Abstract: The free vibration samplings rising from a running structure or machine excited by impulses, such as an aircraft in its flutter flight testing, are generally of multiple modes and high noise level. An optimal ensemble average method is presented by which a selected interesting structural modal element will be highlighted and the noises depressed greatly.The method is based on the similarity between the free vibration spectra and frequency response function in showing the vibration characteristics of structures. The optimized averaging implies that the original phases of the selected modal elements in all the free vibration samplings should be nearly the same. This fulfillment can be judged by making the selected modal resonant peak of the averaged free vibration spectra have its maximal value.From the optimal average free vibration spectra, a purer modal free vibration of the selected mode can be picked out furthermore, which makes the modal parameter identification become very simple and highly accurate at the same time.

Key words: structural vibration, modal analysis, signal analysis, flutter, dynamic testing