Acoustic Metamaterials for Enhanced Wave Control

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Craig, Steven R.
Shi, Chengzhi
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Acoustic metamaterials have redefined the limits of acoustic wave control with composite structures that realize effective material properties that go beyond those of natural materials. These extraordinary material properties enable imaging beyond the diffraction limit, negative effective sound speeds, and acoustic cloaking. Metamaterials continue to be a hot topic in the scientific community, as these resonant structures push the boundaries of acoustic wave control with unprecedented functionality. The primary goal of this work is to advance the prevalence, practicality, and scope of acoustic metamaterial research with novel materials that uniquely tailor wave fields for a variety of acoustic-based applications. Each chapter uses foundational metamaterial physics to advance our understanding of acoustic wave control with composite structures. The first section develops the theory and performs simulations for a non-Hermitian complementary metamaterial (NHCMM) with tunable active feedback loop circuits that improve the acoustic transmission through an intact human skull. This lays the foundation for ultrasonic brain imaging and neural therapies that require high frequency acoustic waves to penetrate deep within the brain. With a similarly designed metamaterial, we compare the accessible range of the effective density and bulk modulus for unit cells with symmetric and asymmetric feedback loop circuits. The asymmetric circuits result in a Willis coupled response that dramatically broadens the metamaterial’s attainable parameter range. We also demonstrate asymmetric wave transmission at high efficiency with passive Willis coupled metagratings for acoustic beam steering at extreme angles. Lastly, we use transformation acoustics to correct focused and self-bending acoustic beams that become distorted in anisotropic media. These developments advance acoustic-based technologies for biomedical imaging, noise control, underwater communication, and structural acoustic applications.
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