LINEAR vs LOG – Scaling New Heights!



When examining spectral machine vibration data, we are often unable to see the trees for the forest!  This can easily be resolved thru the use of a little known Y-axis scaling option, log (abbreviation for logarithmic … more later!) versus the traditional linear scaling.   The concept of normalizing the Y-axis in linear units is well known and of particular relevance when comparing several spectra on the PC screen, however many vibration analysts would be delighted to see the additional detail gained using log scaling, and even more thrilled to learn that our amplitude can remain in engineering units, i.e., mm/s, in/s, g’s, etc.  The benefits are significant particularly when one considers the influence of the “transmission path” in the measurement, and also the issue of a single dominant component in the spectrum.  Now, at the risk of alienating some readers, we will explore the use of decibel notation (dBV) as it is fundamental to a type of “normalization”, that is, to an agreed upon reference quantity.  Furthermore, there are also times when scaling the X-axis logarithmically can benefit the vibration practitioner, and although admittedly these occasions are uncommon in machine vibration diagnosis, in some signal processing cases log X-axis scaling is the standard.


Studied Mechanical Engineering and German Language at Temple University. Has worked with Pruftechnik for 6.5 years doing alignment and vibration services, technical support, and sales throughout a variety of industrial environments. Currently a category 3 vibration analyst through Mobius Institute. Looking to achieve category 4 in 2019.