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This work focuses on the length correction due to radiation effect of a ductdiscontinuity or at the surface of a perforated plate in the linear acousticdomain and at large wavelengths. Two results are obtained from the comparisonof models used in the literature for different geometries of ductdiscontinuities or perforations. First, the mode-coupling at the discontinuitybetween two ducts or a perforated plate and air is negligible for lowfrequencies. Second, all consistent models of the length correction proposed sofar, for any geometries except slit-like, are similar when plotted as afunction of the square root of the perforation rate (the ratio of the ductcross-section areas for ducts). These two results imply the effect of thethickness of the ducts or the perforated plate can be accounted for in thelength correction expression independently from other geometric variables. Asingle approximate formulation for all geometries of perforations or ductdiscontinuities, except slit-like geometries, is proposed for practical uses.

Length correction of 2D discontinuities or perforations at large wavelengths and for linear acoustics