• Henry Ashburn

Good Vibrations

By: Henry Ashburn, MCH Acoustical Consultant

 

Do you feel that? No, I'm not talking about an earthquake, I'm talking about the heavy weight drops from the gym upstairs!

While many designers are acquainted with airborne sound and common noise control approaches, vibration is typically less understood and often overlooked. Vibration and vibration isolation are important considerations to minimize disturbance due to MEP equipment, fitness facilities, transportation, and for proper operation of sensitive equipment in laboratories, hospitals, and manufacturing facilities.

To satisfy criteria and ensure project success, equipment with vibrational energy such as fans, motors, pipes, ducts, etc. should be properly vibration isolated. Specific requirements for proper vibration isolation depend on many factors such as RPM, horsepower, size, location, and weight of equipment, as well as span lengths and natural frequency of supporting structure.

Apart from satisfying criteria, proper vibration isolation will also minimize potential negative effects such as secondary emissions (e.g., rattling from lighting fixtures, ductwork, piping, and other connected elements), or structureborne transmission and re-radiation of energy from walls and ceilings.

Vibration is normally measured with accelerometers and reported in acceleration or velocity. Like airborne sound, which is commonly measured and analyzed from 20Hz to 20,000Hz, vibration energy is frequency-dependent. However with vibration, frequencies of interest typically range from 1Hz to 100Hz.

One example of vibration criteria is the base-response for highly sensitive human comfort in a building, which is generally indicated by the plotted red line in the figure below:


Adapted from ANSI S2.71-1983 (Reaffirmed 2012)


This combined axis base-response depicts a frequency-dependent perception threshold in both acceleration (graph’s y-axis) and velocity (graph’s diagonal axis), which serves as a guideline when analyzing vibration concerning highly sensitive human comfort.

Vibration-sensitive equipment such as electron microscopes, laser-based equipment, or wafer-fabrication machinery is often significantly more sensitive; in some cases, nearly 1,000 times more sensitive than the combined axis base response for highly sensitive human comfort. The “VC Curves” were originally established by Bolt Beranek and Newman (BBN) in the 1980s, and are an attempt at standardizing criteria for application across equipment manufacturers.

Adapted from Eric Ungar

“From Guess to Gospel – The Curious History of Floor Vibration Criteria”

Sound and Vibration Magazine, October 2016


Consult your project acoustician for guidance on vibration and vibration isolation!

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