Visual-Width of an Ultrasonic Gaussian Beam on the Schlieren Photograph and Explanation to the Nonspecularly Reflected Sound Field

On the schlieren photograph, a continuous ultrasonic Gaussian beam and its nonspecularly reflected beams Chin. Phys. Lett. 16 (1999) 819 always have limited visual-width, although the theoretical spatial distribution of the sound field is a continuous function. To study this problem, the first step is to investigate the visual-width of the beam on the photograph related to the sound pressure at the centre of the beam by the threshold of the optical system caused by the refraction of light; the second step is to explain the visual-width of nonspecularly reflected field. By applying a relevant threshold, checked by the visual width of the incident beam, to cut the theoretical curves of the reflected sound field, one can find the visual-width of the two reflected beams and the gap between them are correspondent to that on the schlieren photograph.