Measurement of Acoustic Powers in the Microwatt-Milliwatt Range

Abstract

The recent increase in the use of ultrasonics in medical diagnosis has led to the need for methods to measure very low acoustic power outputs in the megahertz frequency range. A system similar to that described by Kossoff [J. Acoust. Soc. Amer. 38, 880–881 (1965)] has been developed which uses a Cahn electrobalance to measure acoustic radiation force on an absorbing target. At present the system can easily detect power outputs of 30 μW. Sources of error and how they can be eliminated in such measurements will be discussed. Recently questions have been raised about the validity of the classical theoretical expression for Langevin radiation pressure applicable to the case of a plane traveling wave impinging on an absorbing target in an open vessel. However, a critical review of the theory using a formulation based on Euler's momentum theorem leads one to the conclusion that the Langevin radiation pressure is just equal to the classical value of the energy density even when nonlinearities of the propagation medium and distortion of waveform are considered. Experimental tests of this theory are in progress. [This work was supported by the HAS Project at the University of Vermont, and by the National Institutes of Health.]