Resonance is a characteristic of physical systems having a structure that allows energy to flow between various states at a specific, oscillatory rate (resonant frequency). For a stable resonant system at steady state the internal energy is either fixed without losses or the rate of energy input is equal to the energy losses.

Resonance is a characteristic of physical systems having some bounded structure in space that allows energy to flow between various states at specific, oscillatory rates (resonant frequencies). For a stable, resonant system at steady state the internal energy is either fixed and without loss or the rate at which energy enters the system is equal to the rate at which energy is lost.

Examples of resonant systems include mechanical structures like bridges and buildings, electrical, circuits like radio transmitters, musical instruments like the flute, thermoacoustic oscillators,and biological structures like the semicircular canals of the human ear. Resonant systems thus encompass a very broad spectrum in different types of energy flow and are found in many types of physical systems.

Resonance is also seen at all scales of nature from the nucleus of atoms to the expanse of galaxies.

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