Transmission and rainbow trapping of acoustic waves in a fluid medium using gradient-index superlattices

Abstract

Recently, rainbow trapping of acoustic/elastic waves in gradient artificial structures has shown promising applications in energy harvesting and frequency separation. However, the novel phenomenon of acoustic superlattices for underwater acoustic waves has not been explored. In this work, we establish the theoretical model of underwater acoustic waves propagating through the gradient-index superlattice (GISL) by using the transfer matrix method. We show that the combined band structures of infinite sub-superlattices and the transmission curves of finite GISL exhibit Bragg forbidden bands that are wider than those of periodic ones, which implies the occurrence of rainbow trapping. On this basis, the effects of gradient factor, period number, and water thickness are discussed, and the tunability of the trapping range is also explained. Furthermore, simulations with Gaussian beam incidence are performed through finite element methods. The results in frequency-domain and time-domain both indicate perfect performances of energy localization and frequency separation for acoustic waves of different frequencies. Our findings show possibilities for the design of underwater acoustic devices, such as acoustic sensors and frequency dividers.

Broadband acoustic signal enhancement via gradient metamaterials coupled to crystals

“…By designing intricate acoustic metamaterials, they aim to control acoustic field information through negative mass density and negative bulk modulus properties [10][11][12]. New physical phenomenon and mechanisms such as sound absorption [13,14], focusing [15][16][17], directional transmission [18][19][20], and acoustic rainbow trapping [21][22][23][24][25][26] have been generated. Compared with conventional acoustic materials, Zhu et al [27] first proposed a subwavelength gradient grid-type structure that employs subwavelength gaps to selectively capture acoustic signals across various frequency bands.…”

A Nonlinear Gradient-Coiling Metamaterial for Enhanced Acoustic Signal Sensing

“…In recent years, acoustic metamaterial have emerged as a new material with the properties of manipulating sound energy, breaking through the acoustic performance limitations of traditional acoustic structures and materials. Due to their wide range of physical properties and extraordinary performance, such as acoustic bandgap [22], acoustic focusing [23,24], acoustic vortex [25], and acoustic rainbow trapping [26][27][28], acoustic metamaterials have become critical in acoustic signal perception. In contrast to periodic metamaterials, metamaterials or media with gradient refractive index characteristics show flexible control of acoustic waves.…”