  A simple criterion for melting of two-dimensional crystals with soft
long-ranged interactions is proposed. It states that the ratio of the
transverse sound velocity of an ideal crystalline lattice to the thermal
velocity is a quasi-universal number close to $4.3$ at melting. This criterion
is arrived by reference to the
Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young theory of two-dimensional
melting, combined with the observation that the ratio of
transverse-to-longitudinal sound velocities is small in the soft interaction
limit. Application of this criteria allows estimating melting lines in a simple
yet relatively accurate manner. Two-dimensional weakly screened Yukawa systems
represent one relevant example considered.
