Sound absorption measurements for micro-perforated plates : the effect of edge profile

In this study, the effect of perforation edge profile in micro-perforated plates (MPPs) is investigated and compared with the sharp edge case. MPPs are metal plates with small holes (perforations) that absorb incident sound waves using the Stokes boundary layers developed inside and in the vicinity of the perforations. The perforation diameter should be optimized so that the Stokes layers cover almost the entire perforation thus maximizing the absorbed sound energy due to viscosity. Hence, designing MPPs is a tuning problem. The existing analytical model for evaluating the transfer impedance of the MPPs is based on the assumption that the edge profiles of the perforations are sharp. In other words, the angle between the plate surface and the inner surface of the perforation is 90◦;. This case is not very practical to achieve in mass production of MPPs, since the perforations are formed by drilling and / or punching in manufacturing. To investigate the effect of the edge profile on the performance of the MPP, we design samples with different edge profiles such as sharp and 45◦-chamfers on both edges. We carry out open-end reflection coefficient measurements in a semi-anechoic room with 6 microphones. The relative error of this setup is measured to be less than 0.5% for closed-end reflection coefficient between 100 Hz and 700 Hz in the linear regime. The measurements are also compared with a linear numerical model. In our measurements, we observe that the edge profile can have an effect up to 30% in resistance and 18% in reactance.