Acoustic metamaterials and phononic structures

Acoustic metamaterials are artificial structures that present new striking and exotic properties that are not present in conventional or natural materials. Using the concepts of strong dispersion and slow-sound, negative compresibility or negative mass density, these new materials can be designed to obtain interesting properties such as perfect sound absorption, zero-sound transmission, focusing or specific scattering patterns.


Perfect acoustic absorbers

Artificial materials with strong dispersion can be designed by using locally resonant structures. Here, I'm working in the design of perfect acoustic absorbers composed by distributions of Helmholtz resonators. In the slow sound regime, the wavenumber inside the structures can be drastically increased, leading to the posibility of reducing the total thickness of the structure. Moreover, the tuning of the thermoviscous losses allows to critically couple the structure with the exterior medium, leading to perfect absorption.

In the top image, we present the absorption and the refelction in the complex plane of a perfect absorber panel working at 350 Hz. The panel is only 1.1 cm thick. Thus, the structure perfectly absorb an incoming acoustic wave with a wavelength 88 times longer than the panel thickness.

Moreover, slow sound can be generated in a panel in transmission, and using the Fabri-Perot resonances to efficeintly absorb sound.

pdf link doi Ultra-thin metamaterial for perfect and quasi-omnidirectional sound absorption
N. Jiménez, W. Huang, V. Romero-García, V. Pagneux, J. Groby,
Applied Physics Letters, 109 (12), pp 121902, (2016)

pdflinkdoi Quasi-perfect absorption by sub-wavelength acoustic panels in transmission using accumulation of resonances due to slow sound
N. Jiménez, V. Romero-García, V. Pagneux, J. Groby,
Phys. Rev. B, 95, pp 014205, (2017)

pdflinkdoi Broadband quasi perfect absorption using chirped multi-layer porous materials
N. Jiménez, V. Romero-García, A. Cebrecos, R. Picó, V. Sánchez-Morcillo, L. G. Raffi,
AIP Advances, 6, pp 121605, (2016)

Focusing devices

By the spefific control of the anisotropic dispersion of spacially modulated media and metamaterials, the phase of the transmitted field can be specially tailored to produce focusing and beam generation. Here, I'm working on focusing and vortex beam generation by periodic structures and metamaterials.

pdf link doi Nonlinear focusing of ultrasonic waves by an axisymmetric diffraction grating embedded in water
N. Jiménez, V. Romero-García, R. Picó, L. M. García-Raffi, K. Staliunas,
Applied Physics Letters, 107, pp 204103, (2015)

Scattering control by metasurfaces

The phase of the reflected waves by an spatially modulated media can also be tailored to obtain an specific phase pattern, leading to the precise control of the scattering of acoustic waves. Using metasurfaces based on Helmholtz resonators ultrathin sound diffusers can be designed. Another innovative application of this phenomena is to reduce the acoustic load and vibrations in the launch-pad during the litft-off of a rocket. We are appliying these concepts in collaboration with the European Space Agency (ESA)

pdf link doi Metadiffusers: Deep-subwavelength sound diffusers
N. Jiménez, T. J. Cox, V. Romero-García, J. Groby,
Scientific Reports, 7 (1), pp 5389, (2017)

pdf link doi Acoustic load reduction at lift-off by using sonic crystals at the launch pad
L. M. García-Raffi, L. J. Salmerón-Contreras, I. Herrero-Durá, R. Picó, J. Redondo, V. Sánchez-Morcillo, A. Cebrecos, N. Jiménez, V. Romero-García, K. Staliunas,
Submitted draft, -, pp -, (2016)

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