Panel Damping Loss Factor Estimation Using The Random Decrement Technique

Abstract

The use of the Random Decrement Technique (RDT) for estimating panel damping loss factors ranging from 1% to 10% is examined in a systematic way, with a focus on establishing the various parameters one must specify to use the technique to the best advantage. Throughout, loss factors are estimated in full or 1/3rd octave frequency bands with standard 1/3rd octave center frequencies. The full octave filters, which are more computationally efficient than the 1/3rd octave filters, are chosen in the experimental analysis of the damped plates with varied loss factor levels. Two computational models are examined: a single degree of freedom oscillator and a computational model of a uniform rectangular panel. The panel computational model is a finite element model of a rectangular plate mechanically exited at a single point. These models are used to establish a systematic process for evaluating: the appropriate narrow band filter selection; trigger conditions; record length required as a function of frequency and damping level; the averaging scheme; and, the curve-fitting scheme for assigning loss factors in narrow frequency bands. Loss factor estimates for three damped plates are computed using the "optimized" Random Decrement estimation algorithm and compared with estimates from the Impulse Response Decay Method. For the highly damped plates, RDT out-performed IRDM because the loss factors from IRDM are underestimated. For lightly damped plates, RDT and IRDM are consistent in most frequency bands.