Full definition
Graphical representation of vibration amplitude vs. frequency obtained by Fast Fourier Transform (FFT). Enables precise diagnosis: 1x RPM = unbalance, 2x RPM = misalignment, 0.4-0.5x RPM = looseness, defect frequencies = bearing faults (BPFO, BPFI, BSF, FTF). Measured in mm/s (velocity) or g (acceleration). Equipment: SKF Microlog, Emerson CSI, Fluke 810. ISO 10816 defines severity by machine type.