Causality Constraint as a Design Tool for Sound Absorption Metastructures #

Ping Sheng

14:30 Tuesday in 4Q05.

Part of the Multiple wave scattering session.

Abstract #

As much of metamaterials’ properties originate from resonances, the novel characteristics displayed by acoustic metamaterials are necessarily narrow bandwidth and highly dispersive in nature. However, for practical applications broadband is often a necessity. Furthermore, it would even be better if acoustic metamaterials can display tunable bandwidth characteristics, e.g., with an absorption spectrum that is tailored to fit the noise spectrum. In this talk we present a designed integration strategy for acoustic metamaterials that not only overcomes the narrow-band Achilles’ heel for acoustic absorption, but also achieves such effect with the minimum sample thickness as dictated by the law of nature. The three elements of the design strategy comprise: (a) the causality constraint, and (b) the determination of resonant mode density in accordance with the input target impedance. Here the causality constraint relates the absorption spectrum to a minimum sample thickness, derived from the causal nature of the acoustic response. In this talk I present the design strategy by realizing two structures in which one acoustic metamaterial structure, comprising 16 Fabry-Perot resonators, is shown to exhibit near-perfect flat absorption spectrum starting at 400 Hz and extending to the limiting frequency of measurement. The sample has a thickness of 10.86 cm, whereas the minimum thickness as dictated by the causality constraint is 10.55 cm. A second structure demonstrates the flexible tunability of the design strategy by opening a reflection notch in the absorption spectrum, extending from 600 to 1000 Hz, with a sample thickness that is only 3 mm above the causality minimum. I present some examples of successfully commercialized products based on the above design strategy.