07-24U

Within the modified proton ordering model with taking into account linear in strain $\varepsilon _{6}$, $\varepsilon _{4}$ and $\varepsilon _{5}$ contributions to the energy of proton system within the four-particle cluster approximation for short-range and the mean-field approximation for long-range interactions we have calculated corresponding thermodynamic potentials, transverse dielectric, piezoelectric, elastic properties, and also absorption factor and ultrasound speed for the clamped and the free KD$_2$PO$_4$ type crystals. Numerical analysis of the obtained results is performed, and the set of the theory parameters providing the best fit to the available experimental data for M(H$_{1-x}$D$_x)_2$XO$_4$ (M = K, Rb, X = P, As) ferroelectrics is found.