A vibrational generator including an inertial body and a housing means which are relatively movable and where hydraulic fluid is directed into the inertial body through a controlling valve of the inertial body and directed alternately into one or other working chamber which causes the movement of the housing means and any attached load. There is also disclosed a wave form shape detectable from the pressure or flow rate of the hydraulic fluid so that the relative frequency of the drive as compared to resonance can be determined and used for control to maintain resonance. The apparatus has particular application in the frequency range of 20 Hertz to 1000 Hertz.
