Vibration exciter

A vibration exciter adapted for location in a cavity in a member to be vibrated, the cavity having a cavity wall surrounding and spaced from an axis and to which a portion of the exciter can be mounted, the exciter comprising a motor coil assembly adapted to be aligned with the axis and having a coil rigidly fixed to a tubular member, the motor coil assembly having an outer peripheral face adapted to face the cavity wall, a magnet assembly disposed concentrically of the motor coil assembly and arranged for axial movement relative thereto, the magnet assembly having opposed generally disc-like pole pieces each having a periphery, the periphery of one of which pole pieces is disposed within and adjacent to the motor coil assembly, and the periphery of the other of which pole pieces is formed with a flange arranged to lie adjacent to and surround the motor coil assembly, the flange having apertures through which the outer peripheral face of the motor coil assembly is adapted to be fixed directly to the cavity wall in the member to be vibrated.