Nonlinear acoustic resistance of perforated plates in the transition regime

With this study, we provide a global link between linear and non-linear acoustic resistance of a perforation as a function of orifice geometry and Strouhal number. High amplitudes of acoustic pressure induce flow separation at the edges of the orifices. This is a non-linear mechanism increasing the acoustic resistance of a perforation. Quasi-static models have been used to predict the acoustic resistance in the non-linear regime, yet the linear to non-linear transition has been out of focus so far. We carry out open-end impedance measurements in a semi-anechoic room with four different samples. These samples are selected to have different orifice geometries and edge profiles, so that the conclusions would be general. The most prominent result of this study is that the change in the non-linear resistance can be expressed as a function of the Strouhal number only, if the resistance is corrected for the vena-contracta factor which can be deduced from steady flow measurements.