Effects of Volumetric Modes on the Optimization of Micro- Perforated Multi-Layer Absorber Panels
Résumé
Acoustic wall-treatments composed of thin, lightweight micro-perforated multilayer panels (MPMPs) offer a potential solution for broadband absorption of ambient noise, with applications in building acoustics and transport systems requiring constraints on mass and thickness. A MPMP with a negative porosity gradient, induced by a decrease in perforation diameters, can ensure, after optimization, absorption greater than 0.9 over three octaves beyond 140 Hz, while maintaining sub-wavelength thickness. However, such a configuration presents practical limitations linked to the vibrating response of thin panels, which can degrade the broadband performance, and also to the difficulty of producing cylindrical micro-perforations with diameters less than 200 microns on millimetric thicknesses. This study examines the effect of the first volumetric modes of thin micro-perforated plates on the absorption, reflection and transmission loss of functionally-graded MPMPs. The optimization effort focuses on the distance between perforations, rather than their diameter, as well as on the depth of the cavities and thickness of the panels in order to maximize the total absorption of MPMPs from 100 Hz to 1 kHz. The impedance translation method and a spectral analysis of the MPMPs' scattering matrix have been implemented. Their effective properties are also examined. The optimization process succeeds in compensating for the absorption losses induced by the volumetric modes of the panels, except for those due to the first mode, which is strongly coupled to the acoustic resonances. An optimal decrease in perforation diameter remains the most effective strategy for merging resonances at low and medium frequencies, while an adequate increase of the holes pitch is more suitable for medium frequencies. These mechanical and geometrical parameters must be taken into account when optimizing MPMPs, depending on the frequency range targeted.
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