When we think of sound, we usually think of the airborne variety: tiny pressure fluctuations of air molecules that are converted to neural impulses in our brains. This conversion allows us to enjoy music, alerts us to a crying baby, and allows us to communicate with each other. However, sound can travel through other media besides air. Liquids like water, or solids like steel and concrete, can also transmit sound and can do so very effectively. Once a sound has entered a solid, we typically refer to it as vibration.
Vibration can be introduced into a building structure by many paths. Motor-driven mechanical units create a certain amount of vibration during normal operation. Without proper isolation, this vibration makes its way easily into the structure of a building. Once vibration has entered a structure, it can travel a long distance and cause rattles in ceiling grids, lights, conduit, or anything else that isn’t fastened down. If this happens in a bedroom, boardroom, or any other sound-sensitive space, we have a problem! To reduce vibration transmitting to a structure, proper isolation is necessary. Vibration isolators come in many forms: metal springs and mounts, elastomeric pads, and compressed fiberglass are just a few examples.
The goal of any vibration isolation technique is to reduce the transmissibility of the system. Transmissibility is defined as the ratio of the output of the system to the input. In a system consisting of a motor on a concrete slab, the vibration produced by the motor would be the input, and the output would be the vibration of the slab. When we add well-chosen vibration isolators to the system, the output is reduced, thereby decreasing the transmissibility of the system. Ideally, we want to reduce vibration entering a structure by 90% or more. However, one can’t just go to the isolator store, pick up any old spring, and expect it to reduce vibration; careful calculations are required to determine the characteristics an isolator should have for a given system. Two of the more important ingredients are deflection and mass.
Deflection is the distance a spring or pad is compressed under load. Metal springs are a simple way to provide high deflections (typically 1″ – 4″), while neoprene pads provide much smaller deflections than springs (usually around 0.1″ to 0.3″). Higher deflection is needed for equipment with low rotational speeds, such as cooling tower fans; pads are suitable for high-speed motors and transformers. There is a “sweet spot” aspect to deflection; over- or under-loading a spring or pad allows vibration to pass right through as if it wasn’t there. Load calculations must be made to ensure the right amount of deflection is achieved.
Another way to reduce transmissibility is to incorporate mass into a system as an inertial element. Typically, these inertia blocks consist of several inches of concrete. It takes a lot of energy to induce motion in a 2,000 pound concrete block – much more than it takes to excite a lightweight wood structure. By installing vibrating equipment on an inertia block and installing the inertia block on spring mounts, much of the energy produced by the equipment is isolated from the structure.
Careful analysis and vibration-reducing techniques like these will prevent these shakes, rattles and rolls, before they start. So next time you’re asked – “What’s shakin’?”, you can answer “Not much!”