Investigation of Micro-Capillary Plates to Enhance Low-Frequency Sound Absorption

International audienceTheoretical, numerical and experimental studies are presented that investigate the wideband absorption properties of unbacked and rigidly-backed micro-capillary plates (MCPs) under normal plane wave incidence. MCPs are characterized by a perforation ratio greater than 50% and channels diameters between 4μm and 50μm, thus leading to a thin porous absorber with a regular distribution of at most one million of micro-channels per cm2. They are fabricated by micro-chemical etching process. Unlike micro-perforated panels, the MCPs have a high perforation ratio that ensures minute reactance, thereby reducing their transfer impedance to a pure resistance over a broad frequency range. Moreover, their micrometric channels diameter allows to tailor their resistance to achieve constant target absorption over a wide frequency band. This property has been validated through finite-element simulations that also confirmed isothermal conditions in capillary channels. An optimal value of the micro-channels diameter can be derived that achieves maximal viscous dissipation of the acoustical energy under plane wave anechoic load. Kundt tube experiments confirmed the ability of optimized MCPs to achieve wideband flat absorption performance that exceeds 0.85 up to a Helmholtz number of 0.3. They also showed the potential of non-optimal acoustically-transparent MCPs to achieve a low-frequency anechoic termination