A STUDY OF THE INITIAL DECAY RATE OF TWO-DIMENSIONAL VIBRATING STRUCTURES IN RELATION TO ESTIMATES OF LOSS FACTOR

Abstract

The aim of this study was to obtain a better understanding of the process of the initial decay of energy in relation to the estimates of damping loss factor; to investigate the spatial variation of the initial decay rate in order to obtain more reliable estimates of damping loss factor from the decay rate method; and to compare the spatial averages and spatial variation of damping loss factor between the decay rate method and the power input method. The initial decay rate of energy was experimentally investigated on uniform plates. The energy mean free path time was introduced as a factor to characterize the lower limit of a decay interval for fitting an initial decay slope, and to determine the initial point of a decay curve. It is concluded that the initial decay slopes can sometimes be determined within a very short decay interval (e.g., less than 10 dB) provided that the corresponding time interval is larger or much larger than the energy mean free path time. Additionally, the effect of the direct field on the initial decay can be ignored provided that the response point is far enough from the drive point. An analysis of the spatial variation of initial decay rate showed that an increase in the number of modes per band, and light and moderate damping can reduce the spatial variation. High damping may increase the spatial variation. For a frequency band with more modes, fewer response points need to be used to obtain a stable estimation of spatial variance. It was confirmed that the damping loss factor determined from the decay rate method was in general in good agreement with the power input method. The comparison of the spatial variances of damping loss factor between the two methods showed that the decay rate method gives a more reliable estimate of the damping loss factor of the plates. It is concluded that the decay rate method is to be preferred to the power input method when determining the damping loss factor of a system.