08-20U

Within modified proton ordering model of ferroelectric compounds of KH$_{2}$PO$_{4}$ family with taking into account linear on strain $\varepsilon _6 $ contribution into energy of proton subsystem, but without taking into account tunneling, within the four-particle cluster approximation we study transverse dielectric, piezoelectric, elastic and dynamic characteristics of ND$_4$D$_2$PO$_4$ type antiferroelectrics. Sound velosity and sound attenuation in crystals are calculated. At the proper set of the parameters of theory in paraelectric phase correct quantitative description of the available experimental data for ND$_{4}$D$_{2}$PO$_{4}$ and NH$_{4}$H$_{2}$PO$_{4}$ is obtained.