Abstract
Used as building biomaterials for centuries, cereal straws are known for their remarkable acoustic performances in sound absorption. Yet, their use as fibrous media disregards their internal structure made of nodes partitioning stems. Here, we show that such nodes can impart negative acoustic bulk modulus to straw balls when straws are cut on either side of a node. Such metafluid inspired by cereal straws combines visco-thermal diffusion with strong wave dispersion arising from quarter-wavelength resonances within straws. Large spectral bandgaps and slow sound regimes are theoretically predicted and experimental data from impedance tube measurements on an idealised 3D-printed sample layer are in good agreement with the theoretical model. Perfect absorption is achieved at wavelengths 13 times larger than the thickness of the metafluid layer, and slow sound entails an increased density of states causing a cascade of high absorption peaks. Such features could lead cereal straws to serve as cheap acoustic bio-metamaterials.
Highlights
Recent developments in manufacturing techniques, such as Fused Filament Fabrication and Stereolithography technologies, have considerably broadened the range of possible architectures for artificial materials[1]
We theoretically show that their collective resonances entail an effective negative bulk modulus in the metafluid inspired by the cereal straws (MCS) around the quarter-wavelength resonators (QWR) fundamental frequency resulting in both band gap opening and strong wave dispersion
The metafluid inspired by cereal straws has been theoretically and experimentally reported for sub-wavelength perfect sound absorption
Summary
The outside walls of the straw are supposed to be corrugated periodically with the period c in the direction e1 and the semi-elliptic profile of height hc in a longitudinal cross-section, see Fig. 2(d) This corrugation is not fundamental to the singular behaviour of the metafluid, it is an inherent side-effect of the layer-by-layer 3D-printing process using Fused Filament Fabrication technology. It must be accounted for theoretically in the present case of tightly-packed straw arrangements, for which the corrugation height hc is of the same order as the spacing 2 − 2ro between two neighbouring straws
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