**3.1 Harmonic (low) frequency range**

Basically this subsection deals with absolute vibration spectral components of frequencies determined by *f* = *nf*0, where *f*0 results from rotational speed, and to some extent also with relative vibration vectors or orbits. In practical applications, components corresponding to *n* > 4 are seldom accounted for; this means that we are dealing with first four harmonic and sub-harmonic (*n* < 1) components. As each of these is typically influenced by a number of condition parameters, it is convenient to speak in terms of possible malfunctions and faults rather than frequencies.
