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A high-acoustical Q object that vibrates in a resonant mode and has a moderately rough surface illuminated by dispersed laser light shows patterns analogous to Chladni (strewn sand) patterns. These patterns appear as regions in which the laser speckle is similar to that of a rough surface at rest, whereas the rest of the surface has fewer and smaller dark regions between bright speckles. The explanation for this is that regions that undergo the greatest variation of tilt (e.g., usually around nodes) blur the bright speckles. The effect is readily observed visually and may be photographed with some difficulty. Some examples with flat plates, along with the corresponding Chladni sand patterns at the same resonant frequencies, are given. The laser-speckle patterns usually occur at antinodes and are thus complementary to most Chladni patterns. Laser-speckle patterns can be used to provide a real-time noncontact method for vibration analysis, to study flat surfaces in a nonhorizontal position, to eliminate the shift of resonant modes caused by additional weight, and to study curved surfaces, as is shown with a cylindrical can.
© 1971 Optical Society of America
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