CHOOSING THE RIGHT LOCATION
The best way to ensure a quiet environment for your sensitive measurements is to choose a quiet environment in the first place. Selecting the proper location can occur at several stages.
Deciding where to build a research laboratory should take many factors into consideration if one is trying to avoid acoustic, vibration, or EMI noise from affecting sensitive equipment. Regions with significant levels of seismic activity should be avoided if possible, with oceans and other large bodies of water being natural culprits of generating noise. It is advisable to find a building built on bedrock as they are much more stable than those with foundations on sand, clay, or reclaimed land. Other sources of concern for environmental noise include large-scale mining or construction operations, busy streets, airports, and mass transportation lines like railroads and subways.
Buildings can be built with varying levels of stability and structural integrity. The architects and construction companies involved in developing a new building should take into consideration the stringent noise requirements of instrumentation being used inside the building to help avoid future delays resulting from the noise. This information exchange should happen at the beginning of the building design phase.
Tall buildings are subject to horizontal sway, so it is better to place labs in short buildings with wide foundations. If possible, locate the lab on the ground floor of the building or on well-supported floors near the foundation of the building. If possible, labs should be constructed on slabs that are isolated from the rest of the building foundation as the lab itself will then become decoupled from majority of the noise imparted by the building itself.
Location Within the Building
Individual scientists will rarely have input on the site and design chosen for a building, but sometimes they will have the freedom to choose where their labs are located within the building. In this case, the basement is the ideal location. In buildings with multiple stories, basements are less subject to the horizontal sway and building resonances, which commonly plague the higher floors. In addition to the lower noise floor, basements usually receive less foot traffic than the other floors, leading to less external noise from affecting sensitive equipment. If a basement location is not available, lower floors are better than higher floors due to decreased levels of low frequency vibration.
Attention should be paid to the location of machinery within the building. Heating, ventilation, and air conditioning (HVAC) machinery are a notorious source of intra-building noise. Water pipes and elevators can also represent localized sources of noise. Understand where the building’s mechanical rooms are and avoid them at all costs. Other areas to avoid include machine shops, manufacturing areas, and loading docks. You should also note external sources of vibration such as parking lots and busy streets. Try to locate your lab as far from those external noise sources as possible.
If your lab space is located above the ground floor, avoid the center of the floor. Floors which aren’t supported throughout exhibit a ‘trampoline effect’ where the amplitudes of vibrations are greater in the center of the unsupported floor. This effect is minimized if you are positioned near a load-bearing wall or column. Also, you should avoid corridors which will have significant levels of foot traffic or sound (such as meeting rooms and break rooms).
Clean rooms should be avoided, if possible. Clean rooms generally require air handling equipment which causes air current disturbance. They usually have many machines and instruments contained inside. Also, raised floors are not very stable from a vibration perspective. If you must place precision instruments within a clean room, a special clean room support which bypasses the raised floor should be utilized.
Location Within the Room
Once an appropriate room has been found, there are still some measures to take to minimize the effects of environmental noise. Position the instrument away from areas with high foot traffic, such as hallways or gathering points. Avoid doorways, as the impact from door slams can introduce impact disturbance. Avoid air vents which can push air onto instruments and cause temperature fluctuation. Avoid the middle of the floor and position your instrument near a load-bearing wall or pillar. However, do not push the instrument’s workstation directly against the wall as vibrations in the wall can directly couple into the system.
ISOLATING THE SOURCE
Sometimes the preventative measures listed above will fail or be impractical. When noise presents itself, the first step is to diagnose the issue. Environmental isolation systems provide a degree of reduction, but they do not completely eliminate the noise itself. Thus, it is preferable to stop troublesome noise at the source rather than at the sensitive instrument. Another reason you should isolate the source is that one source of noise can affect many different instruments. Preventing the noise from transmitting to the surrounding area can be simpler and more cost-effective than isolating each sensitive instrument individually.
Once you’ve located the source, the best solution is to turn it off or remove it from the premises. Failing that, you can decouple it from the rest of the building. This can be accomplished by moving it to another site or placing it outside using cable or duct extensions. You may also be able to isolate the machinery’s slab from the rest of the building.
Other methods for isolating the source of noise include changing the characteristics of the system by adding mass or improving its support structure. For vibration sources, you can utilize low end viscoelastic feet or spring mounts which will significantly reduce noise transmission. For acoustic sources, you can construct a room or soundproof hood around the piece. For EMI-generating items, you can build a makeshift faraday cage around it to dissipate much of the fields it is generating.
DECOUPLING THE INSTRUMENT
If isolating the source of noise has failed, the next step is to decouple the sensitive item from the source of the vibration. Mechanical decoupling will greatly reduce the amount of energy that is transmitted. This is often pursued at the stage of building a facility. Oftentimes an isolation slab will be installed in a room where sensitive instruments are to be used. The isolation slab is independent of the rest of the building foundation, so the chances of noisy equipment transmitting vibrations to the sensitive equipment are minimized. Unfortunately if this is not considered in the building stage it can be quite costly to install an isolated slab as a retrofit.
If the troublesome noise is being transmitted through a cable or hose, this should be disconnected, replaced, or weighted (see our Tutorial on cable management).
IMPROVING THE SUPPORT STRUCTURE
Sometimes the effect of environmental noise is exacerbated by flimsy support structures. Weak supports can introduce unwanted resonances into the system which amplify incoming vibration. You should ensure that the instrument is being supported by a sturdy, rigid structure. Card tables and basic office furniture are unacceptable as supports. Rigid steel weldments are ideal. Lab benches and lab tables are usually constructed with rigidity in mind.
The next step is to add weight to the system. Increasing the mass of an object increases its impedance, meaning it will require more energy to move the system. This method can be quite effective at minimizing the effects of high frequency vibrations, as high frequency vibrations are usually lower energy. Adding mass will also change the resonant frequency of a system, which can change the characteristics of the system itself to make it less sensitive to a given noise source. This approach is employed by instrument makers when they integrate large pieces of granite with their instruments. This can be an effective way to reduce an instrument’s sensitivity to environmental vibration, but because it requires a lot of mass this method can often be unwieldy and impractical.
If the offending noise is acoustics, look for any protruding pieces which may be acting as conductors for acoustic noise. If EMI is a concern, make sure that the support is properly grounded.
The next step is the utilization of basic dampers. Dampers function by lowering the amplitudes of mechanical resonances excited by incoming vibrations. This category includes viscoelastic pads, sorbothane mounts, and rubber feet. While these simple dampers can be effective for many applications, often they leave harmful resonances undamped or do not provide sufficient reduction in amplitude. On the other hand, they are usually very inexpensive so a trial-and-error process is feasible.
For acoustic noise, the simplest damper would be a physical barrier. Cubicle walls and acoustic curtains provide a basic level of reduction. You can also construct a box around the instrument. Cardboard, plywood, and Styrofoam are commonly used in rudimentary enclosures. Of course, putting your instrument in a box presents challenges in terms of accessibility, visibility, and ergonomics.
For EMI issues, a basic Faraday cage can be constructed out of sheet metal. A steel box can be costly and bulky.