Manipulating nonlinear sound waves using non-Hermiticity and active control. Nonlinear and Active Sound Absorption

Project Information

NASA

Grant agreement ID: 101077954

DOI

10.3030/101077954

EC signature date 19 December 2022

Start date 1 March 2023

End date 29 February 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 449 935,00

EU contribution

€ 1 449 935,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - NASA (Manipulating nonlinear sound waves using non-Hermiticity and active control. Nonlinear and Active Sound Absorption)

Reporting period: 2023-03-01 to 2025-08-31

Summary of the context and overall objectives of the project

The project explores how to absorb and control intense sound waves by:
1. Understanding Nonlinear Effects: Developing accurate models that describe how strong sound waves lose or exchange energy, create new frequencies, or exhibit unpredictable behaviors.
2. Designing New Absorbers: Creating and testing miniature structures that absorb a wide range of sounds (20 Hz–20 kHz).
These include:
• Helmholtz Resonators, efficient passive absorbers used in aviation and heavy industry.
• Electroacoustic Resonators, which use microphones and loudspeakers as smart, tunable elements.
• Thin Efficient Absorbers (Wire Meshes), lightweight structures offering improved sound dissipation and easy integration with active systems.
3. Active Control: Reininjecting electrical signals to counteract nonlinear effects and fine-tune the acoustic response for maximum efficiency.


By the end of the project, prototype “smart” absorbers are tested under realistic conditions. These devices perform better than current technologies, especially for strong and variable noise. The results have the potential to lead to quieter aircraft engines, safer industrial systems, and more comfortable cities. Beyond acoustics, the findings inspire advances in other fields where waves play a role — such as electromagnetism, elasticity, and quantum systems

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During this period, the project makes major progress in understanding and controlling how strong, low-frequency sound behaves in complex acoustic systems. The work combines advanced modelling, controlled experiments, and new device designs to explore how nonlinear effects and active control can be used to absorb, guide, and manipulate intense sound in air with unprecedented precision.

1. Nonlinear Waves and Acoustic Metamaterials

The project studies the propagation of intense low-frequency waves in engineered acoustic structures that respond nonlinearly when subjected to large pressure amplitudes. Using new mathematical models and dedicated measurements, the project reveals how energy redistribution, harmonic generation, and waveform distortion occur in air-filled resonant networks. By carefully tuning geometric and physical parameters, the project shows that these systems can guide and transform strong sound in a stable and predictable manner, even under where standard linear acoustics techniques fail.

2. Active Control

A central achievement is the development of a compact experimental platform that allows sound to be transmitted in one direction only — achieving acoustic nonreciprocity at low frequencies. By coupling active elements (such as loudspeakers) through asymmetric electronic feedback, the project aims at demonstrating several key effects:
• amplification of incoming sound from one side, while strongly blocking it from the other
• nearly perfect absorption from one direction, reaching high efficiencies
• and surprising phenomena such as simultaneous realization of coherent perfect absorption and acoustic lasing (spontaneous emission of sound) in the same system.

3. Perfect and Nonlinear Absorption

The project extends the concept of critical coupling — the condition for perfect sound absorption — to transient and nonlinear regimes relevant for intense sources. The precise treatment and canceling to noise at high pressure levels is a big challenge of this project, in terms of modeling and the taming strong sound,

These results are supported by detailed modelling of nonlinear dissipation mechanisms in air-filled Helmholtz resonators and electroacoustic absorbers.


4. Engineered Acoustic Lattices and Wave Manipulation

The project further investigates how geometry and controlled time modulation shape low-frequency wave behavior in structured air-based systems. The designs demonstrate tailored absorption in asymmetric networks, isotropic propagation in two-dimensional acoustic lattices, and controllable dispersion in time-varying media. These findings show how low-frequency sound can be sculpted with a degree of control previously reserved for higher-frequencies.

Tese achievements open new routes for designing smart acoustic materials capable of managing intense low-frequency noise and vibration in air, with applications ranging from industrial noise reduction to next-generation acoustic wave technologies.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The NASA project achieves major advances in controlling how sound waves behave in complex environments. By combining active electroacoustics, innovative material design, and concepts from modern wave physics, the research reveals new ways to absorb, direct, and manipulate intense sound with high precision.

1. Active and One-Way Control of Sound

A central achievement is the experimental creation of a compact device that allows sound to travel in only one direction. Using two loudspeakers connected through electronically tuned feedback loops, the team demonstrates striking effects:
• Directional amplification: waves arriving from one side are amplified, while those from the opposite direction are blocked (up to 42 dB isolation).
• One-sided absorption: sound coming from one direction is almost fully absorbed (up to 96%), while waves from the other are reflected.
These experiments link acoustics with topological physics, showing that sound can mimic exotic wave behaviors previously observed only in quantum and optical systems.

2. Nonlinear sound

The project investigates how acoustic geometry and resonant design influence nonlinear sound propagation in air. Engineered structures composed of air-filled cavities, ducts, and subwavelength resonators exhibit strong amplitude-dependent responses, enabling the formation of stable, reshaped waveforms that maintain their structure even at high sound levels. In two-dimensional acoustic networks, the inclusion of tuned resonators produces nearly isotropic propagation, allowing circular and ring-shaped wave fronts to emerge. Theoretical developments further introduce strategies for designing customized perfect absorbers capable of capturing all incoming low-frequency sound through tailored incident wave profiles.

The project investigates how acoustic geometry and resonant design influence nonlinear sound propagation in air. Engineered structures composed of air-filled cavities, ducts, and subwavelength resonators exhibit strong amplitude-dependent responses, enabling the formation of stable, reshaped waveforms that maintain their structure even at high sound levels. In two-dimensional acoustic networks, the inclusion of tuned resonators produces nearly isotropic propagation, allowing circular and ring-shaped wave fronts to emerge. Theoretical developments further introduce strategies for designing customized perfect absorbers capable of capturing all incoming low-frequency sound through tailored incident wave profiles.

Together, these breakthroughs redefine how intense air-borne sound can be controlled and absorbed, paving the way for smarter, more efficient technologies in noise reduction, acoustic metamaterials, and advanced wave-control systems.