The present invention may be embodied as an eccentric vibration system comprising a major eccentric member, a minor eccentric member, first and second motor assemblies, and a transmission system. The transmission system operates in a first mode when a torque of the first motor assembly is less than a torque of the second motor assembly and in a second mode when the torque of the first motor assembly is greater than the torque of the second motor assembly. When the transmission system operates in the first mode, longitudinal vibratory forces generated by the major eccentric member substantially cancel longitudinal vibratory forces generated by the minor eccentric member. When the transmission system operates in the second mode, longitudinal vibratory forces generated by the major eccentric member are added to longitudinal vibratory forces generated by the minor eccentric member.