Random decrement technique

The random decrement technique (RDT) extracts a structure’s free-decay signature, and hence its natural frequency and damping, from its response to random excitation. Many segments of the measured response are aligned on a common trigger condition (a fixed level crossing or level-plus-zero-slope) and averaged; the random forced part cancels while the deterministic free-decay response remains, giving the random decrement signature. Fitting an exponentially decaying sinusoid to that signature yields the modal frequency and damping ratio. In marine use it identifies roll damping from full-scale or model motion records and hull-girder modal damping from ambient vibration, without needing a measured input force.