Ambient vibration refers to the low-amplitude, broadband vibration that a structure experiences continuously from environmental sources — wind, traffic, microtremors, and human activity. It is the excitation source used in operational modal analysis (OMA) for identifying structural modal properties without controlled input.
Ambient vibration is the low-amplitude, broadband vibration that all structures experience continuously from natural and anthropogenic sources. These sources include wind (the most common and generally the dominant source for tall structures), traffic (on bridges and adjacent roadways), microtremors (low-amplitude ground vibration from distant natural and human sources), and human activity (footsteps, machinery, HVAC systems). Unlike earthquake loading, ambient vibration is small in amplitude — typically producing strains well within the linear elastic range of the structure — and stationary over time, making it ideal for modal identification without disturbing the structure's operation or causing damage.
Ambient vibration is the primary excitation source for operational modal analysis (OMA) and, more broadly, for output-only modal identification of full-scale structures. Because ambient vibration is naturally broadband (containing energy across a wide frequency range) and always present, it allows identification of multiple modes without the need for controlled excitation — which is impractical for large structures in service. The measured response (typically acceleration) is processed using frequency-domain or time-domain methods (peak picking, FDD, SSI, ERA) to extract modal properties. The amplitude of ambient vibration is typically small — accelerations on the order of 0.1–10 milli-g — which requires sensitive instruments and careful signal processing, but ensures that the identification occurs in the linear elastic range.
Ambient vibration is widely used for several purposes. In structural health monitoring, ambient vibration testing provides a baseline characterization of a structure's modal properties, against which future changes (from damage, aging, or repair) can be compared. In model updating, ambient vibration data is used to calibrate finite element models of existing structures. In seismic assessment of existing structures, ambient vibration testing provides a non-destructive method for identifying structural properties and verifying analytical models. In earthquake engineering research, ambient vibration is used to characterize site response (through microtremor measurements) and to monitor the effects of nearby construction or ground motion on structures. Key challenges in ambient vibration testing include: (1) the low amplitude of the response, which requires careful instrumentation and noise reduction; (2) the need for long recording durations to obtain good statistical estimates of modal properties; (3) the unknown and variable nature of the input, which limits the ability to identify absolute modal scaling; (4) the influence of environmental variability (temperature, wind, occupancy) on identified properties; and (5) the sensitivity of the identification to sensor placement and data quality. In Iran, ambient vibration testing is increasingly used for assessment of existing structures, particularly bridges, historical buildings, and critical facilities, and for site characterization in seismic hazard studies. Modern practice increasingly uses continuous ambient vibration monitoring — with automated identification — as part of permanent SHM systems.