Flicker noise ($S(f)\propto 1/|f|$)

Low-frequency noise with an approximately inverse-frequency power spectral density over a specified range. It can dominate the flat spectrum of a weakly confined mechanical coordinate or an electronic readout at low frequencies. A $1/f$ law cannot hold with finite total variance over unlimited frequency range, so its measured bandwidth and low-frequency cutoff matter. An observed excess alone does not uniquely identify the microscopic source.