Sub-harmonics can best be described as...

Sub-harmonics are essentially frequency components that occur at a fraction of the fundamental frequency, typically represented as whole-number divisions such as one-half times the fundamental frequency (1/2x), the fundamental itself (1x), and so forth. This definition illustrates how sub-harmonics relate to the primary frequency of a system, highlighting that they are lower frequency components than the fundamental.

Understanding this concept is vital in vibration analysis, as sub-harmonics can indicate specific types of mechanical issues. For example, if a rotating machine emits vibrations at a frequency of 1/2x, it may suggest that there are balance issues or alignment problems within the system.

The other choices refer to different aspects of harmonic frequencies or misrepresent the concept of sub-harmonics. While some options mention harmonics, they do not align with the definition of sub-harmonics as frequency components that are fractions of the fundamental frequency. Focusing on the correct representation allows for accurate identification and diagnosis of the underlying issues in vibration analysis.