http://ieeexplore.ieee.org/iel5/5/31047/01444179.pdf

Fundamentals of acoustics

Causal nature of the acoustic response, for any materials or structures, dictates an inequality that relates the absorption spectrum of the sample to its thickness. We present a general recipe for constructing sound-absorbing structures that can attain near-equality for the causal relation with very high absorption performance; such structures are denoted optimal. Our strategy involves using carefully designed acoustic metamaterials as backing to a thin layer of conventional sound absorbing material, e.g., acoustic sponge. By using this design approach, we have realized a 12 cm-thick structure that exhibits broadband, near-perfect flat absorption spectrum starting at around 400 Hz. From the causal relation, the calculated minimum sample thickness is 11.5 cm for the observed absorption spectrum. We present the theory that underlies such absorption performance, involving the evanescent waves and their interaction with a dissipative medium, and show the excellent agreement with the experiment.

Optimal Sound Absorbing Structures

Hybrid composite meta-porous structure for improving and broadening sound absorption

Early results have shown the potential of Deep Learning (DL) to disrupt the fields of optical inverse-design, particularly, the inverse design of nanostructures. In the last three years, the complexity of the optical nanostructure being designed and the sophistication of the employed DL methodology have steadily increased. This topical review comprehensively surveys DL based design examples from the nanophotonics literature. Notwithstanding the early success of this approach, its limitations, range of validity and its place among established design techniques remain to be assessed. The review also provides a perspective on the limitations of this approach and emerging research directions. It is hoped that this topical review may help readers to identify unaddressed problems, to choose an initial setup for a specific problem, and, to identify means to improve the performance of existing DL based workflows.

/pdf/deep-learning-a-new-tool-for-photonic-nanostructure-design-4gl1q07w82.pdf

Deep learning: a new tool for photonic nanostructure design

We introduce a multicoiled acoustic metasurface providing quasiperfect absorption (reaching 99.99% in experiments) at an extremely low-frequency of 50 Hz, simultaneously featuring an ultrathin thickness down to λ/527 (1.3 cm). In contrast to the state of the art, this original conceived multicoiled metasurface offers additional degrees of freedom capable of tuning the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation, and experimental demonstrations for this unique absorber concept, and discuss its particular physical mechanism. Furthermore, based on the same conceptual approach, we propose a broadband low-frequency metasurface absorber by coupling unit cells exhibiting different properties.

Extreme low-frequency ultrathin acoustic absorbing metasurface

A severe limitation of current acoustic metasurfaces remains in their modest tunability to meet multifrequency requirements and alterable functionalities on demand. Here, a reconfigurable curved acoustic metasurface for acoustic cloaking and illusion is reported. The structure is composed of an array of tunable helical units to break this limitation and realize continuously versatile sound manipulations. We theoretically, numerically and experimentally investigate the channel length represented by the helical depth, which is used to achieve full 2\ensuremath{\pi} phase shift continuously over the frequency range from 2 to 7 kHz. As pragmatic examples, we present by full-wave numerical simulations the concept of a curved metasurface for the continuously tunable acoustic multifunction, including broadband carpet cloaking and ground illusion at a wide working band. Then, we experimentally demonstrate these functionalities by showing an excellent effect to restore the disturbed reflective field from a cloaked object or to mimic an arbitrary shaped ground.

/pdf/reconfigurable-curved-metasurface-for-acoustic-cloaking-and-512f7xa4zk.pdf

Reconfigurable curved metasurface for acoustic cloaking and illusion

https://hal.archives-ouvertes.fr/hal-03043155/document

Pillared elastic metasurface with constructive interference for flexural wave manipulation

We introduce a multi-coiled acoustic metasurface providing a quasi-perfect absorption (reaching 99.99% in experiments) at extremely low-frequency of 50 Hz, and simultaneously featuring an ultrathin thickness down to {\lambda}/527 (1.3 cm). In contrast to the state of the art, this original conceived multi-coiled metasurface offers additional degrees of freedom capable to tune the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation and experimental demonstrations for this unique absorber concept, and discuss its physical mechanism which breaks the quarter-wavelength resonator theory. Furthermore, based on the same conceptual approach, we propose a broadband lowfrequency metasurface absorber by coupling unit cells exhibiting different properties.

Krupali Donda

Papers

Extreme low-frequency ultrathin acoustic absorbing metasurface

Reconfigurable curved metasurface for acoustic cloaking and illusion

Pillared elastic metasurface with constructive interference for flexural wave manipulation

Extreme Low-Frequency Ultrathin Acoustic Absorbing Metasurface

Ultrathin acoustic absorbing metasurface based on deep learning approach