Linear and Logarithmic Amplitude Scales
It may seem to be best to look at vibration spectra with a linear amplitude scale because that is a true representation of the actual measured vibration amplitude. Linear amplitude scaling makes the largest components in a spectrum very easy to see and to evaluate, but very small components may be overlooked completely, or are at best difficult to assign a magnitude to. The eye is able to see small components about 1/50th as large as the largest ones in the same spectrum, but anything smaller than this is essentially lost. In other words, the dynamic range of the eye is about 50 to 1
Linear scaling may be adequate in cases where the components are all about the same size, but in the case of machine vibration, beginning faults in such parts as bearings produce very small signal amplitudes. If we are to do a good job of trending the levels of these spectral components, it is best to plot the logarithm of the amplitude rather than the amplitude itself. In this way, we can easily display and visually interpret a dynamic range of at least 5000 to 1, or more than 100 times better than the linear scaling allows.
To illustrate different types of amplitude presentations, the same vibration signature will be shown in linear and two different types of logarithmic amplitude scales.
It might be said that the dynamic range of the eye, when looking at linear spectra, is about 34 dB.
Linear Amplitude Scaling
Logarithmic Amplitude Scaling
dB Values vs. Amplitude Level Ratios
VdB Levels vs. Vibration Levels in ips
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