The discontinuous stochastic characteristics of elementary magnetization processes can give large contributions to the anomalies of power losses as shown by a recent model. It has also been shown that a link exists between local power losses and the power spectrum ϕ( d/2, ω) of the variation with time of the flux through a sample cross-section equal to the square of its thickness d. A review is given of the experimental techniques developed to measure ϕ( d/2, ω). In one method the Fourier analysis is performed of the e.m.f. detected between contacts placed on the lamination surface and separated by a distance d. In another technique, a frequency analysis is made of the optical signal obtained from a high sensitivity Kerr effect apparatus, which permits one to follow domain dynamics over very small regions of area d 2. Measurements of the aftereffect of the permeability are further reviewed, in which a frequency analysis is performed of the decaying permeability signal, after the application of short demagnetizing pulses. This method is particularly useful when the magnetic aftereffect is characterized by a broad spectrum of activation energies, as typically in amorphous materials.