the spectrum of the measurement signal in accordance with fig2 c when the approximate rotating frequency is known for example in this case the frequency 10 hz it is possible to search for a spectrum peak from the spectrum of the measurement signal in the vicinity of said frequency i e 10 hz or one of its multiples n 10 hz i e 20 hz 30 hz 40 hz etc when the spectrum peak is found it is possible to separate said frequency component from the measurement signal for example by means of a narrow band pass filter wherein the result obtained is a periodical measurement signal that follows the rotating frequency of the object in a frequency window determined by the band pass filter said frequency window being determined by means of cut off frequencies in the following the calculation of the synchronized time average is described in more detail with reference to the flowchart in fig4 that illustrates the application of the method this averaging can be carried out in the following way in the first stage step 41 the allowed variation e 1 for the rotating frequency f during the measurement greatest allowed variation f u2212 e 1 u2212 f e for the rotating frequency f as well as the greatest error e 2 allowed for the given rotating frequency f e greatest error allowed for the rotating frequency f.