Vibration criteria
Criteria for floor vibration are specified based on the space’s use and occupancy. Many of these evolved from the isoperceptibility curve, which represents the threshold of vibration perception for humans (foot-to-head vibration direction) over the 1 to 80 Hz frequency range.
The y-axis is the root-mean-square (RMS) vibration acceleration level—typically computed for a one-second time window. Floor vibration levels having magnitudes greater than this curve will be perceptible to most people. The flat portion of the curve, between 4 and 8 Hz, represents the frequency range where people are most sensitive to vibration.
The isoperceptibility curve is commonly referred to as the “ISO base curve” because it is defined in the International Organization for Standardization (ISO) 2631-1, Mechanical Vibration and Shock–Evaluation of Human Exposure to Whole-body Vibration: Part 1: General Requirements. It serves as the basis for many vibration specifications (Figure 1).
Floor vibration specifications for spaces housing sensitive equipment, or procedures are commonly expressed in units of RMS velocity (e.g 10e-9 in/s or µin/s). The thresholds for sensitive equipment are specified as fractions of the base curve (also referred to as the ISO–operating theatre curve in this context, as it is commonly specified for the design of general surgical suites).
Each curve is assigned a vibration class, representing a category of equipment/procedures appropriate for the specified environment—in other words, vibration levels in the environment should not exceed the assigned threshold. Vibration classes C through E are slight modifications of the base curve, with more stringent requirements at frequencies between 1 and 8 Hz, accounting for the heightened sensitivity of ultra-low vibration equipment in this frequency range (Figure 2).
In commercial multi-tenant laboratory buildings, owners may specify, and in some cases, promote the vibration performance of the building as a strategy to attract tenants. Marketing materials may include references to the standard vibration criteria, such as indicating a Class A vibration performance (2,000 micro-in. per second) in sectors A, B, and C, and Class B performance (1,000 micro-in. per second) in sector D. These criteria are commonly recognized by laboratory operators, providing a clear indication of the types of equipment and procedures the building can effectively accommodate.
