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Metamaterials exploit sub-wavelength microstructures to yield novel macroscopic material properties. Recently, the notion of a metacluster has emerged, which is a collection of resonators that interact in order to modify and possibly enhance the resonance. They can also be employed to modify and tune the far-field scattered response. This is particularly important with regard to metamaterial design. In the context of elastodynamics, Cotterill et al. [Proc. R. Soc. A 478(2263), 20220026 (2022)] considered the case of void metaclusters, thus permitting the modification of the so-called giant monopole resonance in elastodynamics. Here, we consider one of the original resonant configurations of metamaterial science in Liu et al. [Science 289, 1734 (2000)]; this structure consists of coated cylinders of circular cross section and gives rise to a strong dipole resonance for sufficiently soft coatings. We consider the nature of the interaction of two such identical resonators in close proximity, which we term the co-dipole metacluster. We show that, contrary to the giant monopole case, the frequency at which the resonance occurs is unchanged as compared to a single resonator. The amplitude of the resonance itself is enhanced significantly, however, by up to 5.7 times the enhancement observed when considering two non-interacting resonators. Furthermore, although the nature of the resonance remains dominated by a dipole response, both the enhancement and the far-field scattered response are now significantly affected by the incidence angle, in contrast to the single resonator. [ABSTRACT FROM AUTHOR]

Recent work in the acoustic metamaterial literature has focused on the design of metasurfaces that are capable of absorbing sound almost perfectly in narrow frequency ranges by coupling resonant effects to visco-thermal damping within their microstructure. Understanding acoustic attenuation mechanisms in narrow, viscous-fluid-filled channels is of fundamental importance in such applications. Motivated by recent work on acoustic propagation in narrow, air-filled channels, a theoretical framework is presented that demonstrates the controlling mechanisms of acoustic propagation in arbitrary Newtonian fluids, focusing on attenuation in air and water. For rigid-walled channels, whose widths are on the order of Stokes's boundary layer thickness, attenuation in air at 10 kHz can be over 200 dB m−1; in water it is less than 37 dB m−1. However, in water, fluid-structure-interaction effects can increase attenuation dramatically to over 77 dB m−1 for a steel-walled channel, with a reduction in phase-speed approaching 70%. For rigid-walled channels, approximate analytical expressions for dispersion relations are presented that are in close agreement with exact solutions over a broad range of frequencies, revealing explicitly the relationship between complex phase-speed, frequency and channel width. [ABSTRACT FROM AUTHOR]