a transducer stack includes rings of piezo - electric ceramic alternating with metal electrodes along a threaded central shaft extending from a titanium back plate . a spacer or threaded element threaded on to the shaft holds the ceramic rings and electrodes in compression against the back plate . the transducer stack is mountable eccentrically to a horn of an ultrasonically - vibratable tool , away from an axis of an elongate waveguide extending from the horn . the transducer stack may vibrate in a flexural mode perpendicular to the waveguide , generating torsional mode ultrasonic vibrations in the horn and waveguide , or in a flexural mode parallel to the waveguide , generating longitudinal mode ultrasonic vibrations in the horn and waveguide . the transducer stack may be tuned by adjusting the length , mass and / or position along the shaft of the threaded element , the back plate and / or a second threaded element mounted to the back plate . motion sensors may be mounted to the horn . torsional vibrational modes of the horn and waveguide may be identified by motion amplitude of a peripheral sensor exceeding that of a near - axial sensor . the operation of the transducer stack may be controlled in response thereto .