Expressions for the transient and frequency response of materials showing exponential distributions of transition rates are summarized and related to recent work in the field. Responses arising from temperature-independent distributions (class I) are contrasted to those that depend in a simple way on temperature (class II). The distinction between dielectric and conducting systems is particularly emphasized, and the different temperature dependences of their frequency power-law exponents for class-I situations are discussed in detail. Besides presenting model frequency-response curves, it is shown that an exponential distribution of activation energies can lead to conductance frequency response very similar to that recently found by Lee et al. for two glasses and several ionically conducting single crystals.
