Abstract: Ultrasonic velocities have been measured on single phase Bi_1.7PB_0.3Sr₂Ca₂Cu₂O_8+x and Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_10+x polycrystalline samples. Anomalous changes were observed near 200K, indicating that some structural phase transition might occur. The elastic constants of these samples have been determined and compared with those of YB₂Cu₃O_7-y, and BaTiO₃. It is suggested that the interlayer coupling of these materials will dramatically decrease when the c-axes of their unit cells increas whereas their interatomic bonding forces are very close. Being corrected to void-free state, the Debye temperature of Bi-based materials is estimated as 270±20K. The temperature dependence of Debye temperature for these materials is also calculated and reported for the first time