A non-linear impedance model for micro-perforated liners

Abstract

Perforated plate liners are widely used to reduce noise emissions from turbofan engines. Micro-perforated plates (MPP), with a porosity below 5% and apertures diameter below 1mm, are of particular interest for this purpose since they also help to reduce the flow drag, compared to conventional perforates. In the nonlinear regime without flow, existing models such as those of Guess or Maa are not suitable for MPP liners, i.e. when the acoustic velocity becomes high in the perforations. The effective velocity through the perforate is a function of the impedance and conversely. Therefore an iterative procedure is required to obtain accurate impedance predictions in the non-linear regime. In the present study, we propose to modify Guess and Laly impedance models by implementing an iterative procedure to compute the acoustic velocity. Good correspondence with impedance tube measurements is observed for sound pressure level up to 150 dB for MPP with a porosity and a neck diameter down to 1.4% and 0:3mm respectively. The modified model can be adapted to predict the impedance when either a white-noise source or a sine-sweep source is used in the experimental setup. In addition, we illustrate that considering only the absorption coefficient is misleading and that it is important to assess the resistance and reactance to properly evaluate the accuracy of an impedance model.