Steven E. Fick
NIST, Gaithersburg, MD 20899
Direct measurements of time-average spatially integrated output power radiated into reflectionless water loads can be made with high accuracy by exploiting the radiation pressure which causes an absorptive target intercepting the entirety of an ultrasound beam to experience a force proportional to the total beam power, which, in turn, is readily determinable as accurately as the radiation force can be measured in isolation from confounding forces arising from such effects as buoyancy, surface tension, and vibration. A radiation force balance (RFB) constructed at NBS in 1974 is the first and only RFB known to be designed to employ simultaneous mechanical and electrical signal processing based on temporal modulation of the incident ultrasound at a frequency well above those characteristic of confounding phenomena. Equipped with purpose-built electronics, the RFB is operated by manually equalizing the radiation force and a counterforce generated by an actuator calibrated against reference masses using dc current as the transfer variable. Improvements made to the RFB during its one overhaul in 1988 have nearly halved its overall measurements uncertainty, and extended its capabilities to include measuring the output of ultrasonic systems with arbitrary pulse waveforms.