Helmholtz-type resonance absorbers constitute noise control devices widely used in many areas that have attracted attention in the last years due to the advancement of acoustic metamaterials. Micro-perforated panels placed over a backing cavity work on the same principle at low frequencies. They can provide important absorption values but confined in a narrow frequency band. To overcome this limitation, unbacked configurations have been considered, but care has to be taken for a proper selection of their constitutive parameters. In this work, freestanding micro-perforated plates with holes diameters down to 10 micrometers and high perforation ratio are shown to be good candidates as wideband low-frequency sound absorbers. Several micro-capillary plates, classified as a function of the Knudsen number, are studied analytically and experimentally. Most of the porous microsystems that use gases work in slip-flow regime whose properties differ considerably from the classical continuum regime. Results showed that unbacked micro-capillary plates can achieve absorption values greater than 0.7 up to 7 kHz with an absorption plateau above 0.85 up to 4 kHz under normal incidence. Dependence of their performance to the backing load is analyzed. They could be used as low-frequency noise dissipation devices with applications as calibrated anechoic terminations.
This study was funded in Spain by the Ministerio de Economía y Competitividad project TRA2017- 87978-R, AEI/FEDER, UE, and the mobility program ILINK+2018. It was supported in France by the ANR VIRTECH (ANR-17-CE10-0012-01). The authors would like to thank J. Kergomard, Emeritus Director of Research at CNRS-LMA, for fruitful discussions, Dr. P. Ecker from GIDS GmbH for kindly borrowing us one of the MCPs and Dr. L. Sabatier, Research Engineer at CNRSLMA, for X-ray imaging the MCPs.