We present a study of the smectic response function close to the smectic A to nematic transition, as a function of temperature. The progressive evolution from an overdamped to an underdamped behavior is evidenced, and we can deduce a temperature dependance for the compressional elastic constant of octylcyanobiphenyl (8 CB) B α (TNA - T)2x with 2 x ≈ 0.61 ± 0.05