Active acoustic metamaterial with tunable effective density using a disturbance rejection controller

A class of active acoustic metamaterials (AAMMs) is developed with desirable controlled distributions of effective dynamic densities. The proposed AAMM consists of an array of acoustic cavities separated by piezoelectric boundaries and arranged to form a one-dimensional wave guide. The stiffness of the flexible piezoelectric boundaries is controlled to generate desirable density distribution along the wave guide. In this paper, a disturbance rejection control strategy is formulated to achieve a closed-loop control of the effective density of this class of acoustic metamaterials while rejecting the effect of the wave pressure disturbances. The time response characteristics of a unit cell of the AAMM are investigated for various parameters of the controller in order to demonstrate the merits of the proposed controller. A natural extension of this work is to include active control capabilities to control also the bulk modulus distribution along the metamaterial in order to build practical configurations of acoustic cloaks.