Noise in a vibration signal can creep in from background vibration sources such as noise generated by the machine being monitored, or from adjacent machinery.
Typically the noise signal has low amplitudes and is close to zero in the spectral data; however, in some instances the noise floor can appear to be raised and this is something that should be noted during analysis.Below are two figures, one showing spectral data without noise, and the other showing a raised noise floor.
Spectral data without noise in the signal
Spectral data with a raised noise floor
Some noise can be expected in the data, however if a noise floor is present it may be caused by either severe bearing wear or by resonances in the structure. In some instances, the noise floor may also indicate poor data collection.
For severe bearing wear, the noise floor will rise higher as the wear becomes worse. Structural resonances on the other hand usually manifest as humps in the spectrum. They have a large base with peaks sitting on top of them. Resonances tend to be directional and the noise floor may only appear in data from a single axis, and over a limited band.
Be sure to investigate the cause of a raised noise floor as it may indicate severe issues and can lead to failures.