Ultra-wide bandgap in active metamaterial from feedback control

To widen the attenuation bandwidth in a mass-in-mass metamaterial chain, active feedback control is employed within the unit cell. The optimal control via Linear Quadratic Regulator is used to ensure the stability of the solution as well as adaptive tuning of the system. The key concept is to obtain a wider bandwidth by reducing the displacement amplitude of the outer mass of each unit of the metamaterial. Transmittance for finite number of units and dispersion diagram for infinitely long metamaterial chain are obtained using the backward-substitution method and Bloch–Floquet’s theorem, respectively. Results evidenced that an ultra-wide uninterrupted attenuation band starting from very low free wave frequency can be obtained. The dispersion diagram primarily depends on the frequency ratio of the resonating unit with the external main unit. The effect of the mass ratio is insignificant while active feedback control is implemented. This unique phenomenon implies that for a very low mass ratio also a wider bandwidth can be achieved. This enables to design a lightweight structure having wider bandwidth in low-frequency regime.

Sound Transmission Comparisons of Active Elastic Wave Metamaterial Immersed in External Mean Flow

Using the active feedback control system on the elastic wave metamaterial, this research concentrates on the sound transmission with the dynamic effective model. The metamaterial is subjected to an incident pressure and immersed in the external mean flow. The elastic wave metamaterial consists of double plates and the upper and lower four-link mechanisms are attached inside. The vertical resonator is attached by the active feedback control system and connected with two four-link mechanisms. Based on the dynamic equivalent method, the metamaterial is equivalent as a single-layer plate by the dynamic effective parameter. With the coupling between the fluid and structure, the expression of the sound transmission loss (STL) is derived. This research shows the influence of effective mass density on sound transmission properties, and the STL in both modes can be tuned by the acceleration and displacement feedback constants. In addition, the dynamic response and the STL are also changed obviously by different values of structural damping, incident angle (i.e., the elevation and azimuth angles) and Mach number of the external fluid with the mean flow property. The results for sound transmission by two methods are compared, i.e., the virtual work principle for double plates and the dynamic equivalent method corresponding to a single one. This paper is expected to be helpful for understanding the sound transmission properties of both pure single- and double-plate models.

Sound transmission tuned by active feedback control attached to elastic wave metamaterials immersed in water

Elastic wave metamaterials have been widely exploited with their dynamic superior properties and outstanding acoustic responses. However, it is difficult to directly manipulate sound pressure in low frequencies. In this study, we propose a new kind of elastic wave metamaterial which consists of vertical and lateral resonators as well as orthogonal stiffeners. The active feedback control system is applied to extend to the tunable scope for both lower and higher frequency regions and change the characteristics of acoustic-structure coupling. Its effective mass density is also discussed with different feedback constants. In order to present effects of the fluid-solid interaction, we considered that the elastic wave metamaterial is immersed in different fluid medium and its sound transmission loss (STL) is calculated. This work provides a feasible method for creating mechanical/acoustic models with multi-functional potentials.