AbstractStatic and dynamic heterogeneity of disordered system is one of the current topics in materials science. In disordered ferroelectric materials with random fields, dynamic polar nanoregions (PNRs) appear at Burns temperature and freeze into nanodomain state below Curie temperature (T_C). This state is very sensitive to external electric field and aging by which it gradually switches into macrodomain state. However, the role of PNRs in such states below T_C is still a puzzling issue of materials science. Electric field and aging effects of uniaxial ferroelectric Sr _x Ba_1−x Nb₂O₆ (x = 0.40, SBN40) single crystals were studied using Brillouin scattering to clarify the critical nature of PNRs in domain states below T_C. On field heating, a broad anomaly in longitudinal acoustic (LA) velocity at low temperature region was due to an incomplete alignment of nanodomains caused by the interaction between PNRs. A sharp anomaly near T_C was attributed to the complete switching of nanodomain to macrodomain state owing to the lack of interaction among PNRs. After isothermal aging below T_C, the noticeable increase of LA velocity was observed. It was unaffected by cyclic temperature measurements up to T_C, and recovered to initial state outside of a narrow temperature range above and below aging temperature.