I've written before about temporal variability in vibration environments, and I recently gave a talk on the subject at IEST's ESTECH conferece. The issue is becoming important enough that there is discussion of addressing it in one of IEST's upcoming standards.
The problem is that no realistic environment is truly "stationary". This is especially true for many research-oriented environments (think: nanotechnology labs) for which low-vibration environments are critical. I've been asked to write some more about this for a working group, and I think the place to start is to think about the timescales of interest. So, when we say that an environment isn't perfectly stable, what exactly do we mean?
This environmental vibration variability can occur on many timescales. In many cases, it's driven by what most people call "cultural vibrations": those vibrations generated by the activities and movement of people in the area.
On the milli-second timescale, “near-instantaneous” transients might result from cars hitting potholes or the slam of an office door. Car and subway pass-by events are often seconds in duration. Long freight trains might generate impacts lasting minutes. Rush-hour and general transportation impacts typically create hours-long cycles of somewhat-higher and somewhat-lower average vibration levels. And these cycles are indeed important: when we perform campus-scale surveys of vibration sensitivities, one of the questions we ask of research groups is whether they sometimes work at night to avoid interference.
But even longer timescales can be relevant. In some campus settings, reduced local traffic leads to lower weekend vibration levels, during which researchers might schedule their most sensitive experiments. Conversely, weekly visits to the building for gas deliveries and trash pickup can cause semi-regular spikes. Sometimes, the impact might be so regular that it can itself appear as a signal: an emergency generator test occurring, say, every Tuesday at noon could be said to produce a discrete 1.6µHz signal (once per week, or every 604,800 seconds). At the most-extreme timescale, eventually there will be an earthquake.
Extraordinary timescales are rarely relevant. And computing the return period on extremely rare events (like earthquakes) is notoriously fraught, and probably irrelevant to any contemporary lab uses, anyway.
But timescale is an important parameter in considering vibration impacts. And while there are technical reasons to consider timescale (is my apparatus even sensitive to milli-second-scale excursions? what are the chances that I'm even doing something sensitive at the moment when the transient occurs?) economic and practical considerations can be just as important. If your lab executes experiments that take huge budgets and months of planning to pull off, then even rare events might be a real threat, if only because the consequences of failure (however unlikely) are so dire.