| A "Spectrum"
is plot of amplitude vs. frequency. The above plot is a spectrum that was
created from a time domain plot using a mathematical principle called the
"Fast Fourier Transform", or "FFT". This plot is often simply referred
to as an "FFT". |
| This
principle states that any periodic signal (what we measure with vibration)
can be broken down into a series of simple sinusoids that, when combined,
will generate the periodic signal we have just analyzed. In practical terms
that means this process can generate the spectrum we see here from a time
domain signal it has analyzed. By plotting amplitude versus frequency
(instead of time), it becomes far easier to analyze. By relying on complex
mathematical processes, however, it also becomes susceptible to generating
what can be misleading information. The plot displays a certain number
of amplitude values (400, 800, 1600, etc.) over a range of frequencies.
The plot seen here tells the analyst that there is: |
-
'A' amplitude
at a frequency of approximately 3534 cpm (58.9 Hz)
-
'B' amplitude
at approximately 7084 cpm (118.07 Hz)
-
'C' amplitude
at approximately 10,633 cpm (177.22 Hz) and so on.
|
| This plot
is the most commonly used analysis tool since, by enabling frequency identification,
it allows for preliminary identification of the source of the vibration. |
