On Modal Parameter Estimates from Ambient Vibration Tests

Modal parameter estimates from ambient vibration testing are turning into the preferred technique when one is interested in systems under actual loadings and operational conditions. Moreover, with this approach, expensive devices to excite the structure are not needed, since it can be adequately excited by human activities, wind, gust, etc. In this paper, the comparison between two differeiit vibration testing techniques is presented. The first approach takes advantage of the Frequency Domain Decomposition, FDD, of the response cross power spectral densities to estimate both the natural frequencies and the "unscaled" mode shapes, whereas the second one, starting from the Hilbert transform of auto power spectral densities and taking account of the cross power spectral densities, allows one to get the frequency response functions. Both the techniques have been used to estimate the modal parameters of two simple structures (a beam and a plate), excited by an acoustical random signal.