A review of underwater acoustic metamaterials

Abstract

Underwater acoustic materials are the key to the noise and vibration reduction, stealth, reception and amplification of underwater acoustic signal. However, conventional underwater acoustic materials usually suffer from the weakness of great bulk, heavy weight and poor resistance to hydrodynamic pressure. The advancement of the acoustic metamaterials (AMMs) has provided new insights into fixing these problems, and a variety of underwater AMMs are proposed accordingly. This article reviews the history and the developments of underwater AMMs and the difficulties that existed in the developments of underwater AMMs as compared with the AMMs in air. The focuses are the three types of AMMs which are closely associated with the underwater acoustic stealth and signal reception & amplification of hull-mounted sonar, i.e., the AMMs for sound absorption, AMMs for decoupling and those for sound focusing. The classification, physical mechanisms and controlling effects of those three types of underwater AMMs are reviewed in details. The AMMs for underwater sound absorption can be divided into local-resonance (LR) and non-resonance types. The LR types take advantages of the local resonance principle with the controlling wavelengths orders of magnitude greater than the physical dimension of a unit. To overcome the intrinsic narrow-band limitations of LR and broaden the bandwidths, efforts have been made to utilize multilayer LR-AMMs or multi-size LRs, attach/embed elastic plate scatter on/into elastically coated plate, or introduce complex lattices with multiple (coupled) LRs, etc. The non-resonance types are based-on non-resonance mechanisms such as viscous loss, heat conduction, macromolecular relaxation or a combination of them. These AMMs mainly include porous metals and nonmetal materials, gradient AMMs and acoustic meta-surfaces, etc. The AMMs for underwater decoupling mainly use acoustically soft AMMs to decouple the vibration of the base structure from adjacent fluid. These AMMs contain the acoustic coating embedded with various cavities or local resonators, gradient coating, re-entrant honeycomb structure or structure with negative Poisson’s ratio and chiral porous coating, etc. The sound-focusing AMMs are categorized into diffractive, refractive and reflective types. The diffractive types focus underwater sound through constructive interference of diffracted waves, which mainly include all kinds of zone plates and fractal lens. The refractive types are mainly based on various acoustic lens and focus sound by controlling the refraction index of the materials. The reflective types focus sound by modulating the reflective waves, including the recently-proposed reflective acoustic meta-surfaces and acoustic coatings with attached/embedded periodically distributed signal conditioning plates, etc. Finally, the conclusions and drawbacks of the above AMMs are summarized. It is pointed out that although the present underwater AMMs have achieved some encouraging results, they are still confronted with several key problems such as the realizations of low-frequency and broadband, omnidirectional, subwavelength, lightweight and hydrostatic pressure resistant sound manipulations, the manufacture and testing of large AMM samples, and the environmental adaptations requirements. Therefore, there is still a long way for the current underwater AMM toward the real application. The trend and outlook toward future underwater AMMs are also presented. It is hoped that this review could provide guidelines to the design and realization of future materials demanded for underwater acoustic stealth and detection.