Improvement of acoustic resistance equations in perforated plate absorbers with thin porous layers

Abstract

The absorption coefficient of perforated plate absorbers largely depends on the acoustic resistance of the backing layer, adjacent to the plate itself. The optimization process of Helmholtz resonators is focused on the maximization of their absorption coefficients, by tuning the acoustic resistance and reactance of the overall absorbing system. In this paper, the comparison of acoustic resistance of a perforated plate absorber for two characteristic cases is shown: one with a thin porous layer glued tightly to the backing of the perforated plate, and the other with a thin porous layer placed near the perforations, without tight contact. The measurements were done in the Kundt's tube, by using four characteristic porosities of the plate (4.4%, 8.5%, 12.9% and 16.9%) and a 2 mm thick porous layer. The porous layer can be simply added to the acoustic resistance in the case of its tight placement to the perforated plate. On the other hand, absorber resistance is much less increased when a loosely placed porous layer is introduced to the perforations, but still shows a positive correlation with the resistance increase in the case of tight placement. Ratios of measured resistances are shown for the mentioned cases, and a range of useful values of those ratios is introduced as an improvement to the analytical expression used for calculating the acoustic impedance. A novel correction factor for calculating acoustic resistance of a perforated plate with porous layer in loose contact is proposed.