Membrane-based acoustic metamaterial with near-zero refractive index**Project supported by the National Natural Science Foundation of China (Grant Nos. 61571222, 11104142, and 11474160), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20161009), the Qing Lan Project of Jiangsu Province, China, and the Six Talent Peaks Project of Jiangsu Province, China.

Abstract

We investigate a one-dimensional acoustic metamaterial with a refractive index of near zero (RINZ) using an array of very thin elastic membranes located along a narrow waveguide pipe. The characteristics of the effective density, refractive index, and phase velocity of the metamaterial indicate that, at the resonant frequency , the metamaterial has zero mass density and a phase transmission that is nearly uniform. We present a mechanism for dramatic acoustic energy squeezing and anomalous acoustic transmission by connecting the metamaterial to a normal waveguide with a larger cross-section. It is shown that at a specific frequency f1, transmission enhancement and energy squeezing are achieved despite the strong geometrical mismatch between the metamaterial and the normal waveguide. Moreover, to confirm the energy transfer properties, the acoustic pressure distribution, acoustic wave reflection coefficient, and energy transmission coefficient are also calculated. These results prove that the RINZ metamaterial provides a new design method for acoustic energy squeezing, super coupling, wave front transformation, and acoustic wave filtering.