Forced vibration is a time-varying disturbance applied to a mechanical system. Vibration sources are generally broken out into three categories: continuous, impulsive, and intermittent.
Vibration can be measured using an accelerometer. An accelerometer is made of a piezoelectric material that gives off a small voltage when it is moved. This signal is amplified and can be processes on a computer or in a sound level meter. This signal can be calibrated using a shaker. The accelerometer is attached to a shaker that moves at a known frequency and amplitude. A calibration factor is determined to insure that the accelerometer, amplifier, computer system is providing an accurate reading.
There are very large accelerometers that are used for low frequency vibration of the ground. There are smaller accelerometers that measure higher frequency vibration. They can be attached directly to a surface to determine the vibration of surface.
Vibration is measured in acceleration (G’s, m/s2, ft/s2) or velocity (m/s, ft/s).
There is an Occupational Safety and Health Administration (OSHA) vibration guidelines for hand-arm vibration and for whole-body vibration.
There are some guidelines as to when vibration is noticeable and disturbing but it is not a regulation.
Sometimes we deal with vibration for structural problems – such as a building being constructed near a heavy rail track. Sometimes vibration is addressed for worker exposure (long term exposure to vibrating equipment that causes a health issue).
Sound Solutions regularly measures the existing vibration levels at the sites of proposed MRI machines (hospitals and clinics). The manufacturers require that the sites meet certain vibration limits so that imaging from the MRIs are not affected.
Vibration measurements include steady state spectrum and time decay after an impact. For the steady state measurements, expected activities are simulated, such as: an elevator operating, people walking in corridors, people in the control room, and HVAC equipment operating.
If the site does not meet the vibration requirements, the vibration source is identified and treated. If further mitigation is needed, a vibration isolation system is installed.
Reducing vibration depends on the source of the vibration and the where it is located. I’ll give a few examples:
Many times the manufacturer will have vibration isolation systems for their systems. MRIs are an example. The manufacturer is a good place to start.
When evaluating rooftop mechanical you need to know the equipment type, horsepower, RPM, floor span. With that information you can determine the isolator base type, isolator type and minimum deflection. There are rubber pads, spring isolations, concrete inertia bases, curb-mounted bases.
For floor-ceiling systems and roof-ceiling systems, mitigation can be applied to the floor/roof and/or to the ceiling. It is best if the floor and ceiling can be isolated. Treating the floor can include adding a resilient layer under a finished floor. Treating the ceiling can include mounting a gypsum board ceiling on springs or resilient channels.
In either case, it is easier to design these treatments than put them in after construction because the usually entail removing the floor or ceiling. The can also include raising the height of the floor.
We have recently worked on several cross fit gyms located next to office spaces. In these gyms, big people often lift heavy weights and drop them on the floor. This impact shakes the adjacent spaces. We have had success installing additional rubber pads in the lifting areas. Rubber mats come in a variety of thicknesses and densities.
We have not found the magic combination of thicknesses and densities for every situation. We have gotten the best results by testing a variety of rubber mats of various thicknesses and densities.
Vibration isolation for plumbing usually entails the way the pipes are mounted to the studs. Wrapping the pipes in closed cell foam and mounting the system to the studs or using resilient mounts (that do the same thing).
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Sound Solutions is an acoustical consulting firm that works throughout the US conducting noise measurements, analysis, and reporting for environmental and architectural noise issues. Please contact us for a free proposal.