Abstract: If a laser Doppler vibrometer is used in a continuously-scanning
mode to measure the response of a vibrating structure, its output spectrum
contains side-bands from which the response mode shape, as defined along the
scan line, may be obtained. With impact excitation, the response is the
summation of a set of exponentially-decaying sinusoids which, in the frequency
domain, has peaks at the natural frequencies and at `sideband' pseudo-natural
frequencies, spaced at multiples of the scan frequency. Techniques are
described for deriving natural mode shapes from these, using standard modal
analysis procedures. Some limitations as to the types of mode which can be
analysed are described. The process is simple and speedy, even when compared
with a normal point-by-point impact test survey. Information may also be
derived, using a circular scan, on the direction of vibration, and angular
vibration, at individual points.
