Wideband sound absorption and transmission through micro-capillary plates: Modelling and experimental validation

The current work presents theoretical and experimental studies that investigate sound propagation through micro-capillary plates (MCP) under a general plane wave excitation and in the no-flow case. MCPs are characterized by micrometric channels radius with Knudsen number greater than 0.001 so that a slip-flow model has been derived for their viscous transfer impedance. It is found that the slip-flow model should be used instead of the continuum model to predict the transfer impedance of MCPs with channels radii lower than 2 tm as well as their absorption coefficient under near-grazing incident excitations. Otherwise, both approaches provide similar results, as confirmed by comparison with finite element simulations. Due to their high porosity, MCPs provide minute reactance and constant resistance that can be tailored to achieve target absorption over a broad frequency range. Plane wave impedance tube experiments have shown that a near-optimal MCP termination can provide a low frequency flat absorption spectrum that stays above 0.7 up to a Helmholtz number of 1.84. Measurements on rigidly-backed MCPs have led to ultra-wideband absorption with a half-bandwidth spanning up to 12 octaves around the absorber Helmholtz resonance. Expressions have been derived to find the optimal channels radius that maximize the MCPs dissipation under general incidence angle and assuming anechoic or rigid backing. The sensitivity of the MCPs optimal transfer resistance to their load impedance has been examined. It provides a design chart to find the MCP optimal parameters that achieve specific broadband absorption value under general incidence and practical load conditions. (C) 2020 Elsevier Ltd. All rights reserved.