ABSTRACT
The reduction of noises, achieved through absorption, is of paramount importance to the well-being of both humans and machines. Lattice structures, defined as architectured porous solids arranged in repeating patterns, are emerging as advanced sound-absorbing materials. Their immense design freedom allows for customizable pore morphology and interconnectivity, enabling the design of specific absorption properties. Thus far, the sound absorption performance of various types of lattice structures are studied and they demonstrated favorable properties compared to conventional materials. Herein, this review gives a thorough overview on the current research status, and characterizations for lattice structures in terms of acoustics is proposed. Till date, there are four main sound absorption mechanisms associated with lattice structures. Despite their complexity, lattice structures can be accurately modelled using acoustical impedance models that focus on critical acoustical geometries. Four defining features: morphology, relative density, cell size, and number of cells, have significant influences on the acoustical geometries and hence sound wave dissipation within the lattice. Drawing upon their structural-property relationships, a classification of lattice structures into three distinct types in terms of acoustics is proposed. It is proposed that future attentions can be placed on new design concepts, advanced materials selections, and multifunctionalities.
ABSTRACT
Helmholtz resonance, based on resonance through a pore-and-cavity structure, constitutes the primary sound absorption mechanism in majority of sound-absorbing metamaterials. Typically, enhancing sound absorption in such absorbers necessitates substantial geometrical redesign or the addition of dissipative materials, which is non-ideal considering the volume and mass constraints. Herein, we introduce a new approach - that is to simply reshape the cavity, without alterations to its overall mass and volume - to drastically enhance sound absorption. This is achieved by bringing the cavity walls close to the pores where additional thermoviscous dissipation along these boundaries can occur. Experimentally validated, with three sides of the cuboid cavity close to the pore and at a particular pore-cavity geometry, a 44% gain in maximum absorption is achieved compared to the original structure. Through numerical simulations, we fully elucidate structure-property relationships and their mechanisms, and propose analytical models for design and optimization. Ultimately, utilizing this concept, we demonstrate a heterogeneously porous broadband (1500 to 6000 Hz) absorber that exhibits an excellent average absorption coefficient of 0.74 at a very low thickness of 18 mm. Overall, we introduce a new and universal concept that could revolutionize the design principles of Helmholtz resonators, and demonstrate its potential for designing advanced sound-absorbing metamaterials.
ABSTRACT
The advent of 3D printing brought about the possibilities of microlattice metamaterials as advanced materials with the potentials to surpass the functionalities of traditional materials. Sound absorbing materials which are also tough and lightweight are of particular importance as practical engineering materials. There are however a lack of attempts on the study of metamaterials multifunctional for both purposes. Herein, we present four types of face-centered cubic based plate and truss microlattices as novel metamaterials with simultaneous excellent sound and mechanical energy absorption performance. High sound absorption coefficients nearing 1 and high specific energy absorption of 50.3 J g-1 have been measured. Sound absorption mechanisms of microlattices are proposed to be based on a "cascading resonant cells theory", an extension of the Helmholtz resonance principle that we have conceptualized herein. Characteristics of absorption coefficients are found to be essentially geometry limited by the pore and cavity morphologies. The excellent mechanical properties in turn derive from both the approximate membrane stress state of the plate architecture and the excellent ductility and strength of the base material. Overall, this work presents a new concept on the specific structural design and materials selection for architectured metamaterials with dual sound and mechanical energy absorption capabilities.
