Comparison of moments and couple-generating forces near discontinuities in structural-acoustic systems

In modeling vibration isolators in structural-acoustic systems, the isolator's dynamic properties are often treated as acting only in the axial direction as moments are often neglected. Furthermore, the size, or scale, of the isolator is often neglected and the isolator is assumed to act at single points on the connected structures. Previous work has shown that concentrated moments can be particularly important when located near a fixed support or a structural discontinuity. This research extends that work to examine the importance of moment scale effects for a system containing a distributed structural discontinuity with its own scale. Moment scale effects are examined by determining the difference in radiated acoustic power for a simple system that is excited by a couple-generating distributed force and a concentrated moment. The distributed force produces a couple that is equivalent to the concentrated moment. As a result, only the scale is being examined. Particular interest occurs when the excitation is located near the structural discontinuity. Based on the cases studied here, moment scale is shown to be important at lower frequencies when the excitation is located near the edges of the discontinuity. At higher frequencies, any overlap of the excitation and discontinuity may warrant the need to consider moment scales.