Sound absorption and transmission through flexible micro-perforated structures
Résumé
This work presents a theoretical and experimental study on sound absorption and transmission through structures made up of single and multiple-layer Micro-Perforated Panels (MPPs). As they contribute to improve acoustical comfort speech intelligibility and comply with lightweight transparency and fibreless requirements increasing applications are found in architectural acoustics or in the aeronautic and surface transport industries. A fully coupled modal approach is proposed to calculate the absorption coefficient and the Transmission Loss of finite-sized layouts made up of multiple flexible MPPs separated by air gaps. Validation results are obtained for single and double-layer thin MPPs against the transfer matrix approach and against measurements performed in a standing wave tube and in an anechoic chamber. Analytical approximations are derived from coupled-mode analysis for the Helmholtz-type and structural resonance frequencies of a single layer MPP structure together with relationships on the air-frame relative velocity over the MPP surface at these resonances. Principled guidelines are provided for enhancing both the sound absorption and transmission properties of mutiple-layer MPP structures through suitable setting of the design parameters.