A. Korpel et al., DIFFRACTION-FREE NATURE OF SCHLIEREN SOUND-FIELD IMAGES IN ISOTROPIC MEDIA, Journal of the Optical Society of America. A, Optics, image science,and vision., 11(10), 1994, pp. 2657-2663
We demonstrate that schlieren images of sound fields do not spread by
diffraction when propagating; they form generalized, diffraction-free
beams. Our experimental results are shown to be in good agreement with
theory and computer simulation.