Session: 01-02-02: Acoustic Metamaterials

Paper Number: 150386

150386 - Top-Down Design of Active Metamaterials for Noise Control and Acoustic Cloaking 

Active acoustic metamaterials consist of periodic arrangements of electronically paired sensors and drivers that produce a programmable acoustic response to impinging waves. They can be designed to replicate the scattering behavior of conventional passive materials and metamaterials, but also have the comparative advantage of unprecedented accuracy on the realization of desired properties and greatly relaxed constraints on the achievable properties and bandwidth. Here, we focus on the development of bulk active acoustic metamaterials composed of subwavelength unit cells, with applications in the realization of transformation acoustics devices as well as compact noise absorbers. We address challenges of past work related to the complex interactions between unit cells, including the modeling of the programming-effective property relationships and the identification of stability-derived constraints.

We fabricated active unit cells with two-dimensional operation that mount into a waveguide such that the acoustic elements interact with, but do not block waves propagating through the interior. The cells are each composed of several microphones to sense the local pressure and particle velocity, speakers to drive monopole and dipole responses, and a microcontroller to relate the inputs and outputs according to a desired programming scheme. Modeled after the behavior of subwavelength fluid cylindrical scatterers, a simple gain implemented between the sensed pressure and monopole response dictates the effective bulk modulus of the metamaterial, while gains between the directional components of the sensed particle velocity and dipole response dictate the effective mass density tensor. The design strategy is completely distributed, i.e., each cell is individually and independently controlled, thus the metamaterial can be scaled without the exponentially growing complexity of a centralized system.

With a bulk metamaterial of interacting cells, we demonstrated the ability to independently program the bulk modulus and mass density for specified values, both less than and greater than those of air. We also showed that the principal axes of anisotropy of the metamaterial could be rotated arbitrarily. The acoustic properties were validated by comparing the scattering of the metamaterial due to incident waves at various angles to the simulated scattering of an ideal medium. Transformation acoustics devices have been fabricated with little success using passive structures, requiring significant adjustment of the theoretically specified property maps at the cost of performance.  We have demonstrated that active acoustic metamaterials have the capabilities needed for a faithful realization.

By adjusting the phase of the active unit cell response, a loss component can be added to the effective properties of the metamaterial. We showcased this with an impedance-matched loss medium that absorbs sound without reflection. Passive noise absorbing structures require significant volume for application at low frequencies or have narrow bandwidth. Active noise cancellation solutions only work in small regions. In contrast, active acoustic metamaterials enable strong broadband loss in very thin absorbers covering large surfaces.

Presenting Author: Dylan Kovacevich University of Michigan

Presenting Author Biography: Dylan Kovacevich is a Ph.D. Candidate at the University of Michigan advised by Professor Bogdan-Ioan Popa. He is expected to defend in May 2025. For his work on active acoustic metamaterials, he has been recognized as a UC Berkley Rising Star in Mechanical Engineering and recipient of the Rackham Predoctoral Fellowship.

Authors:

Dylan Kovacevich University of Michigan
Bogdan-Ioan Popa University of Michigan