Active Acoustical Metamaterials – METACTIF

METACTIF project aims at both conceiving and elaborating acoustical metamaterials exhibiting effective density and compressibility both negative in certain frequency ranges.
Acoustical metamaterials are heterostructures made of local resonators embedded in a homogeneous matrix. Their effective density and/or the compressibility become negative when the resonators vibrate out of phase with respect to an elastic wave propagating in the matrix. But for this to happen, two conditions are necessary:
First, the inclusions have to be much smaller than the wavelength of the elastic wave in order that homogeneisation approach makes sense. The second condition to be fulfilled is that the sound velocity in the material making the inclusions is much smaller (by at least two orders of magnitude) in comparison to the sound velocity in the matrix. This is only the case for very specific polymers.
This is the reason why we propose with METACTIF another approach. We intend to insert within the matrix, inclusions having perfectly controlled shapes and to enforce them to vibrate through an electrical tension if they are made of a piezoelectric material or through an ultra short optical pulse if they are made of a material exhibiting large photoelastic coefficients. If one is successful in simultaneously controlling both the normal modes of the inclusions and their phase, one will mimic the situation described above, the only difference coming from the external energy provided by the electrical tension or by the optical pulse necessary to put the inclusions into vibration.
Our approach will be twofold, theoretical and experimental.
The theoretical part of the project will consist in calculating by finite elements methods, the strains associated to the local resonances. Then we will simulate how is scattered an elastic wave propagating in a membrane or on the surface of a semi infinite medium, with inclusions embedded on them, when these inclusions are vibrating due the electric tension or to the absorption of an optical pulse. We will identify that way the frequency band(s) where the active metamaterial exhibits a density and a compressibility both negative.
The experimental part of the project will consist in elaborating heterostructures with solid background. The geometrical and physical parameters of this heterostructure will be designed according to the results of the theoretical part of the project. Then we will measure the acoustical field scattered by the vibrating inclusions. We will deduce from the 2D mapping of the acoustic field, both the effective density and compressibility of the metamaterial as a function of the frequency of the surface wave propagating in the membrane or on the free surface of the heterostructure.