An optical nearfield measurement technique is described for determining the ultrasonic powers radiated by circular piston transducers under continuous wave and narrow-band pulsed excitation. The experimental apparatus is simple, relatively inexpensive, and also provides information concerning the acoustic pressure waveform and duty cycle. Continuous wave powers as low as 25 μW can be measured. For narrow-band pulsed excitation, using duty cycles common to some medical diagnostic systems, temporal average powers as low as 25 nW should be detectable.
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Ward A. Riley (1980) studied this question.