Engineering Boundary Impedance for Quality Factor Control by Customizable Acoustic Metamaterials

Controlling the quality factor (Q) of a resonant cavity broadbandly is a highly complex task, as it involves dealing with the intricate modal response and the limitations of boundary control degrees of freedom. The resonant behavior of an acoustic cavity often leads to non-uniform decay times of its wave field, making it quite challenging to achieve spatial uniformity. To tackle this issue, we propose a novel approach: designing impedance boundary conditions on the surface of the cavity walls that vary with both frequency and space. This design allows for the generation of a uniform response across a wide frequency range. To implement such boundary conditions, we leverage the capabilities of customizable dispersive acoustic metasurfaces, a technology that has steadily matured in recent years. These metasurfaces consist of integrated multiple resonators, whose collective average response spectrum can be tailored to achieve the optimal form based on the available space and specific requirements. In essence, our efforts focus on utilizing numerous small resonators at the cavity boundary to effectively control and tame the strong resonances of the larger cavity. This abstract physical problem finds a practical application in the control of reverberation time (T
60
) in room acoustics. We will revisit the principles and concepts behind our recent achievement of maintaining a constant reverberation time over a finite frequency band and provide experimental results to validate the effectiveness of our theoretical design. By introducing wideband customizable metamaterials into the realm of room acoustics, our work has the potential to significantly impact various fields, including listening rooms, recording studios, and automotive acoustics. This groundbreaking approach opens up new possibilities for achieving optimal acoustic experience and enhancing the overall auditory experience in these environments.