Measuring Area Vibration Mode Shapes with a Continuous-Scan LDV

The out-of-plane vibration response mode shape of a vibrating surface may be measured very conveniently by using a scanning laser Doppler vibrometer (LDV). Sinusoidal vibration within a rectangular area may be measured by scanning at a uniform rate over the whole area, the mode-shape being recovered by demodulation. This process may be applied to any vibrating surface, but a large amount of data may be generated, and results may be polluted by speckle noise. Alternatively, the area may be covered by sine-scan inputs, simultaneously applied, at different scan frequencies, in x- and y-directions. The frequency spectrum of the LDV output then contains components at discrete frequencies, from which coefficients of a polynomial expression for the mode shape may be derived directly. Both magnitude and phase data are required for each of the spectrum components. The process is only valid if the mode-shape is spatially smooth without, for example, any apertures within the scanned area but it has the particular advantage of representing the mode-shape by a relatively small data set. Repeat measurements at different test frequencies allow standard modal analysis techniques to be employed.