  Sifted colimits (those that commute with finite products in sets) play a
major role in categorical universal algebra. For example, varieties of
(many-sorted) algebras are precisely the free cocompletions under sifted
colimits of (many-sorted) Lawvere theories. Such a characterisation does not
depend on the existence of finite products in algebraic theories, but on the
above fact that these products commute with sifted colimits and another
condition: finite products form a sound class of limits.
  In this paper we study the notion of soundness for general classes of weights
in enriched category theory. We show that soundness of a given class of weights
is equivalent to having a `nice' characterisation of flat weights for that
class. As an application, we give an elementary characterisation of sifted
weights for the enrichment in categories and in preorders. We also provide a
number of examples of sifted weights using our elementary criterion.
