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Source, Path, Receiver |
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Management can’t figure out why production in their (noisy) Dallas
facility is lower than in its similar (but quieter) Oakland
facility. A longtime employee has filed a lawsuit seeking compensation for acute hearing loss. Neighbors Against Noise have started a letter campaign to the City Council complaining of late-night noise from your facility. Worst of all, a delivery truck in Portland has struck a supervisor who did not respond to the back-up alarm. Noise can adversely affect nearly every aspect of business: Productivity, health, safety and public relations. Today, industrial hygienists and others responsible for controlling noise and vibration have more tools and products available to them than ever before. While this affords many options, it can also create much confusion. In this article, we lay out a basic framework and strategies for solving noise and vibration problems with the hope that it will help you deal with them more effectively. Framework. Every noise and vibration (acoustical) problem has three elements: A source, a transmission path and a receiver. In order to mitigate an acoustical problem, you must do one or more of the following: 1. Reduce the source level; 2. Interrupt (impede) the transmission path; or 3. Relocate the receiver. Controlling source levels. Among acoustical engineers, it is widely held that the source level should be reduced if possible. This keeps the genie in the bottle, rather than trying to control the spread of acoustical energy once it is out in the open. Here are some strategies for controlling source levels. Consider equipment noise and vibration when purchasing equipment. Most equipment manufacturers provide sound ratings. There can be substantial differences, and any “savings” had by buying less expensive but noisier equipment can be quickly eroded by dealing with and solving acoustical issues down the road. Review equipment and installation details. Even a “quiet” piece of equipment can be noisy if it’s not installed correctly. Make sure that the equipment is appropriately installed to gain the most benefit from built-in quiet features and that seismic bracing or other rigid connections do not short-circuit the isolation. Keep machines balanced and in good repair. Move equipment. When noise issues arise, most people’s natural inclination is to “get something” to solve the problem. For example, if cooling fans are making too much noise, they want to know if there are quieter fan blades or if they can build a sound barrier around the offending piece of equipment. Before you add anything new to a situation, try to rearrange the things you already have to separate the source and receiver. This will reduce the direct sound levels, especially if equipment can be moved into a mechanical closet or other separate space. Try to vary process and flow conditions. The amount of noise from fluid flow or rotating machinery generally increases with speed, while certain speeds excite natural resonances in the system generating hums or squeals. If you have an issue related to such processes, see if there is any latitude to vary the speed. Slowing things down will usually lower the overall level, but speeding things up might move you off a resonance. Changing process speeds, if possible, is often a very inexpensive way to reduce noise and vibration. Finally, if there are pipes or ducts involved, have someone check for poorly designed configurations like sharp corners and turns without vanes. Active-noise control. This technology can be successfully deployed in some situations, but it is not a panacea for all acoustical issues. It works best in confined, controlled, fairly static environments, or where a “passive” solution like a sound barrier wall can not be constructed. Some examples are in controlling low-frequency noise in HVAC ducts, aircraft engine noise inside the cabin, and electrical transformer noise in substations. At this time, it is not very effective at dealing with situations where the source and receiver are moving relative to each other. Operational controls. In situations where tasks and processes are not continuous, there can be opportunities to have certain workers do their tasks when a machine is down or operating in a relatively quiet mode. This effectively reduces the source noise level for these workers. Impeding the transmission path. This is how most acoustical issues are resolved, and there are many products available to absorb, isolate and insulate acoustical energy. However, even the best product will not help you if used in the wrong situation. The key to getting the right product is to discern the primary transmission path(s) related to the problem. Here are the major paths and the types of products that will help treat them: The direct path. This is the most obvious path – an unobstructed, usually short, distance between the source and the receiver’s ears, for example, noise from a machine at the operator’s position. The first thing you need to do is block this path with a barrier or with hearing protection at the receiver’s ears. No amount of absorption will help this situation if the direct path is not blocked. If a barrier is used, it must be of sufficient extent to control diffracted sound and density to control “pass through” sound. See the December 2005 issue of Facility Safety Management for a good article on successfully implementing hearing protection. The reverberant (indirect) path. Once the direct path is either blocked or the receiver is far enough away (the direct sound level drops off with distance), the reverberant sound level becomes dominant. In a room with little absorption, the acoustical energy simply reflects continuously around the room resulting in a large sound “build up.” Absorption removes the acoustical energy from the space by converting it to heat energy. This only works for reflected sound waves, so it will not help if the direct path is dominant. The effectiveness of absorption is subject to rapidly diminishing returns, so adding a little in a room that has none will make a dramatic difference, but once you have covered about half of the room’s surface area, additional absorption will not improve the results much. Finally, a good practice when applying absorption is to spread it around. Put some on every surface, including the floor if possible. The structure-borne path. This is the “forgotten” path, so, consequently, it is the one that is usually dealt with least effectively. Consider a mechanical room with a fairly noisy pump next to an office. Many people believe that if they apply absorption in the mechanical room, that will reduce audible pump noise in the office. Maybe. While the pump is rotating, it is transmitting vibrational energy to the floor. In a concrete floor, this energy transmits very efficiently to other spaces via the building structure. Once it reaches the office floor and walls, it will cause these to vibrate, and they will then radiate sound into the office space. This path does not include the sound inside the mechanical room, so absorption there will not help. To cut this transmission path, the pump must be mechanically isolated from the floor by rubber pads and/or steel springs. Note that all connecting pipes must also be isolated either from the pump or the structure with flexible connectors and conduit. Once the pump is isolated, it may be the case that noise through the wall to the adjacent office is the dominant path, in which case additional measures, including absorption in the room, might be utilized. Relocating the receiver. While this option might seem obvious, consider the following: People are different. It is well known that people have different sensitivities to noise. Roughly, 10 percent of people are very sensitive (most everything bothers them), 25 percent are very insensitive (nothing, it seems, bothers them), and the other 65 percent react depending on the level, frequency, and “emotional” content of the noise. While it might not be practical to leave the office next to the mechanical room empty, it might be possible to have (sensitive) Jack switch offices with (insensitive) Jill down the hall. Beware of remodels. During remodels, people are often moved into spaces where there were previously no acoustical issues, but now there are. What happened? In some cases, something about the physical space was altered, so there is now more noise. For example, one facility had a floor that was built-up with Styrofoam to make it even with another part of the building. During the remodel, this was removed. After the remodel, noise from a previously inaudible mechanical room below was now bothersome. In other cases, compatible uses are changed to incompatible uses. For example, for years the accounting people sat next to a call center with no problems. Then, the accounting people were moved out and tape recording transcribers were moved in. The conversations from the call center interfered with the ability of the transcribers to hear the often soft-spoken dictations on the tape. These were incompatible space uses. Relocating a receiver can create an acoustical issue as well as it can resolve one. If it falls to you to solve a noise or vibration issue, we hope this information will help you sort out what needs to be done and to deal more effectively with manufacturers’ reps and design consultants. Remember, the most important thing is to first discern the key source(s) and transmission path(s), and only then begin to consider remedies. Always bear in mind that the optimum solution might lie with any combination of the source, path, and receiver elements. FSM Derek L. Watry is a member of the Acoustical Society of America, the National Council of Acoustical Consultants, and an zssociate principal with Wilson, Ihrig & Associates. From its offices in California and New York, Wilson Ihrig supplies clients worldwide with practical, effective designs for industry, transportation, building, and other sectors. (www.wiai.com) |
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