Acoustic Radiation Pressure and its Use in Power and Intensity Determinations

Abstract

The radiation force upon an object placed in a sound field can be used to determine the acoustic power or intensity. Two types of test objects have been extensively used as targets in radiation pressure determinations. The first is either an absorbing or reflecting target which intercepts the entire sound beam. For the case of a traveling plane wave in an open vessel impinging on an absorbing target, the radiation force F equals the acoustic power divided by the velocity of sound c even when nonlinearities of the propagation medium are taken into account. The second type of test object used for calibration is a sphere. For an elastic sphere of radius a surrounded by a nonviscous fluid in a traveling plane wave F = Yπa2I/c. Here I is the local intensity and Y is the acoustic radiation force function. Experimentally the forces on targets have been determined by observations of deflections for pendulum arrangements, measurement of change in target weight using analytical balances, measurement of change in displacement of targets attached to floats, and determination of the force necessary to maintain the target in equilibrium. For measurements of low acoustic powers, it is necessary to minimize fluctuations associated with building vibration, convection currents, and interfacial effects. [Work supported by HAS project at the University of Vermont and by the National Institutes of Health.]