Impedance models in time domain

2 Introduction We use throughout the e+iωt convention. Acoustic liners are usually described in frequency domain by their impedance. If we use a numerical method in time domain, this is to be translated into an equivalent relation. This is not straightforward. For example, it is principally not possible for an non-real impedance to retain this value for all frequencies. We have to extend the impedance as a function in the complex frequency. This requires certain specific conditions. If these are not obeyed, the impedance would be physically impossible. This would manifest itself by, for example, non-physical instabilities. It is remarkable that only relatively recently the frequency dependence of the liner was left unnoticed in a stability analysis in frequency domain [1].