What is meant by Lines of Resolution in a Spectrum?

Vibration Spectrum is a 2 Dimensional Plot of Vibration Amplitude vs. Vibration Frequency - with Amplitude on Y Axis and Frequency on X Axis. Spectrum is usually captured using Data Acquisition Systems which could be in the form of Portable Instruments (like Vibration Analyzers) or Online Systems (like DAQ and Oliver).

Example of Acceleration, Velocity and Displacement Spectrum captured by DAQ and Oliver

The spacing between 2 data points on the X Axis (Vibration Frequency) is decided by the Lines of Resolution.

For example, if our X Axis frequency starts from 0 Hz (Fmin) and ends at 800 Hz (Fmax) and our Lines of Resolution is set to 800 - then there will a jump of 1 Hz between two adjacent frequencies. So, if a Y coordinate is present at 50 Hz X Coordinate - then the next Y coordinate will be present at 51 Hz X Coordinate. Any vibration present at 50.5 Hz will be missed or averaged out.

Now, for the same Fmin of 0 Hz and Fmax of 800 Hz - if the Lines of Resolution was set to 1600 - then the jump between two adjacent frequencies would have been (Fmax-Fmin)/Lines of Resolution = (800 - 0)/1600 = 800/1600 = 0.5 Hz. So, if a Y coordinate was present at 50 Hz X coordinate - then the next Y coordinate would have been at 50.5 Hz X Coordinate and the 50.5 Hz vibration which was missed out or averaged out in previous example would not have gotten missed out.

So, Lines of Resolution determines how detailed your Spectrum will be. Higher Lines of Resolution means a more detailed spectrum. Lesser Lines of Resolution means a less detailed spectrum. But please remember - that increasing the lines of resolution means a larger capture time for the spectrum.

A wise vibration analyst starts with a wide gap between Fmin and Fmax and lower lines of resolution for quick data capture and then gradually narrows the Fmax and Fmin and increases Lines of Resolution for conducting a Good Vibration Analysis

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