Why is my vibration spectrum so large at low frequency?

Q. When measuring a vibration spectrum, I see a large signal at the low frequency end. Is this a real vibration?
A. The issue is caused by the 1/f noise limit of the accelerometer.

The Wilcoxon 731A accelerometer bandwidth is 0.1Hz - 500Hz. At large low frequency vibration amplitudes the bandwidth is the limiting parameter. At the very low amplitudes we need to measure for electron microscope environments, the 1/f noise of the accelerometer is the limiting parameter.
To derive displacement in microns from the accelerometer output the SC11 has to divide its output by frequency squared. Please see page 111 of the SC11 user manual.

This fundamental physics noise limit means that vibration spectra results at low frequencies must be interpreted carefully. Most of the signal you see on spectra below 1Hz is probably not vibration. Vibration specialists call this issue the system noise floor.