For experimental determination of structural loss factor with mechanical excitation, the excitation location — i.e. “where to excite?” — and sensor placement — i.e. “where to measure?” — are quintessential considerations. For a highly-damped panel a significant portion is not experiencing reverberant field conditions, especially in higher frequency bands. That is, localized disturbances “die-out” before they can reflect off boundaries. As energy flows away from the excitation point, the energy the level in the direct field is higher than elsewhere, thereby resulting in higher response levels. Since the level of response is inversely proportional to damping, the loss factor predicted inside the direct field is underestimated. The size of direct field is proportional to frequency and loss factor. Therefore, for a highly damped and/or small plate, loss factor estimation based on randomly positioned accelerometers—which have a higher probability of being inside the direct field—will tend to underestimate damping.

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