Acoustic metamaterials (AMMs) and metasurfaces (AMSs) are engineered structures aimed at achieving unprecedented acoustic properties not available in nature. However, the designs reported so far in the literature often rely on passive resonant structures that present inherent limitations, such as narrow frequency bandwidth of operation and damping precluding the effective novel properties. Active control concepts allowing tunability and reconfigurability have received a surge of interests in AMMs and AMSs designs over the last decade. It has also been shown that further complex wave manipulations are made possible when accurately introducing nonlinearities in such metamaterial constructions.
Very recently, a linear active impedance control scheme formerly developed at LTS2-EPFL has been further developed to achieve a novel active nonlinear control approach. Such control, applied to an Active Electroacoustic Resonato(AER) has been modified to achieve a Nonlinear Active Electroacoustic Resonator (NAER), allowing triggering interestingnonlinear behavior even at low sound pressure thresholds. The achieved NAER, with its sub-wavelength feature, allows improving sound absorption in the low frequency range. However, only cubic nonlinear functions of the diaphragm displacement (ie. nonlinear stiffness) have been achieved so far with such concepts. In a view to enriching nonlinearwave manipulations, different types of achievable NAERs need to be investigated, through the development of ad hoc control frameworks. In this project, different nonlinear active control methods, based on accessible acoustic quantities, are targeted. Considering the developed NAERs as the building unit cells, the main objective of the proposed project is to design active Nonlinear Acoustic MetaSurfaces (NAMS), aiming at different wave manipulations. In particular, the focus will be put on 2 different applications: the first one focusing on improving sound absorption at low frequencies, the second addressing nonreciprocal acoustic devices such as unidirectional transmission devices (diodes) and acoustic topologicalnonlinear insulators, in which cases the combination of nonlinearities and active control can either offer more degrees of freedom for exploring the desired wave phenomena (case of unidirectional transmission) or paves the way for research that has been left largely unexplored in acoustics (case of topological nonlinear insulators).
To this end, with a view to thoroughly analysing the constructed meta-atoms and metasurfaces, some performance criteria need to be rigorously defined at first, adapted to each specific nonlinear configuration. The development of simulation tools will also allow investigating and optimizating the various designs (from the unit cell NAERs to the whole NAMSs), that will be challenged against experimental results, and will also allow unveil the underlying physical principles. Prototypes will be constructed at each stage of development and will be experimentally assessed, through the defined metrics of interest. An iterative design loop with simulations and the early specifications will help refining and optimizing the final prototypes. The proposed design approach facilitates the development of active nonlinear AMMs and AMSs, targeting the improvement of functionalities of existing concepts, and their translation into advanced concepts in other fields to physics.
- X. Guo et al, Improving sound absorption through nonlinear active electroacoustic resonators, Physical Review Applied 13, 014018 (2020)
- X. Guo et al, PID-like active control strategy for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers, arXiv preprint (2021)
This project is funded by the Swiss National Science Foundation under grant No. 200020_200498.