  The deformable pseudospin Mitsui model is modified in order to take into
account interactions of the ordering dipoles of Rochelle salt with dipoles,
associated with switchable crystal defects. Using the Glauber-type kinetics of
the ordering and defect pseudospins, we calculate the linear, second, and third
order dynamic susceptibilities and piezoelectric coefficients of the system.
The defect-assisted dispersion of the dynamic characteristics below 1 kHz is
described. Behavior of the linear and non-linear susceptibilities close to
T_C1,2 is also satisfactorily described by the presented model.
