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Active acoustic metamaterials (AAMM) have been developed to overcome the limited frequency bandwidth characteristics of passive acoustic metamaterials. The AAMM rely in their operation on using piezoelectric active ingredients in a fluid-solid composite structure forming the basic building block of a larger metamaterial periodic arrangement. A prototype of AAMM composite cell is manufactured and active control strategies are implemented on the piezoelectric elements to vary its stiffness in order to control the effective dynamic density of the cell. Acoustic characterization of the developed AAMM cell is carried out by measuring its acoustic impedance and transmission loss and comparing the results with the predictions of a finite element model. The obtained experimental measurements and the predictions of the finite element model are in very good agreement for the considered frequency range. The transfer functions between the reference microphone in the impedance tube and the piezoelectric elements demonstrate the coupling nature inside the cell rendering it to a system of single acoustic properties acting as a single degree of freedom system. The proposed AAMM can be useful in manufacturing the next generation of acoustic cloaks and metamaterials with controllable directivity and dispersion characteristics.

Considerable interest has been devoted to the development of various classes of acoustic metamaterials that can control the propagation of acoustical wave energy through these materials. However, all the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this article, the emphasis is placed on the development of a new class of composite acoustic metamaterials with effective bulk moduli that are programmed to vary according to any prescribed pattern along the volume of the metamaterial. The composite consists of an acoustic cavity, which is coupled with an array of actively controlled Helmholtz resonator to enable the control of the effective bulk modulus distribution along the cavity. The theoretical analysis of this class of multi-cell composite active acoustic metamaterials (CAAMM) is presented and the theoretical predictions are determined when the Helmholtz resonators are provided with piezoelectric boundaries. These smart boundaries are used to control the overall bulk modulus of the cavity/resonator assembly through direct acoustic pressure feedback. The interaction between the neighboring cells of the composite metamaterial is modeled using a lumped-parameter approach. Numerical examples are presented to demonstrate the performance characteristics of the proposed CAAMM and its potential for generating prescribed spatial and spectral patterns of bulk modulus variation.

We report an experimental study of acoustic effects produced by wind impinging on noise barriers based on two-dimensional sonic crystals with square symmetry. We found that the attenuation strength of sonic-crystal bandgaps decreases for increasing values of flow speed. A quenching of the acoustic bandgap appears at a certain speed value that depends of the barrier filling ratio. For increasing values of flow speed, the data indicate that the barrier becomes a sound source because of its interaction with the wind. We conclude that flow noise should be taken into account in designing acoustic barriers based on sonic crystals. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3111797͔A sonic crystal ͑SC͒ is a periodic array of sound scatterers in air. SCs based on two-dimensional ͑2D͒ distributions of cylindrical scatterers have been mainly studied in view of their possible applications as acoustic barriers.1-9 Hexagonal and square configurations of cylinders are those widely studied, but other combinations made of them have been also analyzed.3,4 For the case in which the scatterers are made by solid materials, such as aluminum or concrete, the physical mechanism giving the transmission loss through the barrier is controlled by Bragg reflection between successive planes of scatterers in the barrier. The phenomenon of Bragg reflection creates frequency bands ͑bandgaps͒ at which the sound propagation is forbidden. Cylindrical scatterers with circular section are mainly studied although sections like square, hexagonal, and many others have been also considered looking for stronger and wider complete bandgaps. 8,9 More recently, additional mechanisms of sound reduction have been essayed in order to improve the barrier's performance in the range of frequencies not covered by the bandgaps. For example, the absorption mechanism associated to a porous covering of the scatterers would produce additional sound reduction in a wide range of frequencies.10 Another possibility is using periodic arrays of trees, which also produce a reduction in the transmitted sound by natural means.11 Sound attenuation by outdoor barriers based on 2D SCs have already been demonstrated at realistic size. 6 In this letter we report an experimental study showing that wind impinging on a noise barrier based on a 2D SC produces effects changing its attenuation performance. We analyze here the quenching of SC bandgaps by flow noise, a problem that has not been previously tackled, although, as it is shown below, it has a paramount importance. It is demonstrated that flow noise can completely deteriorate the barrier performance if the speed flow is large enough. We also demonstrate that for barriers too dense ͑i.e., when the distance between cylinders is too short͒ the barrier even becomes in a source of noise for large flow speeds.In order to quantify the attenuation performance of SC barriers under the effect of impinging wind, a series of measurements are developed in a wind tunnel consisting of a duct 6 m long that has been constructed in the...

Acoustic metamaterials are those structurally engineered materials that are composed of periodic cells designed in such a fashion to yield specific material properties (density and bulk modulus) that would affect the wave propagation pattern within in a specific way. All the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a new class of composite one-dimensional active acoustic metamaterials (CAAMM) with effective densities and bulk moduli that are programmed to vary according to any prescribed patterns along its volume. A cylindrical water-filled cylinder coupled to two piezoelectric elements form a composite cell to act as a base unit for a periodic metamaterial structure. Two different configurations are considered. In the first configuration, a piezoelectric panel is flash-mounted to the face of the cylinder, while the other is side-mounted to the cylinder wall, introducing a variable stiffness along the wave propagation path. In the second configuration, the face-mounted piezoelectric panel remains unchanged, while the side-mounted panel is replaced with an active Helmholtz resonator with piezoelectric base panel. A detailed theoretical lumped-parameter model for the two configurations is present, from which the stiffness of both active elements is controlled via charge feedback control to yield arbitrary homogenized effective bulk modulus and density over a very wide frequency range. Numerical examples are presented to demonstrate the performance characteristics of the proposed. The CAAMM presents a viable approach to the development of effective domains with a controllable wave propagation pattern to suit many applications.