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In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency). Examples are the acoustic resonances of musical instruments, the tidal resonance of the Bay of Fundy, orbital resonance as exemplified by some of the Jovian moonss, and resonance in electronic circuits.

An improperly constructed bridge, like the Tacoma Narrows Bridge (Galloping Gertie), can be destroyed by its resonance; that is why soldiers are trained not to march in lockstep across a bridge, but rather in breakstep.

In an electrical circuit, resonance occurs at a particular frequency when the inductive reactance and the capacitive reactance are of equal magnitude, causing electrical energy to oscillate between the magnetic field of the inductor and the electric field of the capacitor.

Resonance occurs because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor and the discharging capacitor provides an electric current that builds the magnetic field in the inductor, and the process is repeated. An analogy is a mechanical pendulum.

At resonance, the series impedance of the two elements is at a minimum and the parallel impedance is a maximum. Resonance is used for tuning and filtering, because resonance occurs at a particular frequency for given values of inductance and capacitance. Resonance can be detrimental to the operation of communications circuits by causing unwanted sustained and transient oscillations that may cause noise, signal distortion, and damage to circuit elements.

Since the inductive reactance and the capacitive reactance are of equal magnitude, ωL = 1/ωC , where ω = 2πf , in which f is the resonant frequency in hertz, L is the inductance in henries, and C is the capacitance in farads when standard SI units are used.

Source: Federal Standard 1037C

While resonance is an important consideration for instrument builders as most acoustic instruments utilize resonators, such as the body of a violin, composers have begun to make resonance the subject of compositions. Alvin Lucier has used acoustic instruments and sine wave generators to explore the resonance of objects large and small in many of his compositions, the complex inharmonic partials of a swell shaped crescendo and decrescendo on a tam tam or other percussion instrument interact with room resonances in James Tenney's Koan: Having Never Written A Note For Percussion, and Pauline Oliveros and Stuart Dempster regularly perform in large reverberant spaces such as the two million gallon cistern at Fort Warden, WA, which has a 45 second reverb.