  The cosmic microwave background anisotropies produced by active seeds, such
as topological defects, have been computed recently for a variety of models by
a number of authors. In this paper we show how the generic features of the
anisotropies caused by active, incoherent, seeds (that is the absence of
acoustic peaks at small scales) can be obtained semi-analytically, without
entering into the model dependent details of their formation, structure and
evolution.
