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Log Linear or dB Scales?

Vibration Experts

Log linear or dB scales?

Log linear or dB scales? Which should you use?

Log and dB scales better for analyzing vibration velocity spectra. Here’s why:

Log or dB scales allow you to see small amplitude peaks in the presence of large amplitude peaks. The small amplitude peaks may be your rolling element bearing defects whereas large peaks may be 1x and vane or blade pass frequencies.

Think about trying to photograph your friend standing next to a large building, where the top of the building needs to be in the photo. In a linear scale this will only work if the building is less than ten times taller than the person. If it is greater than this you can see the top of the building but your friend is cut off at the bottom – or you can see your friend but not the top of the building. When using a linear scale, one must re-scale the graph to see both. This takes time!

In a log or dB scale, the building can be 1,000 times taller and you can still see both the building and your friend in the picture. In other words, you can see large peaks like vane pass along side small peaks like bearing tones.

Because the log or dB scale allows you to view all of the important data in one graph, it also means that you can use a fixed graph scale instead of auto-scaling the graphs. This offers another HUGE benefit.

When all of your graphs are always scaled the same way it gets very easy for your eyes and your mind to pick out patterns and to compare one plot to another – say from different test points, test dates or different axes on the same machine. This drastically speeds up the analysis process. No more rescaling every graph or trying to figure out how high the peaks are!

A benefit of dB over log is that the vertical scale in a dB plot is “linear” and the log part is contained in the unit itself. This means that you can look at a peak in the spectrum and estimate its amplitude without clicking on it. You cannot do this with a log scale because the scale is graduated. (it goes up by factors of 10)

In a dB scale just remember that an increase of 6 dB is a doubling of amplitude. When using the imperial version of VdB (velocity decibels) a good fixed scale to use is 60 – 120 VdB. The reference for (imperial) VdB is 0.0000003937 in/s – your software might ask you to input this reference value. The metric reference for VdB is 10^-8 m/s.

Changing unit scales is a button click away in most software programs. It does not effect how the data was collected or how it is stored, it is simply a way to view the data. Therefore there is nothing to lose by giving it a try. You can always easily switch back to your preferred scaling without effecting the data in any way.

Log linear or dB? I would recommend giving dB (or more specifically VdB) a try.