Bulk Active Acoustic Metamaterials

Abstract

Acoustic technologies are pivotal to our ability to communicate and perceive our environment, with applications ranging from noise mitigation to non-destructive testing. These technologies can generally be divided into those that are passive, manipulating sound waves via the materials through which they propagate, and those that are active, using transducers to sense or generate sound. In this work, we merge these areas with a materials-based approach to the active control of sound. Since the early 2000s, significant progress has been made in expanding the available acoustic material properties through the development of acoustic metamaterials—composite structures with subwavelength periodicity. However, these metamaterials suffer from narrow bandwidth, high energy loss, challenges of integration into devices, and other limitations in performance due to their static and passive nature. In active acoustic metamaterials, the composite building block is replaced with programmable pairs of sensing and driving transducers. This approach offers a path around the limitations of passive structures and opens numerous new opportunities for wave manipulation. The transducers in an active acoustic metamaterial unit cell are configured to replicate the scattering of a subwavelength material element by driving a multipole acoustic response to the locally sensed acoustic pressure and particle velocity. Many of these unit cells, each operating individually, can be assembled to form a bulk material governed by only a small number of parameters—the effective acoustic properties. This is in contrast to the poor scaling of conventional centralized active sound control methods. Active acoustic metamaterials have been previously demonstrated, but only in simple one-dimensional cases, avoiding the majority of challenges fundamental to truly considering them as materials and greatly restricting their application. The goal of this dissertation is to develop a versatile theoretical model and physical platform for bulk active acoustic metamaterials such that they can be easily arranged in a desired geometry and programmed with the acoustic properties needed for extreme wave manipulation. We start by modeling active acoustic metamaterials in the frequency domain as arrays of polarized sources with amplitudes proportional to the local field. We then derive direct expressions relating the gains between the sensed and driven quantities of an active unit cell to the effective acoustic properties of bulk modulus and mass density. We show how the polarized source model can be used to simulate the acoustic behavior of devices with demanding property specifications, using a cloaking shell as an example. The polarized source unit cell is then physically realized as a printed circuit board-mounted assembly of microphones, speakers, amplifiers, and a microcontroller. Two-dimensional arrangements of these active unit cells are programmed to behave as effective acoustic media with specified bulk modulus and mass density tensor. Experimental demonstrations include properties with values less than those of air, high anisotropy, and reflectionless absorption. This is the first active acoustic metamaterial that can be fully programmed with the properties needed to realize transformation acoustics devices. The polarized source model is then expanded to incorporate Willis coupling parameters, enabling new functionality in bulk active acoustic metamaterials, such as non-reciprocal behavior. Finally, the non-ideal experimental frequency response of an active unit cell and its stability consequences for the overall metamaterial are analyzed to determine the limitations on the achievable acoustic properties for a given design.