The invention concerns a method whereby immobile sensors (C1 to C8) are used to measure periodically the vibrating motions of portions (2) of a body (3) rotating in front of them, such as blades, and determine their self-resonant frequencies by spectrum analysis. The invention is characterised in that the sensors are assembled into several arrays in different number and the determination consists in comparing the spectra of the different arrays by various operations selecting and rejecting rays. The method enables to substantially improve unambiguous detection of frequencies by extending the limits thereof.