Abstract
Emission of low-frequency sound by a source is limited by the size of the source relative to sound wavelength. We propose to place the source in a subwavelength enclosure of anisotropic metamaterials which enhances the emission around Mie resonant frequencies of that enclosure [J. Zhao, L. Zhao, and Y. Wu, J. Acoust. Soc. Am. 142(1), EL24–29, July 2017]. The enclosure has a relatively small sound speed along the radiation direction, enabling the resonances to occur at low frequencies. Our numerical simulations provide evidence of the proposed low-frequency enhancement for both monopole and multipole sources. The common Mie resonant frequencies increase with the order of the multipoles. Here, we introduce an extreme anisotropy in the azimuth of the enclosure to degenerate the resonant frequencies of higher-order multipoles down to the same frequencies as the lower-order multipoles. The degeneracy associated with the anisotropy of the enclosure is theoretically analyzed. The results guide the experimental realization of the enclosure for low-frequency enhancement.
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