Simultaneous measurements of acoustic pressure and particle velocity by an optical holographic method

Abstract

An extension of an optical, holographic interferometery method for the measurement of instantaneous acoustic pressure previously reported [J. A. Clark, Holographic Visualization of Acoustic Fields, J. Sound Vib. 56, (1978)] is described. Optical interference fringe patterns are photographed which determine the acoustic condensation or relative density variations associated with a propagating sound wave. If the usual linear equation of state for a liquid is assumed, these same fringe patterns determine the acoustic pressured distribution, but with a different scale factor. Since optical data is available over an extended region of space for a set of different times, it is possible to also determine other independent measures of the acoustic field, simultaneously with the pressure measurements, by spatially or temporally differentiating or integrating the data. A procedure for numerically obtaining the components of particle acceleration by spatial differentiation (assuming the equation of motion) and for then obtaining the components of particle velocity by temporal integration is demonstrated. A second procedure for obtaining pressure and velocity measurements without assuming a linear equation of state is also discussed. This latter approach could be useful in experimental investigations of some nonlinear effects associated with waterborne sound and shock waves. [Research supported by ONR.]