Measurement of acoustic boundary fields by real‐time optical holographic interferometry

Abstract

A real‐time extension of a recently developed optical holographic interferometry method [J. A. Clark, J. Sound Vib. 56(2) (1978)] is reported. In this extension of the basic method, a single hologram of the visualization region in an acoustic test facility is recorded. Then the hologram is developed and reinserted into its original position in the optical system. If the visualization region is illuminated for a short duration while an acoustic wave is propagating through the region, interference fringes can be observed—even directly by eye! —which can be quantitatively interpreted as contours of equal instantaneous pressure. These fringe patterns can also be photographed with a still or motion picture camera. The measurement method is used to investigate scattering of sound at a pressure release boundary such as a free liquid surface or a bubble screen. Motion picture records of the scattering process observed as a function of time, frequency, and source strength will be shown. It is demonstrated for the first time that the instantaneous pressure field can be observed in the region immediately adjacent to a free acoustic boundary. [Research supported by ONR.]A real‐time extension of a recently developed optical holographic interferometry method [J. A. Clark, J. Sound Vib. 56(2) (1978)] is reported. In this extension of the basic method, a single hologram of the visualization region in an acoustic test facility is recorded. Then the hologram is developed and reinserted into its original position in the optical system. If the visualization region is illuminated for a short duration while an acoustic wave is propagating through the region, interference fringes can be observed—even directly by eye! —which can be quantitatively interpreted as contours of equal instantaneous pressure. These fringe patterns can also be photographed with a still or motion picture camera. The measurement method is used to investigate scattering of sound at a pressure release boundary such as a free liquid surface or a bubble screen. Motion picture records of the scattering process observed as a function of time, frequency, and source strength will be shown. It is demonstrated for the ...