity is estimated via calibration prior to imaging to implement the non-dispersive model assumption. Imaging performance is done under these inaccurate assumptions to demonstrate the robustness of minimum variance imaging to familiar sources of imaging artefacts.

Anisotropic Media - Polar Scans

Polar scans are used to assess damages in composites and their anisotropy. Damage estimation in composites has increased complexity due to their anisotropic nature. Early on, an analytical model was developed in Matlab to simulate polar scan images in a Carbon Fiber Reinforced Composite (CFRC) plate sample. The theoretical model was then compared with an experimentally obtained polar scan plot of an undamaged CFRC plate specimen to show that the undamaged composite can be considered homogeneous for the considered frequencies and that the patterns appearing in the scan are due to known acoustic phenomena. Later, the damage in two CFRC samples subjected to tensile and bending loads till failure respectively was imaged using the polar scan technique. The use of the time of flight based estimation of damage is introduced in this study. It was used successfully to complement the damage estimation using the earlier developed amplitude-based technique. Polar Scans are vastly overestimated measurements that are only meaningful in particular circumstances, combined with other investigations. In particular, the equipment used to make such scans is very versatile and has more often been used by our team for other investigations than to make such scans. Until 2015 our lab was the favourite target of a Belgian copycat who meticulously followed our lab's work and used it as inspiration for incremental work, which was a reason to stop updating our website and cease using polar scans. For scientists, it is okay to have fans, but it is not okay to be stalked. Another reason was mentioned earlier and freed more time and opportunities to apply the equipment in other, more exciting investigations that resulted in extensive research papers on damage analysis of fibre-reinforced materials for the automotive industry.

Anisotropic Media - Piezo-electric crystals

When Declercq was a PhD student and was only contractually bound to study the interaction of sound with fibre-reinforced composites, he studied, as a hobby, sound in piezoelectric crystals. Finally, he decided to include this solo work into his thesis and considered it part of his PhD, although it was done independently and in his free time. He still considers it the most prominent work done in his PhD period because of its difficulty. It resulted in a paper in the reputable Annalen der Physik and opened opportunities for research in collaboration with the Vitaly Voloshinov in Moscow. This is why some of Declercq's research was on wave scattering effects in piezoelectric crystals used in acousto-optics, notably paratellurite. In particular, the collaboration with Natalia Polikarpova was fascinating and promising. However, the distance between Metz and Moscow was not practical, which is why the collaboration was not further fueled, although it may be reignited in the future. Still, Declercq is often asked as a referee for this topic by journal editors, and it is also valuable for teaching Signals and Transducers at GT Lorraine.

Finite Element Acoustics

A finite element simulation of a bounded beam in water

Our team was a pioneer in Finite Element simulation of sound propagation and interaction with materials of complex geometry in 2006-2011. In collaboration with Ghent University, we expanded the other team's knowledge from low-frequency body deformations for materials characterization related to impact and classical strain studies to include high-frequency ultrasonic vibrations, i.e. sound propagation. The early efforts resulted in a PhD of Lamkanfi, for whom Declercq has served as voluntary co-advisor while covering 50% of the manuscript's contents. The collaboration was not further fueled for several reasons, such as a drive for independence by the Ghent University Team (while ignoring the GT-Lorraine contribution) and the deontological refusal by Declercq to have his knowledge drained by the Belgian team without financial or publication commitments. The priority of Declercq's team was his loyalty to Georgia Tech and to commit entirely to a significant research grant obtained with the French Institut de Soudure, for which experimental work and finite element simulations were equally important. Nowadays, finite element simulations in acoustics and ultrasonics are no research topics anymore but established tools to understand sound interactions. Investigators in this field often lack understanding of the ongoing physics, so fundamental training and experience in acoustics remain essential. Finite element approaches have replaced earlier fields, such as normal-mode theory, radiation mode theory and specific analytical approaches. The team has developed and exploited the tool in the past years in the context of coda-waves, a robotic inspection of large plate structures (containers, ship hulls, pipes, ...) and accommodates the team's efforts. It has become a 'conditio sine qua non' in modern science.

Electrets for Ultrasonic Transduction

The research on electroactive foams was done in collaboration with Yves Berthelot at the beginning of Declercq career as a faculty member. Declercq's team was, in this context, mainly involved with applications of the foams for nondestructive testing of materials.
In order to ascertain the structural integrity and the reliability of mechanical structures, there was a need to develop a reliable, fast, accurate, nondestructive evaluation (NDE) technique to qualify/quantify the defects inside a component. Traditional ultrasonic inspection can be broadly categorized as contact and immersion-based testing. Both have their limitations, and a promising alternative is an air-coupled ultrasonics technique mainly because it eliminates the need for a couplant and hence in situ scanning and real-time testing becomes possible. The discovery of a new class of piezo-active polymer foams was a very promising avenue to develop new air-coupled ultrasonic transducers because of their excellent impedance matching with air compared to traditional transducers. So, this project aimed at developing transducers using novel porous polymer foam (cellular polypropylene) piezoelectret materials. When a voltage is applied, these materials exhibit a phenomenon similar to the inverse piezoelectric effect. The defining features of the piezo-like polymer foam are small, elliptically shaped and electrically polarized gas bubble voids located inside the polymers.


Acousto-Optics is the study of the interaction of light with sound. Briefly explained, the sound causes a periodical modulation of the optical refractive index (through density variations caused by the acoustic pressure), deflecting light. Because of the periodicity and the interaction's weakness, the light phase and not the amplitude are periodically distorted, whence the sound wave acts like a diffraction grating. As a result, light is diffracted, which means that it splits in different light rays with directions and Doppler shifts determined by the sound wave. Engineers and scientists can use the principle to visualise sound (through Schlieren photography) or control laser light. Therefore, as described in numerous scientific papers, it is essential in laser-guided systems, laser printing, optical communications systems, and many others. Declercq's PhD advisors, Oswald Leroy and Mack Breazeale, were very familiar with Acousto-Optics. In particular, Oswald Leroy was one of the most prominent scientists in the theory of this phenomenon in the 1960s and later, stemming from the Belgian acousto-optics school in Ghent, led by Robert Mertens. Having worked in this background, Declercq is quite familiar with the subject and also made his investigation. In 2014, our lab was the first to use Acousto-optic Bragg imaging of biological tissue.

Archaeo-Acoustics : Chichen Itza

Article in Nature News
Published online: 14 December 2004; | doi:10.1038/news041213-5
Mystery of 'chirping' pyramid decoded
Acoustic analysis shows how temple transforms echoes into sounds of nature
By Philip Ball

A theory that the ancient Mayans built their pyramids to act as giant resonators to produce strange and evocative echoes has been supported by a team of Belgian scientists. Nico Declercq of Ghent University and his colleagues have shown how sound waves ricocheting around the tiered steps of the El Castillo pyramid, at the Mayan ruin of Chichén Itzá near Cancún in Mexico, create sounds that mimic the chirp of a bird and the patter of raindrops1.The bird-call effect, which resembles the warble of the Mexican quetzal bird, a sacred animal in Mayan culture, was first recognized by California-based acoustic engineer David Lubman in 1998. The 'chirp' can be triggered by a handclap made at the base of the staircase. Declercq was impressed when he heard the echo for himself at an acoustics conference in Cancún in 2002. After the conference, he, Lubman and other attendees took a trip to Chichén Itzá to experience the chirp of El Castillo at first hand. "It really sounds like a bird", says Declercq.

Sound structure
But did the pyramid's architects know exactly what they were doing? Declercq's calculations show that, although there is evidence that they engineered the pyramid to produce surprising sounds, they probably couldn't have predicted exactly what they would resemble. Lubman was at first convinced that the pyramid-builders did create the bird-chirp effect intentionally. But that's not necessarily so, Declercq and his colleagues argue. Their analysis of the pyramid's acoustics show that the precise sound caused by the echoes depends on the sound that excites them. Drums, for example, might produce a different type of resonance. The researchers hope that others will make more on-site measurements of El Castillo's acoustics to see what effects other sounds sources induce. Indeed, Declercq heard one such variation during the 2002 trip. As other visitors tramped up the steps of the 24-metre high pyramid, he noticed a flurry of pulse-like echoes that seemed to sound just like rain falling into a bucket of water. Declercq wonders whether this, rather than the quetzal call, could have been the aim of El Castillo's acoustic design. "It may not be a coincidence," he says - the rain god played an important part in Mayan culture.

But perhaps such meaningful interpretations are fanciful. Declercq's team has shown that the height and spacing of the pyramid's steps creates like an acoustic filter that emphasizes some sound frequencies while suppressing others. But more detailed calculations of the acoustics shows that the echo is also influenced by other, more complex factors, such as the mix of frequencies of the sound source.Ultimately, then, it will be virtually impossible to prove that any specific echo effect is intentional. "Either you believe it or you don't," says Declercq. He himself is now sceptical of the quetzal theory - not least because he has now heard similar effects produced by staircases at other religious sites. At Kataragama in Sri Lanka, for example, a handclap by a staircase leading down to the Menik Ganga river produces an echo in response that resembles the quacking of ducks.

Nico F. Declercq, Joris Degrieck, Rudy Briers, Oswald Leroy, "A theoretical study of special acoustic effects caused by the staircase of the El Castillo pyramid at the Maya ruins of Chichen-Itza in Mexico",
J. Acoust. Soc. Am. 116(6), 3328-3335, 2004

Article in New Scientist
Published: 16 September 2009 (magazine issue 2726, page 12.)
Mayans 'played' pyramids to make music for rain god
By Linda Geddes

SIT on the steps of Mexico's El Castillo pyramid in Chichen Itza and you may hear a confusing sound. As other visitors climb the colossal staircase their footsteps begin to sound like raindrops falling into a bucket of water as they near the top. Were the Mayan temple builders trying to communicate with their gods?
The discovery of the raindrop "music" in another pyramid suggests that at least some of Mexico's pyramids were deliberately built for this purpose. Some of the structures consist of a combination of steps and platforms, while others, like El Castillo, resemble the more even-stepped Egyptian pyramids.
Researchers were familiar with the raindrop sounds made by footsteps on El Castillo - a hollow pyramid on the Yucatán Peninsula. But why the steps should sound like this and whether the effect was intentional remained unclear.
To investigate further, Jorge Cruz of the Professional School of Mechanical and Electrical Engineering in Mexico City and Nico Declercq of the Georgia Institute of Technology compared the frequency of sounds made by people walking up El Castillo with those made at the solid, uneven-stepped Moon Pyramid at Teotihuacan in central Mexico.
At each pyramid, they measured the sounds they heard near the base of the pyramid when a student was climbing higher up. Remarkably similar raindrop noises, of similar frequency, were recorded at both pyramids, suggesting that rather than being caused by El Castillo being hollow, the noise is probably caused by sound waves travelling through the steps hitting a corrugated surface, and being diffracted, causing the particular raindrop sound waves to propagate down along the stairs (Acta Acustica united with Acustica, DOI: 10.3813/AAA.918216).
El Castillo is widely believed to have been devoted to the feathered serpent god Kukulcan, but Cruz thinks it may also have been a temple to the rain god Chaac. Indeed, a mask of Chaac is found at the top of El Castillo and also in the Moon Pyramid. "The Mexican pyramids, with some imagination, can be considered musical instruments dating back to the Mayan civilisation," says Cruz, although he adds that there is no direct evidence that the Mayans actually played them.
Francisco Estrada-Belli, an archaeologist at Boston University, Massachusetts, says: "Most if not all Maya pyramids were conceived as sacred mountains, which were the places where the clouds gathered and created rain." However, while the acoustics may have emphasised the metaphor of water, "the fact that there were echoes around them does not mean that they were musical instruments", he says - adding that Mayan texts do not mention such a use.
Elizabeth Graham of University College London points out that the pyramids have been restored. "The authors need to provide a good reason for why they think the restored building surfaces are enough like ancient building surfaces," she says.

Jorge Antonio Cruz Calleja, Nico F. Declercq, „The acoustic raindrop effect at Mexican Pyramids: the architects’ homage to the rain god Chac?“, Acta Acustica united with Acustica, 95, 849-856, 2009

Archaeo-Acoustics : Epidaurus

An ancient theatre filters out low-frequency background noise.
Nature News Published online: 23 March 2007; | doi:10.1038/news070319-16
Philip Ball

The wonderful acoustics for which the ancient Greek theatre of Epidaurus is renowned may come from exploiting complex acoustic physics, new research shows.
The theatre, discovered under a layer of earth on the Peloponnese peninsula in 1881 and excavated, has the classic semicircular shape of a Greek amphitheatre, with 34 rows of stone seats (to which the Romans added a further 21).
Its acoustics are extraordinary: a performer standing on the open-air stage can be heard in the back rows almost 60 metres away. Architects and archaeologists have long speculated about what makes the sound transmit so well.

Now Nico Declercq and Cindy Dekeyser of the Georgia Institute of Technology in Atlanta say that the key is the arrangement of the stepped rows of seats. They calculate that this structure is perfectly shaped to act as an acoustic filter, suppressing low-frequency sound — the major component of background noise — while passing on the high frequencies of performers' voices1.
It's not clear whether this property comes from chance or design, Declercq says. But either way, he thinks that the Greeks and Romans appreciated that the acoustics at Epidaurus were something special, and copied them elsewhere.

Sound steps
In the first century BC the Roman authority on architecture, Vitruvius, implied that his predecessors knew very well how to design a theatre to emphasize the human voice. "By the rules of mathematics and the method of music," he wrote, "they sought to make the voices from the stage rise more clearly and sweetly to the spectators' ears... by the arrangement of theatres in accordance with the science of harmony, the ancients increased the power of the voice."
Later writers have speculated that the excellent acoustics of Epidaurus, built in the fourth century BC, might be due to the prevailing direction of the wind (which blows mainly from the stage to the audience), or might be a general effect of Greek theatre owing to the speech rhythms or the use of masks acting as loudspeakers. But none of this explains why a modern performer at Epidaurus, which is still sometimes used for performances, can be heard so well even on a windless day.

Declercq and Dekeyser suspected that the answer might be connected to the way sound reflects off corrugated surfaces. It has been known for several years now that these can filter sound waves to emphasize certain frequencies, just as microscopic corrugations on a butterfly wing reflect particular wavelengths of light. The sound-suppressing pads of ridged foam that can plastered on the walls of noisy rooms also take advantage of this effect.
Declercq has shown previously that the stepped surface of a Mayan ziggurat in Mexico can make handclaps or footsteps sound like bird chirps or rainfall (see 'Mystery of 'chirping' pyramid decoded'). Now he and Dekeyser have calculated how the rows of stone benches at Epidaurus affect sound bouncing off them, and find that frequencies lower than 500 hertz are more damped than higher ones.

Murmur murmur

"Most of the noise produced in and around the theatre was probably low-frequency noise," the researchers say: rustling trees and murmuring theatre-goers, for instance. So filtering out the low frequencies improves the audibility of the performers' voices, which are rich in higher frequencies, at the expense of the noise. "The cut-off frequency is right where you would want it if you wanted to remove noise coming from sources that were there in ancient times," says Declercq.
Declercq cautions that the presence of a seated audience would alter the effect, however, in ways that are hard to gauge. "For human beings the calculations would be very difficult because the human body is not homogeneous and has a very complicated shape," he says.

Filtering out the low frequencies means that these are less audible in the spoken voice as well as in background noise. But that needn't be a problem, because the human auditory system can 'put back' some of the missing low frequencies in high-frequency sound.
"There is a neurological phenomenon called virtual pitch that enables the human brain to reconstruct a sound source even in the absence of the lower tones," Declercq says. "This effect causes small loudspeakers to produce apparently better sound quality than you'd expect."

Although many modern theatres improve audibility with loudspeakers, Declercq says that the filtering idea might still be relevant: "In certain situations such as sports stadiums or open-air theatres, I believe the right choice of the seat row periodicity or of the steps underneath the chairs may be important."

Nico F. Declercq,
Cindy S. A. Dekeyser, „Acoustic diffraction effects at the Hellenistic amphitheater of Epidaurus: seat rows responsible for the marvelous acoustics“, J. Acoust. Soc. Am. 121(4), 2011-2022, 2007
See also: ‘
The Whistle’, ‘The Economist’, ‘The Washington Post’, ‘The Financial Times’, ‘Het Nieuwsblad’, and many others…

Room Acoustics - Alvar Aalto's discussion room

Although the primary interest of Declercq's lab is ultrasonics, the physics of audio waves in rooms is determined by the same fundamental laws and can, therefore, also be studied. For example, there is an auditorium operating as a discussion room and lecture room at the Municipal Library of Vyborg, Russia (built during Finnish rule when the city's name was Viipuri in Finnish), designed by Aalto and constructed in 1933-35, where the wave-shaped ceiling has been specially designed to enhance the acoustics and make every position within the room acoustically equivalent. No matter where a speaker is standing, he is supposed to be heard equally well all over the room. Therefore, the corrugated ceiling has been constructed to distribute sound optimally, at least from the point of view of a ray theoretical analysis. A numerical study showed that the corrugation is such that the ray approach is not precisely valid due to diffraction effects that must be incorporated. The acquired knowledge is vital for the future construction of similar rooms.


A project involving Laurent Capulongo was initiated several years ago between the ultrasonics lab and that of Capulongo to study acoustic wave propagation in largely dislocated crystals and aimed at a novel high-resolution damage characterization method for Multilayered Composite Crystalline Materials. While the consortium was seeking funding, Capulongo found an opportunity to join the National Lab in Los Alamos, which he liked very much. He continued some of this work with others in the USA. At the same time, the lab of Declercq focussed more on damage investigation of fibre-reinforced composites in the automotive industry as per the commitment made by the Open Lab between GT-Lorraine and Peugeot-PSA in France.