What is "anharmonicity"

n. (context mechanics English) Of or pertaining to the deviation of a system from harmonicity (being a harmonic oscillator)

In classical mechanics, anharmonicity is the deviation of a system from being a harmonic oscillator. An oscillator that is not oscillating in simple harmonic motion is known as an anharmonic oscillator where the system can be approximated to a harmonic oscillator and the anharmonicity can be calculated using perturbation theory. If the anharmonicity is large, then other numerical techniques have to be used.

As a result, oscillations with frequencies 2ω and 3ω etc., where ω is the fundamental frequency of the oscillator, appear. Furthermore, the frequency ω deviates from the frequency ω of the harmonic oscillations. As a first approximation, the frequency shift Δω = ω − ω is proportional to the square of the oscillation amplitude A:

Δω ∝ A

In a system of oscillators with natural frequencies ω, ω, ... anharmonicity results in additional oscillations with frequencies ω ± ω.

Anharmonicity also modifies the profile of the resonance curve, leading to interesting phenomena such as the foldover effect and superharmonic resonance.