a surgical apparatus comprises a body , an ultrasonic transducer , a shaft , and an end effector . the ultrasonic transducer is operable to convert electrical power into ultrasonic vibrations . the shaft couples the end effector and the body together . the end effector comprises an ultrasonic blade in acoustic communication with the ultrasonic transducer . the ultrasonic blade includes a recess region having a plurality of recesses . the recess region is tapered such that the cross - sectional area of the recess region decreases along the length of the recess region . the ultrasonic blade is also curved such that a central longitudinal axis of the ultrasonic blade extends along a curved path . a reference circuit is used to account for voltage drops of unknown values during operation of the surgical apparatus .