Characterization of an experimental wavenumber fitting method for loss factor estimation using a viscoelastically damped structure

This study presents the experimental characterization of a relatively new method used to measure the damping performance of a structure. The method evaluated is the complex wavenumber fitting method in which an analytical description of the response over the structure surface is adjusted to match experimental measurements of the response over the same region. In computing the analytical response, a large number of points on the structure can be considered without much expense. In measuring that response, though, one must consider both experimental complexity and measurement time when considering the number of sensor locations. The goal of the present study is to examine the number of measurement points needed in the complex wavenumber fitting method to obtain a loss factor of sufficient accuracy. To that end, the loss factor for a viscoelastically damped sandwich beam with intermediate bolts is determined for different numbers of measurement points. To facilitate this study, a scanning laser vibrometer is used to measure the response at many points on the structure. Then, the wavenumber is estimated by iteratively fitting an analytical wave description to the response. For various selected sets of response points, it is shown that this approach is sensitive to both the number of measurement points as well as the coherence of that data. Experimental examples are presented to demonstrate those effects. Recommendations for better signal-to-noise ratio using the scanning laser system are also provided.