Ultrasonic pulse‐echo measurements are used to determine the changes inc11E,c33D,c44D,c44E, andc12Ethrough the reversible ferroelectric—antiferroelectric phase transition in a Pb(Ti, Zr, Sn)O3ceramic. The results indicate that after the transition is enforced by hydrostatic pressure the material loses both electrical and mechanical orientation and becomes elastically isotropic.