ACTIVE NONRECIPROCAL ACOUSTIC METAMATERIALS
Baz, Amr M
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In this thesis, the emphasis is placed on the development of a class of active acoustic diodes and metamaterials in an attempt to control the flow and distribution of acoustic energy in acoustic cavities and systems. Such development departs radically from the currently available approaches where the non-reciprocities are generated by hard-wired designs, favoring one transmission direction which is dictated by the arrangement of the hardware and hence it cannot be reversed, or without the presentation of rigorous control theory analysis. The proposed active nonreciprocal acoustic metamaterial (ANAM) cell consists of only one-dimensional acoustic cavity provided with active flexible boundaries. These boundaries are made from piezoelectric bimorphs with the inner layers which interact directly with cavity acting as sensors for monitoring the pressures of the propagating acoustic waves. The outer layers of the bimorphs provide the necessary control actions by direct application of the appropriate control voltage on each layer or by proper connection of nonlinearly activated shunted networks of electrical components such as the switching resistor networks. The control of the switching is carried out using the robust Sliding Mode Control (SMC) strategy. In this strategy, a lumped-parameter model of the ANAM cell is developed to control the strength of the nonreciprocal characteristics of the cell by proper selection of the slope of the switching surfaces. Appropriate optimization strategies are developed to enable a rational selection of the characteristics of the switching surfaces. Numerical examples are presented to demonstrate the effectiveness of the proposed ANAM in tuning and programming the directivity, flow, and distribution of acoustic energy propagating though the metamaterial. Experimental demonstration of the proposed ANAM is presented and includes a comprehensive investigation of the effect of the parameters of the SMC on the system performance. Such investigations are carried out in an attempt to validate the capabilities of ANAM in controlling the non-reciprocity in magnitude and direction. The presented theoretical and experimental techniques provide invaluable tools for designing and predicting the performance of this class of ANAM.