Plenary speakers

Title: The Return of the Helmholtz-resonator: A Story of Acoustic Metamaterials and Advanced Acoustic Resonators

Abstract:

First conceptualised in the 19th century by Hermann von Helmholtz, the Helmholtz-resonator (HR) has been a staple in acoustical engineering with various applications, e.g., as tonal sound absorbers in rooms, duct silencers, or in ported loudspeaker enclosures. Owing to their long history and the well understood physical principles, most of the scientific community in acoustics moved on from HR-related research in the late 20th century. However, with the emergence of acoustic metamaterials (AMMs) in 2000 and the discovery that HR-based AMMs can be used to, for example, achieve negative bulk modulus, a whole new chapter was opened in the history of Helmholtz-resonators. This presentation aims at providing an overview covering the most recent innovative developments in HRs, with a focus on advanced sound control using acoustic metamaterials and smart acoustic resonators. First, the basic underlying physical principles of HRs will be introduced, alongside a summary of common misconceptions and pitfalls in the design of HR-based acoustic systems. Then, the presentation will highlight recent developments in acoustic metamaterials using HRs to achieve broadband subwavelength absorption or enhance reduction of sound transmission. The final part of the presentation will focus on advanced acoustic resonator designs, based on the HR principle, which offer enhanced functionality compared to conventional HRs, such as HRs combined with structural resonators, active HR absorbers, and smart self-tuning HRs.

Curriculum Vitæ:

Dr Felix Langfeldt is Associate Professor and Doctoral Programme Director at the Institute of Sound & Vibration Research (ISVR), University of Southampton. Felix did his PhD in Acoustical Engineering (2018) at Technical University Hamburg, with a thesis on “Membrane-type acoustic metamaterials for aircraft noise shields”. Since then, his research focuses on innovative technologies for low-frequency sound control, involving lightweight acoustic metamaterials, Helmholtz-resonators, and active control. He joined ISVR in 2021, funded by a Walter Benjamin fellowship from the German Research Foundation, and became an academic in Southampton in 2022. Felix is also the Co-Lead of the Special Interest Group on “Acoustic Metamaterials” in the UK Metamaterials Network (UKMMN+).

Title: Multi-scale approach to analysis and design of subwavelength mechanical and acoustic metamaterials on finite size domains

Abstract:

This talk will present the recent advancements in the computational homogenization techniques for modelling elastic and acoustic wave propagation in locally resonant metamaterials on finite size domains in both frequency and time domains, including transient regimes. I will start by describing the main idea of the transient computational homogenization approach that allows the direct simulations of wave propagation and attenuation on finite size domains. The general approach is suited for arbitrarily complex unit cell geometries and material behaviour, including material and geometrical non-linearities. As an example illustrating the emergent behaviour due to non-linearities, energy transfer by an auto-parametric resonance from propagative to evanescent wave will be shown. Next, extensions and applications of the approach will be presented for visco-elastic (lossy) metamaterials, porous materials including fluid-solid interaction and acoustic labyrinthine metamaterials.

Curriculum Vitæ:

Varvara Kouznetsova is an Associate Professor of Multi-scale Solid Mechanics at the Department of Mechanical Engineering of Eindhoven University of Technology. She holds a degree in Applied Mathematics from Perm State Technical University, Russia, and a PhD in Mechanical Engineering from Eindhoven University of technology. Her research focus is on the development of multi-scale techniques applicable to various materials, ranging from high-strength steels to metamaterials, and related physical phenomena, emerging from non-trivial interactions across different space and time scales.

Title: Wave propagation across time-varying interfaces

Abstract:

The metamaterial field has strongly expanded recently with the introduction of the time variation of the mechanical properties referred to as Space-Time metamaterials and which offers new possibilities for waves control. While research has largely focused on modulated materials with bulk properties dependent on both space and time, we rather focus in this work on time-modulated interfaces. More precisely, we consider wave propagation in a 1D medium containing interfaces whose jump conditions are modulated in time. In the case of a single interface, properties of the scattered waves are investigated theoretically and numerically: energy balance, generation of harmonics, impedance matching and non-reciprocity. In the case of a periodic network, low-frequency homogenization is performed for different regimes of the frequency of modulation. When the frequency of modulation is low or moderate, standard homogenization is applied and ends up with a reciprocal effective model with time-dependent effective coefficients. For time-periodic modulations, the occurrence of gaps in wavenumber is illustrated. In the regime of high-frequency modulation, homogenization with a fast-time scale leads to an effective model with constant effective parameters but with a Willis coupling term breaking reciprocity. Comparisons with time-domain simulations illustrate the findings.

Curriculum Vitæ:

Marie Touboul is a CNRS researcher at the POEMS Laboratory (CNRS–Inria–ENSTA Paris, Institut Polytechnique de Paris). Her work focuses on the mathematical modeling and analysis of wave propagation in complex and structured media. She develops and studies models that describe how waves interact with materials and interfaces, with particular attention to time-domain simulations and multiscale effects. Her recent projects involve high-frequency homogenization, time-modulated systems, and sensitivity analysis in acoustic and elastic settings.

Title: Loss-compensated non-reciprocal wave scattering with synchronized self-oscillators

Abstract:

Non-reciprocal wave transmission through linear resonant cavities exhibiting broken time-reversal symmetry is usually impeded by losses. To avoid the attenuation of the transmitted waves, and even to amplify them, one can equip the resonant cavities with saturable gain. This nonlinear gain turns them into self-oscillators, which can be synchronized by the incident waves. When the conditions for synchronization are met, the interferences between the scattered field and the synchronized radiation from the self-oscillating cavities can result in non-reciprocal wave scattering without net power losses. This nondispersive concept does not require actuators and active sensing of the incident field. It is simply based on the adjustment of the cavity gain. A nonlinear extension of the temporal coupled-mode theory is derived to model this synchronization-based concept of loss-compensated non-reciprocal scattering, and it is experimentally validated with acoustic diodes and circulators.

Curriculum Vitæ:

Nicolas Noiray is Associate Professor at ETH Zürich, where he established the laboratory of Combustion, Acoustics & Flow Physics (CAPS) in 2014. He obtained his Ph.D. from the Ecole Centrale Paris in 2007, and then worked in the Gas Turbine Research Division of Alstom until his appointment at ETH. His theoretical, experimental and computational research activities in the fields of Combustion, Acoustics and Fluid Mechanics address fundamental and applied problems. He was awarded a Consolidator Grant and a Synergy Grant by the European Research Council. A key theme of the research performed by his group is the modeling and control of instabilities at various time and length scales.