The fourth semester of the WAVES programme is dedicated to the Master Thesis. This thesis is prepared during an internship with a duration between 4 and 6 months. The internship is hosted either in a company or in a research lab in the acoustics and vibrations area, under the supervision of a local tutor and an academic tutor (part of the WAVES Pedagogical Committee).
The internship results in the writing of the Master Thesis which then defended by the end of September.
Universidade de Coimbra, under the supervision of Luis Godinho
Sound-absorbing materials enhance acoustic comfort by controlling reverberation and reducing noise levels. Porous materials like open-cell foams excel at absorbing medium to high frequencies but are less effective at lower frequencies, while resonant structures such as perforated panels offer durability and customization. Such panels normally use simple perforation shapes.
This study focuses on improving sound absorption by exploring panels with varied internal geometries such as tapered, hourglass, inverse-hourglass, perforations with rounded bubble-shaped extensions, and perforations with embedded resonant structures. The influence of rounding the edges and filling the air gap with porous materials is also investigated. An analytical model based on the transfer matrix method and a finite element model are implemented to analyse the systems. The analytical results show good agreement with the FEM results; however, an angle limitation in a tapered perforation case is identified, which can be due to the 1D simplification assumed in the TMM. FE models are used for examining the structures with embedded resonators. Parametric studies are performed to identify the influence of varying geometrical parameters on absorption.
A design method based on optimisation of the geometrical characteristics of a tapered panel is proposed. Three optimal samples with tapered perforations and one with an embedded torus-shaped resonator with arbitrarily chosen dimensions were 3D printed. The samples were experimentally tested using the procedure presented in ISO 10534-2. A good agreement between the predictions and measurement results was found.
Johana I. Dominguez
Arbane Groupe, under the supervision of Damien Jacquet
Sound spatialization can be achieved by making use of different panning techniques. NESS allows the user to model virtual sound sources based on a combination of DBAP and WFS, which assigns speciﬁc gains and delays to speakers. The software was used to design snapshots for a listening experiment conducted at the École Centrale de Nantes amphitheatre. The experiment design considered three different loudspeaker conﬁgurations and ﬁve different virtual source positions using three different auditory stimuli. The listeners were asked to sit at six different positions on the amphitheatre’s left or right side.
In this article, we show the results of the performance of two sound localization prediction models: TDM and PEM. When compared to real sound perception data from the listening experiment, we evaluated the predicted (models) or perceived (experiment) direction of the virtual source, width, and conﬁdence.
GeoEND laboratory (Gustave Eiffel Univ.), under the supervision of Odile Avraham and Pierric Mora
In the domain of concrete structures, the Nonlinear Coda Wave Interferometry (NCWI) technique emerges as a potent tool for crack detection and structural assessment. Departing from conventional methods, NCWI employs ambient noise for passive pumping, aiming to replace active pump sources and streamline experimental setups. This study aims to explore the feasibility of ambient noise as a passive pump source for NCWI, leveraging its sensitivity to heterogeneous medium changes to enhance concrete health monitoring and infrastructure assessment. Through meticulous data analysis, intricate behaviors driven by factors like temperature fluctuations, pump levels, and traffic noise are unveiled. Visual representations offer insights into velocity variation, decorrelation coefficient, and other dynamics, providing valuable information for crack detection and structural health evaluation. The study further dissects data to uncover nuanced patterns within NCWI measurements, distinguishing behaviors between crack and uncracked zones. Notably, correlations emerge between traffic noise peaks and variations in key parameters, warranting rigorous investigations into potential artificial influences.
This research underscores the potential of Nonlinear Coda Wave Interferometry, particularly in the context of passive pumping through ambient noise, as a promising avenue for precise crack detection and comprehensive understanding of concrete structures. This approach paves the way for advancements in nondestructive testing and infrastructure health monitoring.
Jose Karotte Sani
Institute for Hearing Tech. and Acoustics (RWTH Aachen Univ.), under the supervision of Lukas Aspöck and Pierre-Olivier Mattei
Simulation of sound propagation that is both physically reliable and perceptu-
ally plausible is a crucial requirement in the fields of room acoustic simulation and (real-time) auralization of Virtual Acoustic Environments. While the basics of sound propagation in indoor and outdoor spaces are widely investigated, simulation models are still not able to reproduce the acoustic perception of complex scenarios such as street noise with numerous static and dynamic sound sources and complex diffraction patterns. The thesis aims to utilise simulation tools developed at the Institute of Hearing Technology and Acoustics, RWTH Aachen University, to achieve a plausible simulation of outdoor sound propagation. Issues related to complex acoustic phenomena, such as scattering, absorption and edge diffraction, and optimal simulation parameters are being investigated.
In this study, a hybrid approach in Geometric Acoustics for room acoustic simulation techniques is adapted for outdoor propagation simulation. A combination of physically based synthesis of sound models and anechoic recording is used for source modelling. Propagation in an urban environment is simulated and auralized alongside a visual counterpart. The results of the auralization analysis have been examined, and suggestions for further enhancements have been proposed.
Research Institute for Integrated Management of Coastal Areas (IGIC, Polytech. Univ. Valencia), under the supervision of Rubén Pico and Javier Redondo
This research delves into the intriguing realm of wave propagation within time-modulated metamaterials, leveraging the FDTD method as the simulation tool of choice. It focuses on 1D simulations of wave propagation through a medium with time varyng bulk modulus. The analysis is undertaken by examining the generation of forward and backward waves, their spectral analysis using FFT, power distribution and insertion gain, nature of periodicity including dispersion and band diagrams using plane wave expansion and space-time diagrams showing the evolution of the waveforms in time.
Amplitude Acoustics, under the supervision of Rui Ribeiro and Paulo Amado Mendes
The PU probe technique allows in-situ surface impedance measurements of porous materials with minimal sample preparation and measurement setup. However, this technique comes at the cost of multiple additional factors, which can influence the results if not carefully addressed. In this work, a sensitivity analysis is performed for two different materials, Rockwool and Melamine foam, aiming to provide a set of guidelines on how to perform measurements with the PU probe technique regarding sample size, sound field model and probe location. Below 800 Hz, the influence of these factors proved to be significant. Nonetheless, results showed to be accurate at higher frequencies, yielding errors smaller than those obtained by the impedance tube, in reference to the Johnson-Champoux-Allard (JCA) and the Delany-Bazley-Miki (DBM) equivalent fluid models.
The PU Probe technique was then compared with the current standardized procedures by converting all measured normal incidence sound absorption data into random incidence absorption coefficients of infinite lateral dimensions. This was achieved by a model fitting procedure applied to both Impedance Tube and PU Probe normal incidence measurements. The model fitting procedure enabled the inverse estimation of the main non-acoustical macroscopic parameters of the materials employing the JCA and the DBM models, finding good agreement with the macroscopic parameters determined through direct methodologies. Furthermore, this procedure enabled obtaining broadband normal and diffuse field sound absorption coefficients from the PU Probe measurements. Excellent agreement was found between all measured and reference curves for both materials at normal incidence. In a diffuse field, despite the non-diffuseness found in the measurement chamber, the measured absorption after a size correction was found to be oscillating around the reference curve, from which good agreement between the Impedance Tube and the PU Probe techniques was found.
Rodrigo S. Motta
Laboratory of Mechanics and Acoustics (Aix-Marseille Univ., CNRS, Central Med.), under the supervision of Régis Cottereau and Cédric Bellis
In various branches of engineering, it is essential to rapidly and non-destructively examine whether a structure possesses a specific characteristic, such as a defect, like an inclusion
with distinct properties or a crack. For example, ultrasonic waves are commonly used to inspect gas ducts to detect cracks that could cause failure. In more complex scenarios, such as in petroleum engineering, specific structures like salt domes are actively sought after as they indicate the presence of rock formations that may contain oil reservoirs. This project aims to explore the potential of neural networks in assisting with classifying structures and determining whether they possess a given feature or not. One of the focuses is on understanding the optimal approach for inputting data into the neural network, exploring whether time series data should be directly fed without preprocessing or if preliminary steps can enhance the process. To achieve this, a collection of 2D computational simulations was generated to enable investigation, utilizing both defect-free and defective rectangular samples. Excitation and measurements were executed on the sample surfaces, similar to methods found in non-destructive evaluation. Subsequently, an ANN was created, having its performance, learning, and robustness analyzed.
This project took place in the Laboratoire de Mécanique et d’Acoustique in Marseille, France, and it had financial support from the Centre National de la Recherche Scientifique (CNRS). In addition to the building facilities of the laboratory, it was provided access to the Aix-Marseille University Mésocentre, a high-performance computing server, which contributed to more efficient calculations in this project.
In this report, the scientific context will be presented initially, followed by the description of dataset construction and the development of the artificial neural network (ANN). In the subsequent section, an analysis of the learning process and robustness will be conducted.
ReceNDT GmbH (Linz, Austria), under the supervision of Hubert Norbert
Additive manufacturing (AM) is an advanced fabrication process based on the sequential layering of materials, which is driving a transformative shift in modern manufacturing by creation of the components with enhanced geometrical complexity, reduced material waste, and improved structural integrity. AM of metal components, specifically using Wire Arc Additive Manufacturing (WAAM), involves heating of the metal to its melting point, leading to alterations in the material's chemical structure and creation of structural defects. Consequently, Non-Destructive Testing (NDT) for these materials requires both chemical analysis and physical evaluation through new non- traditional NDT techniques. This master's thesis investigates the suitability and precision of Laser- Induced Breakdown Spectroscopy (LIBS) for monitoring chemical defects in the produced material, while Laser Ultrasound (LUS) is employed to examine structural defects. The research aims at exploring the applicability and accuracy of LIBS and LUS as complementary approaches in the NDE of metallic samples. This thesis comprises four chapters, each addressing different aspects of the experimental methodology for NDE of metal components produced by the WAAM technique.
DICAM - Structural Mechanics (Univ. Bologna), under the supervision of Antonio Palermo
The objective of this study is to investigate how installing a metasurface would affect Scholte waves. A Scholte wave is a type of surface wave that propagates along a solid-fluid interface. Hence, it exists due to the interaction between the solid and fluid. It exhibits a sinusoidal displacement pattern, and it is a dispersive wave. Its dispersion behaviour is characterized by a frequency-dependant phase velocity; hence the different frequencies of the wave will travel at varying speeds.
A metasurface, is a structured material consisting of an array of oscillators engineered to control and manipulate surface waves either Scholte waves or Rayleigh waves by altering their propagation properties at subwavelength scales. The research body consisted of two parts, the dispersion analysis, and the harmonic investigation. Each part consisted of the same four scenarios: the Rayleigh waves, the Rayleigh waves interacting with a metasurface, the Scholte waves, and the Scholte waves interacting with a metasurface.
The first part, initially, the dispersion relation [Frequency vs. Wavenumber] for Rayleigh waves on a free solid surface showed linearity. However, the instalment of an oscillator array to the surface, namely, a metasurface, showed a notable difference to the dispersion relation. At lower frequencies, linearity persisted until the resonance frequency, beyond which wavenumber increased while frequency remained fixed.
After that, the dispersion relation for Scholte waves on a free solid surface showed nonlinearity in the lower frequencies, and linearity in the higher frequencies. This is when the base difference between Rayleigh waves and Scholte waves was observed. However, the instalment of a metasurface on the solid-fluid interface, affected the Scholte waves dispersion relation almost as much as it affected the Rayleigh waves dispersion relation. A key difference between the two dispersion relations was how the two waves reacted when they encountered the bandgap.
The second part, later, a frequency domain analysis to investigate the wave phenomena validated the dispersion plots findings, ensuring their coherence. The study additionally provided evidence of wave attenuation after the instalment of a metasurface, reaffirming the metasurface principle and verifying the emergence of a bandgap in Rayleigh wave-metasurface interactions.
Silicon Austria Labs GmbH, under the supervision of Javad Abbaszadeh
This study consists of two parts. In the first part, PZT Piezoelectric Micromachined Ultrasonic Transducers (pMUTs) were studied under the application of DC voltages. It was shown that their output sound pressure level was improved by 10.7dB and their mechanical
bandwidth increased by 53 per cent under a 20-volt DC bias. The second part of the
study focuses on the performance and application of AlScN pMUTs for object detection.
An elaborate characterisation process was performed on a set of pMUTs and the most
suitable one for air-coupled object detection was selected. The selected pMUT was used
with the Synthetic Aperture Focusing Technique (SAFT) for object detection.
This thesis starts by introducing the environment in which the work was developed. Then pMUTs will be introduced and their application for object detection will be discussed. Next, The major design elements of pMUTs will be elaborately discussed. Then, the results of both parts of this study will be presented. In the end, the conclusions and perspectives will be presented with some points for future work.
David P. Ortega
4a Manufacturing GmbH, under the supervision of Patrick Hergan, Paulo A. Mendes (Univ. Coimbra) and Domenico Foglia (Foliumtec)
Diaphragm materials for accurate sound reproduction in electrodynamic speakers have been a
central research topic since the loudspeakers were created. Nowadays, with the new improve-
ments in designs, manufacturing, and engineering, composite materials are used to outperform
traditional materials and improve acoustic radiation. Therefore, assessing their mechanical and
acoustic properties is essential to prove their advantages and make good product development
and manufacturing decisions.
This work is embedded in the context of composite materials and the main mechanical proper- ties directly responsible for their performance, such as the bending modulus or the loss factor. The main goal is to be able to measure such properties with an experimental setup relying on the impulse excitation technique and the free vibrations of a cantilever beam concept, which is described and compared to other existing setups. The setup is proven to be valid for test- ing specimens with low thicknesses and already provides results with less than a 10% difference compared to the Klippel MPM setup. Furthermore, as dealing with layered composite materials often involves using viscoelastic layers, the mechanical properties of these materials are believed to vary depending on the frequency of excitation. Thus, an additional step is taken to measure the dependency of the aforementioned mechanical properties on the frequency of excitation, as the acoustic application is always targeted. The development of this experimental technique is an ongoing work, only partially validated in this work. However, its problems are pointed out in the discussion section with a proposal for further improvements to the system and other possible techniques to measure these properties.
In the course of the experimental section, and as new testing setups are developed, COMSOL Multiphysics software package is used to prove the concepts before and after the setups are built, having a virtual equivalent model of the physical device which allows further improvement of these setups as well as comparisons and validations.
Univ. Arizona, under the supervision of Sami Missoum
The sound of a trumpet depends on various parameters such as the mouth pressure and
the dynamic properties of the lips of the player. The main objective of this project is to
explicitly identify the regions of the parameter space (i.e., the playing conditions) to play
in tune for a given range of notes.
In this work, a description of the sound generation mechanism in the trumpet is first pro- vided, where the lips act as a basic mechanical oscillator with a single degree of freedom. This oscillator is non-linearly coupled to the air column of the instrument by an airflow equation. All of these components combine to form a model consisting of three ordinary differential equations (ODEs) that must be solved simultaneously. By solving these equations, it becomes possible to obtain the pressure profile for various sounds generated by modifying some of the governing parameters in the acoustics of the trumpet. With the pressure profile it is possible to obtain information regarding the nature of the oscillation, like its frequency and periodicity.
Afterwards, a space of parameters is defined for study. Specifically, two parameters are deeply analyzed: the pressure in the mouth of the trumpet player and the frequency of their lips. By varying these parameters, a wide range of different notes can be produced, even without changing the configuration of the trumpet (i.e., without any of the pistons being touched).
CDM Stravitec, under the supervision of Reinhilde Lanoye and Marina Rodrigues, and Luis Godinho (Univ. Coimbra)
The purpose of this work is to propose and evaluate a methodology for drop
impact testing on dry, lightweight floating floor systems in laboratory, based on the state of the art in the building industry, and targeting approximation to the boundary conditions of an application in situ. The procedure was implemented by means of a mockup of the Stravigym XP system, using experimental data from in situ tests as reference, and general simulations to validate the results.
It is studied how the weight’s mass, shape, height of the drop, and type of impact contact (single-point, double-point) influence the behavior of the system. Given the lack of an standard methodology, we experimented with diﬀerent sources to generate insights on the system’s performance variations; the selection was made to mimic the most commonly dropped weights on gyms and training centers.
It is also addressed how to represent the existing loads of a gym (equipment, furniture, others) in the mockup. We handled by placing sand bags over its surface, which worked normally for the single-contact weights but, caused undesired interactions with the double-contact ones.
The analysis showed that the shape of the body is not relevant but the number of contact points and the stability of the bouncing are, to have results’ consistency when dropping from diﬀerent heights. Regarding repeatability, the approach of keeping the looseness of the top layers is counterproductive due to the soon displacement of the top layers; such feature is key for the system under test, hence the current observations sum to its study.
The current methodology generated relevant information about the functioning of the floating floor on a laboratory environment, as well as some points of comparison to a real escenario, provided the limitations of using diﬀerent slabs — making them two diﬀerent systems. Despite their diﬀerences, they do oﬀer insights to reach better correspondence in the future.
CDM Stravitec, under the supervision of Patrick Carels and Reinhilde Lanoye, and Andreia Pereira (Univ. Coimbra)
The installation of floating floors is a widely used practice to mitigate airborne and impact noise in buildings. Floating floors help mitigate impact noise and vibrations to any adjacent room, especially the room below, by isolating the impact source in the source room. Generally, during the design, both the floating floor (FF) and structural floor(SF) are considered rigid, which means that the floating floor is considered as a single mass supported by springs on a rigid support (structural floor). The system is treated as a single degree of freedom (SDOF) system and the transmissibility curve is used to predict the sound and vibration reduction performance of the installed floating floor; in such a system the mitigation starts at frequencies higher than √2 times the resonance frequency. However, in a lot of cases the floating floor and the structural floor could be flexible, with considerable bending modes influencing the performance of the floating floor especially at low frequencies, making the SDOF approach to overestimate the system performance considerably.
This thesis investigates the low frequency performance of a flexible floating floor installed over a flexible structural floor by using FEM by means of COMSOL Multiphysics. The effect of the thickness of structural floor on floating floor is studied and the results show that it can affect the isolation performance of the floating floor due to the contributions of its bending modes.
The stiffness of the bearings(springs) affects the performance of the floating floor as it does for the simplified SDOF system and is addressed in the thesis. Increasing the stiffness shifts the SDOF mode as well as some of the bending modes of the floating floor to higher frequencies and vice versa. Increasing the stiffness negatively affects the performance by decreasing the insertion loss and vice versa, at frequencies above the SDOF mode.
The negative effects of the bending modes, especially of the floating floor on the performance of the floating floor system can be avoided by arranging the bearings in a specific combination. Three such combinations are studied. The coincidence of bending modes from floating floor and structural floor can result in prominent negative effects in the performance of the floating floor and hence should be avoided.
A simplified 3-DOF system is also developed, to predict the behavior of the floating floor with results in close agreements to those from COMSOL. This simplified system can be used to predict the behavior of the system with three important resonance modes, using MATLAB instead of COMSOL, allowing to save computational time.
4a Manufacturing GmbH, under the supervision of Patrick Hergan and Domenico Foglia (Foliumtec)
The loudspeaker, a ubiquitous device integral to modern life consists of various components crucial to the success of its acoustic performance. Dating back to the early 20th century, the refinement of electrodynamic loudspeaker components has evolved significantly, driven by advancements in materials and signal amplification. The loudspeaker is made up of a conglomerate of parts whose collaborative goal is to transform an electric signal to sound. Among these components, the diaphragm plays a pivotal role in translating the voice coil’s movement into sound. This paper delves into the development of a novel aluminum-carbon-composite diaphragm material for high-fidelity (hi-fi) speaker applications. The research involved meticulous experimentation, encompassing material characterization, mechanical property testing, and the exploration of various fiber orientations. Additionally, parameter variation trials of an in-house thermal pressure mold were performed for elementary deformation of basic layups of the material. The study also explores the intricate trade-offs in diaphragm design, balancing parameters like stiffness, density, and damping. Through comparison between theoretical insights with empirical findings, this research contributes to the ongoing pursuit of high-performance loudspeaker diaphragms, working toward bridging the gap between acoustic theory and real-world applications.
Mohamed Chakib Drias
Saint Gobain Research, under the supervision of Matthieu Gallezot
This master thesis aims to improve the Mechanical Impedance Measurement (MIM) procedure used to evaluate the mechanical properties of plasterboards. The method involves using flexural resonances of a beam excited by a shaker to determine the equivalent Young's modulus and loss factor of gypsum boards. These properties can be frequency dependent. Finite element models are developed based on thin and thick plate formulations to enhance the identification of the equivalent Young's modulus. The standard BA13 plasterboard is used as a case study to examine the influence of the identification model on the measured Young's modulus and sound transmission loss simulations. The identification models are validated against measurements following the MIM procedure on the same panel, and the results are compared to small-scale testing data. The analysis is then extended to other plasterboard references, including standard and complex boards. The simulations reveal that certain assumptions in the MIM procedure (thin beam kinematic model and sample-shaker link boundary condition) are only valid under specific circumstances. The choice of kinematic model impacts the stiffness of the model and introduces variations in the eigenfrequencies. The use of a thin plate kinematic model results in overall stiffening of the model, while a thicker plate model shows softening at high frequencies. Simulating the link between the sample and shaker accurately proves challenging, especially for smaller samples. The fixed constraint assumption is no longer valid when using a thick plate model. Further investigations also show that added mass due to impedance head and attachment screw does not affect the impedance peaks used for characterization. The identification of Young's modulus for different plasterboard references confirms the influence of the identification model on the frequency-dependent modulus variation. Globally, using a softer kinematic model reduces the slope of modulus variation. The discrepancies between identification models become more significant with increasing board thickness. Simulations of sound transmission loss demonstrate that using an averaged value for Young's modulus following NF EN 16703 underestimates the critical frequency, particularly for thicker and more complex boards. Consistency between the identification model and the model used for simulating sound reduction index is necessary.