A percutaneous surgical instrument for de-bulking calculi or drilling bone includes an actuator for generating vibrations at ultrasonic frequencies and a horn coupled to said actuator for amplifying the actuator vibration. A fixed probe is attached to said horn for engaging the calculi and introducing the ultrasonic frequencies thereto. A floating probe is disposed concentric to and under said fixed probe and slidable thereover. A free mass is disposed between the horn and the floating probe for oscillating therebetween, in response to actuator vibration, for causing low frequency impact with the calculi.