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The global goal is to understand the interaction of sound and ultrasound with materials aiming at 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 fiber reinforced composites and other anisotropic materials. In addition medical applications are examined such as the multiple interaction of sound with objects in a blood flow. Investigations cover both linear and nonlinear interactions. One of the objectives is to understand complex situations where different phenomena take place simultaneously. Declercq and his team are specialized in experimental work and theoretical/numerical work.

This research is very important in aerospace industry (e.g. NDE of parts and composite structures), telecommunication (e.g. the use of phononic crystals for filtering of signals), acoustics of architectural masterpieces (e.g. diffraction in Chichen Itza – Mexico, and medicine etc.)

The work produced by Declercq and his team has been widely covered in the media around the world (Nature, Washington Post, New York Times, The Economist, and many local newspapers in most countries).

Below are a couple of topics studied or currently under study by the lab. This page is under construction and will be updated before the end of the year to offer a short but accurate overview of the lab's competences in terms of studied topics.

Finite Element Acoustics

Finite Element Acoustics

Low and high resolution SAM

BioMedical Research

BioMedical Research

Non-Linear Acoustics

Non-Linear Acoustics

Coda Waves

Coda Waves

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…

Ultrasonic Diffraction - anisotropic media

Ultrasonic Diffraction - anisotropic media

Room Acoustics - Alvar Aalto's discussion room

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 built in 1933-35, where the wave-shaped ceiling has been especially 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. The corrugated ceiling has therefore been constructed to distribute sound optimally, at least from the point of view of a ray theoretical analysis. The numerical study presented here shows that the corrugation is such that the ray approach is not exactly valid due to diffraction effects that must be incorporated as well. A detailed description of the sound distribution as a function of the position of the sound source and the receiver is presented in a recently published paper for different situations including the exact configuration of Aalto’s auditorium. The used approach is constrained in terms of frequencies by the corrugation dimensions of the auditorium. Practically it means that the theory can predict correct results up to 800 Hz, which is 70% of the frequencies commonly of interest in speech or music. Additional effects caused by windows, pillars near the windows and downstand beams hidden above the ceiling are not considered. The acquired knowledge is important for future construction of similar rooms.

Anisotropic Media - composites

Anisotropic Media - composites

Anisotropic Media - Wood

One of the most common biological composites is wood material. This natural orthotropic like material is characterized by a high anisotropy determined by the special 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 the 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 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 for a combination of both. The birefringence of shear waves have a particular interest for the fine definition of anisotropy. A more global appreciation of wood anisotropy than the values of individual velocities is given with acoustic invariants. The stability of calculation of acoustic invariants versus different propagation angles confirms the validity of the chosen model for the tested material. Wood species having high density and any important organized structure in the millimeter scale exhibit a high ratio of invariants. The acoustic behavior of tropical wood species is less anisotropic than that of species from a temperate zone having low density.

Voichita Bucur, Nico F. Declercq, "The anisotropy of biological composites studied with ultrasonic technique , Ultrasonics 44 (1), 829-831, 2006

Anisotropic Media - Minimum Variance

Ultrasonic guided waves are capable of rapidly interrogating 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 artifacts 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. This paper reports the application of minimum variance imaging under slightly inaccurate model assumptions, such as are expected in realistic environments. Specifically, 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 not only slightly anisotropic and dispersive but also supports multiple propagating modes, and additionally, the scattering field is dependent on incident angle, scattered angle, and frequency. An isotropic propagation velocity is estimated via calibration prior to imaging to implement the non-dispersive model assumption. Imaging performance is presented under these inaccurate assumptions to demonstrate the robustness of minimum variance imaging to common sources of imaging artifacts. This work has recently been published: James S. Hall, Peter McKeon, L. Satyanarayan, Jennifer E. Michaels, Nico F. Declercq, and Yves H. Berthelot, „Minimum variance guided wave imaging in a quasi-isotropic composite plate“, Smart Materials and Structures 20, 025013 (8p), 2011

Anisotropic Media - Polar Scans

Polar scans are used to assess damages in composites. Damage estimation in composites has increased complexity due to their anisotropic nature. In the first part of the study, an analytical model was developed in Matlab(R) 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 as homogeneous for the considered frequencies and that the patterns appearing in the scan are due to known acoustic phenomena. In the second part of the study, the damage in two CFRC samples that were subjected to tensile and bending loads till failure respectively, were imaged using the polar scan technique. The use of time of flight based estimation of damage is introduced in this study and is used successfully to complement the damage estimation using the earlier developed amplitude based technique.
reference: L. Satyanarayan, John. M.Vander Weide, Nico. F. Declercq, „Ultrasonic Polar Scan Imaging of Damaged Fiber Reinforced Composites“, Materials Evaluation 68(6), 733-739, 2010

Anisotropic Media - Piezo-electric crystals

Anisotropic Media - Piezo-electric crystals

Electrets for Ultrasonic Transduction

The reasearch on electroactive foams is primarily guided by Prof Yves Berthelot. Declercq's team is mostly 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 is a need to develop a reliable, fast, accurate, non-destructive 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 air-coupled ultrasonics (ACU) mainly because it eliminates the need for a couplant and hence in situ scanning and real time testing becomes possible. The recent discovery of a new class of piezoactive polymer foams is a very promising avenue to develop new air-coupled ultrasonic transducers, because of their excellent impedance matching with air compared to traditional transducers.\pard\pardThis project aims 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. A scanning electron micrograph of a cross-section can be seen at left. The literature on these new foams indicates that the electromechanical coupling is not well understood. Predictions are based on a crude 1D model. To provide a better understanding of the foams and their performances as sensors and actuators, a 3D-model has been developed to predict the macroscopic electro-mechanical coupling of the material as a function of the microstructures (size, orientation, and geometry of the micro-inclusions). The model is based on effective medium theory. An interesting prediction is that the d33 piezoelectric coupling coefficient drastically increases when the eccentricity of the micro-inclusions increases thus providing useful guidelines for the manufacturing of the foams.project also focuses on the experimental characterization of electro-mechanical coupling of the polymer foams using a laser Doppler vibrometer to measure the normal surface displacements [6]. Figure 1 below shows the broadband response of the film, flat up to 250 kHz, with a resonance around 300-400 kHz. Ultrasonic C-scans of the polymer films show that there are regions with fewer inclusions, which leads to a locally lower value of the piezoelectric coupling coefficient d33, as measured by the laser vibrometer and shown in Figure 2. Characterization of the heterogeneity is important in the design of high-performance transducers




Acoustic wave propagation in largely dislocated crystals; A novel high resolution damage characterization method for Multilayered Composite Crystalline Materials.
This project is done in collaboration with Laurent Capulongo

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