Acoustic Metamaterials: Theory and Application

Abstract

In this paper the concept of the metamaterial is explained. Using the interaction between the seismic (Rayleigh) wave and the ground-forest system the phenomena of wave stop is clarified. The example of ‘seismic metamaterial’ in architecture is also shown. The subunit of metamaterial is modeled as a mass-in-mass system. On the mass 1 a small mass with linear elastic spring is connected. The resonance conditions for the two-degrees of freedom system excited with a periodical trigonometric force are obtained. The term of effective mass is introduced. The effect of the negative effective mass on excitation wave spread is shown. The real metamaterial subunit is made of a metal sphere surrounded with rubber on which a thin silicon layer is settled. The change of vibration direction of the core in comparison to the outside layer and wave give the transformation from the positive to negative effective mass. The subunit has only one very narrow frequency band gap. In the paper we considered the subunit with nonlinear elastic connection between basic and added masses. Namely the added mass with nonlinear spring acts as a nonlinear vibration absorber. If the excitation has the form of the Jacobi elliptic function and the nonlinear spring property is a cubic function of deflection the solution the frequency of vibration of the system depends on the amplitude of vibration. For certain parameters of the excitation function, the calculated parameters of spring have to be used. It means that the properties of absorber have to be choosing in accordance with excitation parameters. The obtained frequency band gap for frequencies is larger for the nonlinear than for the linear subsystem. The realization of metamaterials by using the subunits incorporated in the basic material or honeycomb structure is shown. The metamaterials are not natural but artificial materials usually composites. The explanation of acoustic metamaterial in noise reduction is considered. The metamaterial produced at the University of Novi Sad, Serbia, is experimentally tested at the Laboratory of Nuclear Sciences in Bucharest, Romania. The experimental rig is shown. The obtained results of energy variation in the metamaterial excited with the white noise are plotted. The results are in a good agreement with theoretically obtained ones.