Geometric correction factor for elliptic holes in perforated plates and its role in sound absorption coefficient

A comprehensive analysis was carried out in order to analyze the influence on the sound absorption performance of elliptical holes in perforated plates (PP) and micro-perforated plates (MPP). The analysis was carried out numerically using two end correction expressions (by Mechel, 1995, and Laurens et al., 2014) for elliptical holes and two impedance models (by Bauer, 1977, and Ingard, 1994). Starting from the above mentioned formulas, two geometric correction factors (GCF) were derived for the end correction of elliptic holes, in terms of an eccentricity related symmetry factor. The impedance models were used to numerically compare how the GCFs influence sound absorption characteristics, using a typical configuration of a thin perforated panel backed by an air gap and a solid wall. The two GCFs show similar behavior for the high and low limit of eccentricity values but differ for medium values. Using Mechel?s GCF, low eccentricity ellipses can be assumed as circular perforations whereas high eccentricity perforations show a GCF<1; on the other hand, Laurens? GCF predicts a value >1 in a wide range of eccentricities. In PP applications the eccentricity affects the frequency response of the system using both GCFs. Concluding, both GFCs models predict that the performance of the resonant absorber may be influenced by the elliptical geometry of the holes, which affects significantly the end correction; however, experimental measures should be performed to define the validity range of each expression.