  Problems involving disordered systems are usually analyzed for systems with
random disorder. However, there are many systems in which the main disorder
involves clusters with correlated differences between their properties and
those of the average system. A new approximation, the average trace
approximation, is proposed for calculating the diagonal elements of the Green
function, and hence the density of states, in such systems. As an example,
application of the method to a simple cubic array of harmonic oscillators shows
that correlation in the disorder leads to a peak in the low frequency density
of states, a result confirmed by computer simulations.
