  Within the framework of a modified proton ordering model of the KH_{2}PO_{4}
family ferroelectric crystals, taking into account a linear over the strain
\varepsilon_{6} contribution into the proton system energy, we obtain an
expression for longitudinal dynamic dielectric permittivity of a mechanically
clamped crystal using the four-particle cluster approximation and the dynamic
Glauber approach. At a proper choice of the model parameters, we obtain a good
quantitative description of available experimental data for these crystals.
