The simplest model for describing an inhomogeneous structure is a double layer arrangement, where each layer is characterized by its permittivity  and its conductivity .  The relaxation time for such an arrangement is given by
Importantly, since the materials' conductivities are in general frequency dependent, this shows that the double layer composite generally has a frequency dependent relaxation time even if the individual layers are characterized by frequency independent permittivities.
A more sophisticated model for treating interfacial polarization was developed by Maxwell , and later generalized by Wagner  and Sillars.  Maxwell considered a spherical particle with a dielectric permittivity  and radius  suspended in an infinite medium characterized by . Certain European text books will represent the  constant with the Greek letter ω (Omega), sometimes referred to as Doyles constant.