What Is An FFT (Spectrum) Plot

Y-Axis Units: Amplitude
	X-Axis Units: Frequency (number of cycles per minute or per second)

Whereas a Trend is amplitude values versus time, a "Spectrum" Plot is amplitude versus frequency.  A spectrum, a.k.a. an "FFT", allows you to assess severity (with the amplitude) and helps identify the source (with the frequency). This is the most commonly used analysis tool and is usually sufficient protection for general speed machinery.