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 10 to 9 m/s or µm/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).
For commercial multi-tenant lab buildings, owners may specify, and some may also choose to advertise, the vibration performance of the building as a means of attracting tenants. Sales literature can include reference to the generic vibration criteria as an indicator of the types of laboratory use that can be accommodated
(e.g. Vibration performance Class A [50 micro-metres per second] in sectors A, B, and C. Vibration performance Class B [25 micro-metres per second] in sector D). These criteria are widely understood by lab operators and effectively convey the types of equipment and procedures the building can support.
Vibration performance assessment
Repurposing floors to support laboratory use often requires an assessment by a vibration engineer to qualify the performance of the existing structure and to determine the feasibility of achieving the project vibration goals. An assessment should include measurements at various locations on the floor(s) and must consider all relevant site vibration sources. With this information, it is possible to produce a map of vibration classes for various areas of the building. The base performance provides the owner and design team with an indication of how close or how far the current performance is to target criteria, and if there is a need for remedial measures.
It is standard practice to measure floor responses to various pedestrian pacing rates (e.g. slow, moderate, fast), as well as to perform dynamic modal tests to identify vibration frequencies and damping ratios. Where environmental sources are a concern (e.g. railway traffic), the worst-case impacts to performance should be quantified and feasibility of mitigation verified. Any major building services to remain following renovations should also be considered as these sources can be particularly problematic for many laboratory tools (e.g. MRI, high-resolution microscopes, NMR, etc.).
