Method and a measuring device for measuring and evaluating a measurable signal emitted by a system being investigated during operation or in response to an external excitation. The device includes a signal receiving unit, an output unit and an evaluating unit. The latter includes elements for Fourier transforming the time dependant signal captured by the receiving unit and characteristic of the system as a whole, the signal is transformed directly in its captured form; elements for calculating the power density function of the Fourier transform; elements for calculating an invariant quantity in the form of the exponent α of the power density function versus the frequency, as well as elements for providing data indicative of the invariant quantity obtained to the output unit.