Ultrasound Instrument shaft assembly includes ontology and extends from ontology to distal end. Shaft assembly includes acoustic waveguide. The instrument further includes that end effector includes ultrasound knife. Ultrasound knife carries out sound communication. The acoustical waveguide instrument further includes a sensor, for sensing at least one shaft assembly and/or end effector. End effector is configured as the power of activation variation based at least one characteristic sensed by the sensor.
