# Spectrum Analysis of Vibration Data – Synchronous, Non-Synchronous and Sub-Synchronous Vibration

Trying to decipher raw vibration data without additional tools can be extremely complicated; the vibration signal will be a waveform that can be a summation of various independent sources vibrating at different frequencies. Thankfully, the waveform can be mathematically converted into spectral data using a Fast Fourier transform (FFT) that separates it into the independent frequencies.

A machine’s running speed is typically referred to as the 1X frequency. Vibration can be divided into three categories depending on its relationship to the 1X frequency:

1. Synchronous vibration
2. Non-synchronous vibration
3. Sub-synchronous vibration

Any vibration that occurs at the 1X frequency, or a whole number multiple of it is referred to as a synchronous vibration.

Any vibration that is greater than the 1X frequency, but is not a whole number multiple of it is called a non-synchronous vibration.

Vibrations at frequencies lower than the 1X frequency are called sub-synchronous vibration.

Spectral data can be very useful for determining the source of a vibration using the above information. The table below shows the various sources for different types of vibration.

Table 1: Sources of synchronous, non-synchronous and sub-synchronous vibration

The source of vibrations at different frequencies can be determined depending on the specific application e.g. fanscompressorswashing machines, etc., and it is important to categorize vibrations in relation to a machine’s running speed.