A Note on the Interpretation of Laser Speckle Patterns of Vibrating Panels to Yield Information Concerning Mode Shapes

It is often important to know the mode shape of a vibrating panel for the purpose of structural and noise analysis. Holographic techniques, besides being expensive and often difficult, have limitations on the amplitudes and areas they can deal with. This note first describes the formation of laser streak patterns and then demonstrates how, by a new interpretation, the complete mode shape of a panel may be found. Although this method does not yield amplitudes directly, its real time viewing capability enables the points of maximum amplitude to be readily identified and measurements made by conventional gauge methods if required. In combination with suitable reflective paints areas larger than 10 m 2 may be viewed and photographed, and amplitudes from a few wavelengths to well in excess of 50 accommodated. This technique constitutes a cheap and effective tool in vibration mode analysis.