Asymptotic modelling of phononic box crystals

TL;DR

This paper introduces phononic box crystals as a new class of subwavelength acoustic metamaterials, deriving simple asymptotic wave equations that accurately predict their dispersion and sound field behavior, revealing unique nonlinear effects.

Contribution

It develops an asymptotic modeling framework for phononic box crystals, providing analytical dispersion relations and insights into their wave manipulation capabilities.

Findings

The entire acoustic branch is subwavelength in frequency.

Phononic box crystals exhibit nonlinear dispersion effects like dynamic anisotropy.

The framework accurately predicts localized defect modes.

Abstract

We introduce phononic box crystals, namely arrays of adjoined perforated boxes, as a three-dimensional prototype for an unusual class of subwavelength metamaterials based on directly coupling resonating elements. In this case, when the holes coupling the boxes are small, we create networks of Helmholtz resonators with nearest-neighbour interactions. We use matched asymptotic expansions, in the small hole limit, to derive simple, yet asymptotically accurate, discrete wave equations governing the pressure field. These network equations readily furnish analytical dispersion relations for box arrays, slabs and crystals, that agree favourably with finite-element simulations of the physical problem. Our results reveal that the entire acoustic branch is uniformly squeezed into a subwavelength regime; consequently, phononic box crystals exhibit nonlinear-dispersion effects (such as dynamic anisotropy) in a relatively wide band, as well as a high effective refractive index in the long-wavelength limit. We also study the sound field produced by sources placed within one of the boxes by comparing and contrasting monopole- with dipole-type forcing; for the former the pressure field is asymptotically enhanced whilst for the latter there is no asymptotic enhancement and the translation from the microscale to the discrete description entails evaluating singular limits, using a regularized and efficient scheme, of the Neumann's Green's function for a cube. We conclude with an example of using our asymptotic framework to calculate localized modes trapped within a defected box array.