An excitation apparatus suitable for use downhole having interengaged masses at least in part confined or guided so as to be movable as an interengaged assembly on an axis, one mass (“rotatable mass”) being rotatable relative to the other mass about the axis to cyclically vary the axial length of the assembly of the interengaged masses, a rotary drive, and an interposed spring (of any kind) between the rotary drive and the rotatable mass able to transmit torque from the rotary drive to the rotatable mass yet vary in its extent responsive to the interengaged masses.