I use the Johnson-Champoux-Allard (JCA) model to predict a sound absorption coefficient of a rigid material. I've seen a lot of articles where this procedure is done, all the formulas are provided. Unfortunately, what I get is, for example, α = 1.5281 + 0.2485j (at 1500Hz) while it should be a real number 0 < α < 1 instead. I understand that the real part of impedance represents acoustic resistance and the imaginary part - acoustic reactance. But how do I combine these to achieve an absorption coefficient?