Internships

Gabriela Czuprynska

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.

Nazan Erdogan

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.

Michal Kubicki

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.

Marco Ribera

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.

Dmitry Solodov

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.

Nadeen Ayyash

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.

This internship has led to the research article
Zeighami Farhad, Quqa Said, De Ponti Jacopo Maria, Ayyash Nadeen, Marzani Alessandro and Palermo Antonio Elastic metasurfaces for Scholte–Stoneley wave control, Phil. Trans. R. Soc. A.38220230365, 2024.

Hossein Heidrai

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.

Maria Perez

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).

Julio Rangel

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.

Naeem Ullah

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.

Teodor Wolter

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.

Marwa Al-Esseili

4a Manufacturing GmbH, under the supervision of Patrick Hergan, and Paulo Amado Mendes (Univ. Coimbra)

The development of high-quality loudspeakers is crucial for achieving superior audio ex- periences. Central to this technology are the diaphragms, such as tweeter and compression driver domes, which play a vital role in sound reproduction. Traditional materials, while effective, often face limitations in balancing acoustic performance and mechanical durability. To address these challenges, this thesis focuses on the exploration and optimization of com- posite materials, specifically carbon fiber reinforced polymers and aluminum composites, aiming to enhance both sound quality and material resilience.
Based on a combination of experimental research, analysis of secondary data, direct ob- servation and discussion with experts, this thesis investigates the impact of advanced com- posite materials on loudspeaker performance. Rigorous tests were employed, including the Impulse Excitation Technique to characterize mechanical properties such as Young’s mod- ulus and bending stiffness. Additionally, acoustical tests were conducted to assess breakup frequencies and analyze sound radiation behavior across the frequency range. The study also integrates principles from signal processing and materials science, deepen the under- standing of the relationship between material properties and acoustic behavior.
Key findings from this study highlight that pre-impregnated fiber simplifies the mate- rial preparation process, ensuring consistent quality. The anisotropic nature of carbon fiber plays a significant role in its acoustic performance. However, it’s crucial to acknowledge that multiple material properties and setup conditions can influence the measurements. For future research, isolating each factor will enable a more detailed comparison and under- standing of their individual impacts.

Fawad Ali

Laboratory of Mechanics and Acoustics, under the supervision of Cédric Maury, and Teresa Bravo (CSIC Madrid)

Low-frequency noise mitigation presents substantial challenges due to the inherently long wavelengths, which require bulky acoustic devices for effective solutions. Unaddressed, this noise detrimentally impacts both human and ecological systems, for instance noise emission from a boat engine not only disturbs nearby environment but also disrupts marine bioacoustics. Among strategies like active noise cancellation and dissipation devices, Acoustic Black Hole (ABH) technology has emerged as a novel solution. ABH phenomena, first observed in tapered elastic beams, exploit the reduction in flexural waves propagation speed to near-zero at the beam’s apex, a result of gradual spatial reduction in bending stiffness. This principle can be replicated in acoustic wave propagation by employing metamaterials to progressively reduce compressibility, thus enabling wave trapping with negligible reflection. While ABH offers potential for efficient HVAC and automotive mufflers, it introduces challenges including increased drag and limited performance in low frequency range.
The current thesis addresses these issues by incorporating an optimized microperforated panel (MPP) in the ABH structure to enhance dissipation across 200-800 Hz range, simultaneously reducing drag. Aeroacoustic performance in the presence of mean flow is also considered. The research is segmented into three key objectives: Firstly, drag analysis within introduced design parameters using Computational Fluid Dynamics (CFD) tools like COMSOL Turbulent Flow and ANSYS Fluent. Secondly, enhancement of low-frequency acoustic dissipation using the Finite Element Method. Finally, aeroacoustic evaluations and time-domain acoustic analysis under 30 m/s mean flow conditions are conducted. For drag calculations, the study employs solving conservation equations within a hexa-structured computational domain, applying fully coupled numerical schemes for efficient convergence. To account for circulating and jet flow, the Reynolds-averaged Navier–Stokes equations are resolved using the Realizable k − ϵ model. Similarly, the acoustic evaluations focus on the dissipation curve area, derived via frequency-domain linearized Navier-Stokes equations, accounting for visco-thermal effects. Also, the aeroacoustic impacts and time-domain acoustic analyses are obtained by using broadband noise source model and implicit time-domain CFD, with turbulence handled via Large Eddy Simulations (LES) and the Wall-Adapting Local Eddy (WALE) subgrid scale model, respectively. High-fidelity interpolations, primarily Second-Order Upwind except in high-gradient scenarios where Third-Order MUSCL schemes are applied to ensure numerical stability and accuracy.
Findings indicate that high perforation ratio adversely affects drag performance, while the acoustic dissipation patterns are significantly influenced by Helmholtz resonance and ABH effects. Aeroacoustic analyses reveal MPPs as primary contributors to noise, with distinct tonal peaks at 860 Hz due to resonant cavity interactions. In mean flow scenarios, the MPP-ABH model’s transmission loss peaks shift to higher frequencies with reduced amplitude and flattened curve, demonstrating the flow’s impact on acoustic behavior. An optimal ABH configuration with a 15 cm square channel, fifteen cavities each 9 mm wide, separated by 1 mm spacing, following profile (M5), and 0.5 mm thick MPPs of 0.7 mm diameter and 4.5 mm spacing, was identified, offering manufacturability and enhanced performance.

Viktoriia Boichenko

GEOMAR, under the supervision of Prof. Jens Greinert, Dr. Mario Veloso and Christian Kanarski

This master’s thesis focuses on developing a theoretical model for the acoustic frequency response of multi-bubble compounds, such as air bubbles or methane gas bubble seepage from natural reservoirs. The objective is to simulate and analyze the acoustic behavior of these compounds in water columns and to reconstruct the acoustic response of the received bubble signals. The research begins with a review of existing single- and multi-bubble acoustic response models, integrating these with dynamic bubble movement and multiple scattering effects between bubbles. Initial models are simplified and progressively refined based on empirical data from previous studies of Anderson and Thuraisingham formulas for the backscattering strength of a single bubble. Furthermore, the model is extrapolated for multiple bubbles involving the multiple scattering effect. Experiments is conducted in a water tank with a bubble generator to estimate multi-bubble responses, with data used to try to validate and adjust the theoretical model. The final model can be incorporated into existing simulation software, such as KiRAT, to demonstrate its application as a digital twin for underwater column simulations and for generating SONAR data to train AI systems.

Yu-Lin Lai

Tymphany Acoustic Technology, under the supervision of Jessica Wild, and Prof. Francisco Castells

Nowadays, there is a wide range of electroacoustic transducers on the market, making it challenging for products to stand out to consumers. This work focuses on the design and optimization of an automotive subwoofer which emphasizes features such as a shallow mount, excellent acoustic performance, and competitive cost.
The goal is to develop an 8-inch, 2 Ω subwoofer with a 1.5-inch motor, capable of handling around 150W of power. The target acoustic performance includes an sound pressure level (SPL) of 87 dB, a BL factor of over 7.6 (T·m), and an Fs of approximately 40 Hz. Initially, simulations using FINEMotor software were conducted to optimize the component design for good acoustic performance. Next, more advanced simulations were performed using COMSOL Multiphysics, including detailed sound pressure level (SPL) curves, impedance curves, BL curves, and Kms curves, taking into account the detailed geometry of the subwoofer. Afterward, all components were confirmed with suppliers to ensure manufacturability and feasibility. The Thiele-Small (TS) parameters were recalculated based on theory and cross-checked with the simulations to validate the results.
Once the optimal sample based on the simulation was built, its acoustic properties were measured using Audio Precision and KLIPPEL equipment. The experimental results are then compared to the simulation outcomes to analyze performance and guide the development of next versions.
In conclusion, the experimental results generally aligned with the simulations, achieving the targeted acoustic performance. However, a dip in the SPL curve was observed, which, upon investigation, is due to the basket design and definitely requires improvement. Future work will focus on enhancing the SPL and reducing Total Harmonic Distortion (THD), and progressing to power and reliability testing to ensure a high yield ratio for this automotive subwoofer.

Karan Manoj

IGIC Research Inst. for Integrated Management of Coastal Areas, under the supervision of Dr. Victor Espinosa

The design of an acoustic underwater communication and positioning system for an Autonomous Underwater Vehicle (AUV) is always a challenge, due to the restrictions imposed by sea water as a propagation medium. The attenuation of acoustic signals by sea water is a function of frequency and also the environmental properties of the acoustic channel. In shallow water channels, acoustic signals are reflected by the sea floor and sea surface, and so the same signal may be received multiple times with different arrival times and amplitudes. These aspects must be considered when designing signals to be used for communication and positioning purposes.
In the present work, various types of signals were studied for use in underwater communication and positioning systems. As a novelty, sine waves modulated with Maximal Length Sequence (MLS) envelopes were proposed. They offer the possibility of encoding information in the envelope, and identification of the emitting beacon if each beacon uses a different carrier frequency.
A theoretical comparison of signals was performed, on the basis of their ease of detection in noise, and Signal-to-Noise Ratio (SNR). The influence of signal amplitude and duration on these parameters was evaluated. In addition effect of reflected signals on the received signal strength was also studied.
An analytical model was developed to calculate signal propagation and losses in an underwater positioning system. It uses the method of Received Signal Strength Indicator (RSSI), which estimates the distance of an emitter by its signal strength. To account for losses due to the sensitivity of each device in the system, they were first calibrated separately, and their calibration curves were incorporated into the analytical model. The predicted signal strength of this model was compared to actual measurements of the strength of signals propagated in the sea.
It was found that although chirp signals were the best for communication theoretically, they did not propagate as well as the novel MLS carrier signals did. This is because MLS carrier signals have most of their energy concentrated at their carrier frequency, so they can travel larger distances without significant attenuation. In contrast, chirp signals have their energy spread evenly across a frequency range, so they are prone to greater attenuation with distance. It was also concluded that Time Stretched Pulse (TSP) signals were the most resistant to interference from reflected signals, so they can be used for positioning, which requires a stable and predictable decrease of signal strength with distance.
The model developed in this work could be used for the preliminary design of an underwater communication and positioning system. It allows different signals to be tested, and effect of each device on the system performance can be understood. A new device can easily be introduced in the model by including its calibration curve.

Simon Masson

IGIC Research Inst. for Integrated Management of Coastal Areas, under the supervision of Dr. Victor Espinosa and Noela Sánchez (CONICET)

In this work, we investigate the application of new generation broadband echosounders for the seabed characterization. Single beam and multibeam echosounders are well known and widespread for bathymetric studies and for the sea bottom characterization. However, there are other types of echosounders that have an alternative approach based in their broad frequency content and which application have been only scarcely studied for seabed classification and object detection inside the sediments. These broadband scientific echosounders are more commonly used for fisheries acoustics. In collaboration with the Patagonic studies centre CONICET of Puerto Madryn (Argentina) and the Universitat Politècnica de València in Gandı́a (Spain), we investigate new signal analysis techniques and algorithms for sediment characterization, as well as the detection of buried objects in soft sediments. We consider here different models for the underwater acoustic propagation and the interface with the seabed. We include several losses due to absorption and spreading in the water. Then, we introduce different concepts related to signal processing that apply to our echosounder signals.
In particular, we detail how to deal with broadband signals where frequency-dependent corrections need to be applied. We also develop the deconvolution inverse problem. This lets us compute the impulse response of the seabed, without the transduction from the echosounder nor the propagation in the water column. The information of the seabed lets us obtain a characterization of the sedimentation content of the seabed as well as a detection of buried objects. The different steps of the process are detailed in one example, and then presented for measurements obtained during the study, in the ocean and from a boat. We summarize the results of this study with openings and perspectives for further investigations.

Monisha Muralidharan Menon

GeoAzur, under the supervision of Dr. Anthony Sladen and Dr. Diego Mercerat

Instrumentation of the sea-bottom is complicated and not widespread. The inability of electro-magnetic waves to propagate beyond a certain depth has made continuous monitoring a challenge. The varying pressure and temperature with depth presents difficulties that are not easily handled by instruments. With the use of DAS technology, these problems are partly overcome. Researchers are closer to understanding submarine structures and the composition of the sediments that cover the DAS cable.
Distributed acoustic sensing(DAS) is a technology that makes use of telecom optic fiber cables to record seismic and other dynamic events in its vicinity. The anomalies in the cable caused during cable manufacturing allows back-scattering of optical signals. The strain gradient along the cable due to variation in pressure or temperature, aids the back-scattered signal in recording events and understanding the environment the cable is laid on. This makes DAS a robust technology which can be deployed in harsh environments. DAS is currently being used in monitoring ship traffic, tracking and movement of marine mammals among many applications.
Over the course of the master thesis, the aim was to analyse the seafloor off the coast of Marseille. The study is conducted using an 81.5 km long telecom cable that was deployed by Alcatel Submarine Networks(ASN) as part of the 2Africa project from Marseille towards Africa.
The project aims to bring internet connectivity to parts of Africa by extending optical fibers using a series of optical repeaters. The telecom cable was recording events as part of testing and research for 2 months beginning october 2022 before the commencement of Telecom operations. The analysis of the seafloor is performed using this data. Since the start of recording, there have been earthquakes, UXO explosions and quarry blast events, that have originated at large distances of up to 320 kilometres from the cable. The seismic or hyrdoacoustics waves of these events were captured by the cable based on the intensity of magnitude of the event. 10 such events along with 2 windows of ambient noise recordings were analysed. Considering that the events generate low frequency waves, the recordings are studied in the frequency range of 2 Hz to 20 Hz. The data reveals the behaviour of the seafloor and the cable to different types of excitation and offers information on the source and the medium. The structure of the seafloor composed of sedimentary basins offers insight into hazards that may occur during high magnitude earthquakes such as ground motion amplifications and soil liquefaction. These hazards are noticeable in regions containing thick sedimentary deposits where site effects dominate. Three sedimentary basins were identified along the length of the cable. These structures are imaged and analysed in the time and frequency domain. Auto-correlation functions and power spectrum density are used to understand its characteristics.

Hedi Merhaben

Laboratory of Mechanics and Acoustics, under the supervision of Dr. Zine Fellah, Dr. Erick Ogam, Dr. Cédric Maury, Abdellah Bouchendouka and Christian Meso

This thesis investigates the acoustic characterization of porous materials, focusing on determining high-frequency parameters such as tortuosity and porosity. Conducted at the Laboratory of Mechanics and Acoustics (LMA) in Marseille, the study utilizes ultrasonic transducers to analyze various foams provided by an automotive industrial group. The primary objective is to develop and validate methods for accurately estimating these material parameters, which are crucial for applications in fields such as automotive sound insulation.
The research employs both direct methods and inverse problem approaches to extract the desired parameters from the transmission of ultrasonic waves through the materials. Significant emphasis is placed on the sensitivity of these methods to various experimental conditions, such as the angles of insonification. The results indicate that the tortuosity parameter can be reliably estimated with reasonable accuracy, with deviations between developed methods and inverse problem results ranging from 4% to 11%, depending on the material’s attenuation properties.
However, challenges remain in accurately determining porosity. The results for porosity were inconsistent and often outside the expected range, suggesting that the parameter’s sensitivity or negligible influence in the transmission mode may require further investigation, potentially in reflection mode or using alternative experimental setups.
Overall, this study provides a promising approach to estimating tortuosity in porous materials but highlights the need for further research to overcome the limitations encountered in porosity estimation.

Gustavo Nunes Queiroz

SLAM Acoustique, under the supervision of Philippe Schmid, and Prof. Luis Godinho (Univ. Coimbra)

When constructing buildings near railways infrastructures, assessing the vibration levels within these structures is crucial to ensure both safety and comfort. As the number of such buildings continues to grow, it becomes increasingly important to have a method for evaluating ground-borne vibrations that minimizes computational time. This study begins with a review of the fundamental principles of vibration transmission and the impact of these vibrations on buildings. Various predictive engineering approaches, including the FTA, SBB, BAM, CSTB, and DB models are analyzed highlighting their strengths and limitations in different contexts. The primary objective of this study is to implement a simplified method for calculating ground-borne vibrations in building structures efficiently. It focuses on the development and application of a 2D Finite Element Method (FEM) model that incorporates soil-structure interaction (SSI) as a spring-damped system to simulate the dynamic response of buildings to railway-induced vibrations. A key aspect of this research is the use of real-world vibration data, used as the excitation input in the model, specifically measured vibration data from a site in Saint-Cloud, France, where a building will be constructed 20 meters away from a tram line. To enhance the accuracy of the model, two different approaches, based on the works of Lopez et al. and Auersch for calculating stiffness and damping, are employed allowing for a more nuanced representation of soil-structure interaction. The FEM model is applied to various structural scenarios, ranging from simple two-story buildings to complex multi-story frames. The results of these simulations are compared with those from FTA empirical method.

Valerie Ritter

Müller-BBM Building Solutions GmbH, under the supervision of Dr Eckard Mommertz, and Prof. Paulo Amado Mendes (Univ. Coimbra)

Echoes, which are the audible repetition of sounds resulting from strong delayed reflections, present a significant challenge in the acoustic design of multipurpose halls. These echoes can disrupt speech intelligibility and diminish the overall acoustic experience, necessitating effective prediction and mitigation strategies. This thesis investigates methods to predict and eliminate rear wall echoes in multipurpose halls by employing a combination of room and electroacoustic simulations.
The study begins with an in-depth analysis of echo perception, incorporating psychoacoustic principles and various echo criteria, including those proposed by Dietsch and Kraak [1], as well as advanced loudness-based methods by Frey [2].
Simulations under varying conditions, including different rear wall materials and reverberation times, are used to identify key factors influencing echo generation. These simulations are validated through in-situ measurements in a real multipurpose hall, ensuring the reliability of the findings. The research highlights the critical role of reverberation time and rear wall acoustic treatments, demonstrating how varying levels of absorption and diffusion can effectively mitigate echoes.
Furthermore, the study evaluates the effectiveness of different post-simulation methodologies, comparing results using both the well-known Dietsch and Kraak echo criterion and sophisticated loudness-based methods. The latter offers more accurate echo prediction by taking into account the subjective perception of sound, making it a superior tool for assessing unwanted echo disturbances.
Finally, the findings offer practical strategies for optimizing acoustic design in multipurpose halls, particularly through tailored treatments for rear walls. This work enhances the theoretical understanding of echo mitigation and provides concrete solutions that acousticians and engineers can implement in the design and refurbishment of multipurpose spaces using a loudness-based model for realistic echo prediction.

Stelios Tsampourakis

LIS-lab, under the supervision of Prof. Hervé Glotin.

Conventional techniques for monitoring insects such as trapping, sweep netting and others are time consuming and prone to human error. More recently, acoustic techniques for monitoring insect populations have been successfully applied for species detection, population estimations and mappings of their distributions, while additionally providing the advantage of non-invasive monitoring and greater detection distance compared to traditional methods. Although limited to insects that emit sound, these techniques have shown increased popularity, especially due to advances including the rise of autonomous recording units (ARUs) which have led to Passive Acoustic Monitoring (PAM) techniques that allow for the monitoring of ecosystems at large temporal scales. The amount of data generated by PAM have introduced difficulties, which are currently being addressed by the rapidly evolving field of Machine Learning (ML), with Deep Learning (DL) techniques showing the way forward. The recently emerged field of Ecoacoustics includes all of the above under its umbrella, by combining traditional bioacoustic knowledge with advanced methods, forming a multidisciplinary bond between ecologists, acousticians, engineers and data scientists.
In this work, an experimental study was conducted in order to assess the effectiveness of automatic detection algorithms and localisation techniques, by utilising five time-synchronised recording units with GPS/PPS synchronisation. The annotation procedure involved four different insect species of the Orthoptera order (grasshoppers) by utilising the Raven Lite software. The classification of insect mixtures was attempted with up to three concurrent vocalisers of different species (interspecific) by utilising the YOLOv5s Deep Learning classifier model based on the Convolutional Neural Network (CNN) architecture, achieving good detection accuracy. A pipeline was then developed for unmixing the vocalisations of multiple sources through sound source localisation, by utilising two-stage localisation methods and a hyperbolic algorithm, which calculates the time delays of arrival (TDOAs) between pairs of microphones from their cross-correlation functions. This was achieved through the OpenSoundscape (OPSO) framework and a custom approach, showing promis- ing preliminary results.
The developed pipeline offers the advantages of modularity and scalability, allowing for its seamless integration with future experimental setups involving different a number of microphones, sources or even involving more species, and could even possibly generalise to simultaneously involve multiple taxa. Such techniques can help in monitoring animal populations and potentially be used for estimating animal density and abundance, in which occurrences of mixtures of multiple sources is often the norm.

Silvija Zvirblyte

Threshold Acoustics, under the supervision of Scott D Pfeiffer.

Architectural acoustics plays a pivotal role in the design and functionality of concert halls, where the quality of sound significantly impacts the audience experience. The geometry and shape of a concert hall are primary factors in determining its acoustic performance. These elements influence how sound waves travel, interact with surfaces, and ultimately reach the listeners ears. By carefully considering the halls geometries and shapes, architects and acoustic engineers can create environments that enhance sound clarity, richness, and spatial distribution, ensuring that every seat in the house enjoys optimal auditory conditions.
Before diving into acoustical context of concert halls I would like to highlight that music venues are not only about an architecture perspective of the space, but about music architecture from itself. The essence of this statement lies in the understanding that there is no definitive ’right’ or ’wrong’ way to design a concert hall. Rather, its suitability is contingent upon factors such as the musical genre being performed, the arrangement of instruments, stylistic considerations of the era, and numerous other elements. This research endeavors to explore the realm of architectural acoustics, with a particular empha- sis on concert halls. The objective is to gain a comprehensive understanding of the complexity involved in the design and optimization of concert hall environments as well as understand the difference in sound perception depending on the concert hall design.
Firstly, it will conduct a review of existing literature on architectural acoustics, focusing on acoustic fundamentals, anomalies, treatments, and design. Furthermore, the research will delve into the calculation methodologies and simulation techniques employed in the analysis and design of concert hall acoustics. This includes a comprehensive exploration of computational methods and specialized software tools utilized for modeling concert hall geometries and simu- lating acoustic properties. The research will investigate various acoustic treatment options and applications aimed at optimizing the acoustic environment within concert halls.