In periodic forced vibration, the term fundamental refers to the lowest frequency component present in a harmonic train. For example, most rotating machines have frequency components at multiples of shaft rotational frequency and the fundamental component is usually at rotational frequency (e.g. at 30 Hz in a machine rotating at 1800 rpm).

In structural vibration, the term fundamental natural frequency is used to denote the frequency of the lowest mode of vibration. If the system can vibrate independently in several directions, the lowest mode in each direction might be referred to as fundamental. The use of the term fundamental in structural vibration does not imply that natural frequencies are integer multiples of each other.

See also: Frequency, Harmonic, Missing Fundamental, Subsynchronous.

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Subjects: Noise & Vibration Physics