Realizing Acoustic Cloaking and Near-Zero Density with Acoustic Metastructure
This chapter proposes an acoustic metastructure which has the property of near zero density and is developed for acoustic invisibility cloaking. This acoustic metastructure sustains the characteristics of the reported acoustic cloaks derived by transforma
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Realizing Acoustic Cloaking and Near-Zero Density with Acoustic Metastructure
This chapter proposes an acoustic metastructure which has the property of near zero density and is developed for acoustic invisibility cloaking. This acoustic metastructure sustains the characteristics of the reported acoustic cloaks derived by transformation acoustics, and is also able to overcome the defect of topologically-optimized acoustic cloaks. Different from the traditional acoustic metamaterials which have complex (inhomogeneous or anisotropic) components of micro or subwavelength scales, our acoustic metastructure for invisibility cloaking is only made of singlepiece homogeneous elastic copper in an accessible layout, including one pressure absorber and one pressure projector connected by an isolated energy channel. The elastic material can be regarded at a certain resonant frequency as an effective density-near-zero (DNZ) structure. Due to the mechanical resonance of the elastic structure, the phase velocity of sound waves in the cloaking setup almost reaches infinite value, and consequently, extraordinary sound transmission (EST) is expected [1, 2]. The cloaking performance by our acoustic metastructure is explained by simplified theoretical spring-mass model and verified by COMSOL numerical simulation in two-dimensional unbounded space as well as in curved waveguides. The theoretical explanation of the DNZ property of our proposed metastructure is to be elaborated using the spring-mass models at Sect. 5.2. In short, DNZ is the consequence of systematic resonances, which result in simultaneous vibration of the entire structure. Therefore, it appears that the vibrational status at one end of the structure can reach to the other end without taking time. The seeming infinite speed of sound can be further paraphrased as the DNZ property. One limitation of our design is the 1D functionality, i.e., the capability to work only for normal incident waves.
© Springer Science+Business Media Singapore 2016 J. Zhao, Manipulation of Sound Properties by Acoustic Metasurface and Metastructure, Springer Theses, DOI 10.1007/978-981-10-2125-1_5
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5 Realizing Acoustic Cloaking and Near-Zero Density …
5.1 Metastructure for Acoustic Cloaking Made by Copper To illustrate the concept of our design, we compare it with acoustic cloaking based on coordinate transformation in Fig. 5.1a, which renders an object invisible by distorting its ambient flow. The scheme of the proposed DNZ acoustic-metastructure for cloaking in Fig. 5.1b can produce EST, to hide arbitrary inserted objects as well as to preserve wavefronts and phases. Conceptually, the flow at the front of an object is concentrated into the energy channel by an absorber. Then, acoustic energy is coupled out by a projector to the
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Fig. 5.1 Comparison of a traditional acoustic cloaking based on coordinate transformation and b our cloaking by a density-near-zero metastructure for extraordinary sound transmission. c Our metastructure consists of one single-piece copper with
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