By: Taylor Blaine, MCH Senior Consultant
Today is International Noise Awareness Day and the one-year anniversary of this Tech Blog series. A common type of noise in buildings is structural vibration from mechanical systems that radiates from room surfaces as airborne noise.
So, how do vibrations turn into sound or noise that we hear?
Objects vibrate, and so do buildings
Most objects vibrate when a force is exerted on them, such as being struck, shaken or otherwise disturbed.
An old-fashioned alarm clock with bell alarm will get much louder when you place it on a table, because the table reradiates vibrations as sound. The same type of thing happens when you place active mechanical equipment into a building, except that the vibrations may be much more intense and many of the building surfaces may reradiate those vibrations as sound.
Common building system equipment will generate vibration
Common vibration-generating sources in buildings include HVAC, electrical, or plumbing equipment. These sources tend to operate at specific speeds, such as the RPM (revolution per minute) of a fan.
These speeds need to be converted to Hz (Hertz, cycles per second) for comparison to most criteria for vibration, as well as for design of vibration isolation systems.
For example, a fan operating at 1000rpm has a fundamental driving frequency of 16Hz. You can calculate the driving frequency as shown below:
Sometimes this is complicated by variable speed drives, in which case there can be concerns for various speeds of a particular piece of equipment.
Once a building structure is excited into motion, the vibrations do not decay quickly and can readily travel throughout the structure. These structural vibrations can sometimes be felt, and can cause audible primary and secondary noise emissions.
Human sensitivity to feelable vibration
There are various vibration level criteria related to human sensitivity, and which are frequency dependent.
[Related to the topic of vibration and associated criteria, MCH Acoustical Consultant, Henry Ashburn, provides a general overview here: Good Vibrations (mchinc.com).]
The left side of Figure 2, below, shows approximate sensitivity and response of humans to various levels of vibration. The feelable vibration curves represent typical responses, although reactions may vary per individual and for different ways in which a person might sense vibration (standing, seated, fingertips, etc.). The example data spectrum shows that the vibration of a floor generated from nearby heavy walking would be perceived between “barely perceptible” and “distinctly perceptible”.
Primary airborne noise emissions from vibrating structure
The vibration of a large building surface (e.g., floor slab) can radiate airborne sound, and this audible airborne sound sometimes can be more problematic, even at lower vibration levels, than the feelable vibration of the surface.
Figure 2, shown to the right, includes example measured data which shows that resultant vibration acceleration levels from walking on a floor at 5ft away (i.e., large surface set into vibratory motion) would be expected to produce audible airborne sound levels. The Vibration-induced Noise Criterion (VNC) curves show the acceleration levels of a large radiating surface (e.g. floor or ceiling or wall of a room) that could produce airborne radiated sound corresponding to Noise Criterion (NC) curves. NC curves are a common standard used to describe noise environments for a variety of functional areas. In the shown examples, the vibratory excitation of the floor from walking is measured near -45dB RE 1G at 100Hz, which corresponds to approximately VNC-53, and in turn to NC-53 airborne sound level.
Secondary noise emissions from vibrating lightweight elements
A vibrating structure can also cause shaking of less massive elements, such as lights, ceiling tiles, etc., which can result in shaking/rattling noises.
The middle of Figure 2, between 16Hz and 63Hz bands, shows potential vibration levels which may cause secondary noise emissions. In the example shown, secondary noise emissions from walking on the floor are “Highly Likely” to cause secondary noise emissions.
Vibration isolation systems prevent equipment vibrations from transmitting into structure
Vibration isolation systems and building structures can be designed to significantly reduce the amount of force that is transmitted into supporting structures. Properly selected vibration isolation systems minimize potential negative effects related to human sensitivity to vibration, minimize re-radiation of energy from large vibrating surfaces (walls, floors, ceilings, etc.) and minimize potential for vibration of connected lightweight structures (lights, ducts, etc.).
Note that these isolation systems should be designed according to engineering principles, beginning with the operating speeds of the equipment (in Hz), the weight of the equipment, and some parameters related to the supporting structure of the building.
Consult your project acoustician for guidance for proper selection vibration isolation systems!