Glossary term
Resonance
Large-amplitude response that occurs when excitation frequency is near a natural frequency.
Definition
phenomenonLarge-amplitude response that occurs when excitation frequency is near a natural frequency.
Resonance occurs when periodic or broadband excitation efficiently transfers energy into a system mode, producing amplified response near a natural frequency. It appears in structures, rotating machinery, acoustic cavities, electrical circuits, control loops, and fluid-structure interactions.
Resonance is amplified response caused by efficient energy input near a mode of a system. A forcing frequency does not need to match a natural frequency exactly; the amplification depends on damping, forcing shape, modal participation, boundary conditions, and how close the excitation is to the mode’s frequency.
For a lightly damped single-degree-of-freedom system, response grows near the natural frequency and the peak becomes sharper as damping decreases. High Q factor means the system stores energy effectively and dissipates little per cycle, so resonance is narrow and strong. Higher damping broadens and lowers the peak.
Engineering consequences
Resonance can cause fatigue cracks, fastener loosening, excessive noise, inaccurate sensor readings, rotor instability, control-loop oscillation, acoustic amplification, or failure of electronics under vibration. It can also be useful: resonators, filters, oscillators, tuned mass dampers, ultrasonic devices, and musical instruments rely on controlled resonance.
Engineers manage resonance by shifting natural frequencies, changing mass or stiffness, adding damping, isolating vibration, avoiding excitation harmonics, balancing rotating machinery, modifying control bandwidth, or adding absorbers. In some systems, operating through a resonance during startup or shutdown is acceptable only if dwell time and amplitude remain controlled.
Diagnosis
Modal analysis, frequency response testing, run-up coast-down data, order tracking, impact testing, and spectral analysis can identify resonant modes. A useful diagnosis must distinguish resonance from direct forcing, imbalance, misalignment, looseness, aliasing, or control instability.
Common mistakes
A common mistake is assuming resonance is present whenever a vibration is large. Large response can also come from high forcing amplitude, impacts, transient events, or poor measurement placement. Another mistake is treating a natural frequency as fixed when support stiffness, temperature, fluid fill, preload, wear, or attached equipment can shift it. A good review states mode shape, excitation source, damping estimate, operating speed or frequency range, response limits, and mitigation method.