The aim of good acoustic control in facilities design is to balance sounds appropriately rather than to produce a completely quiet environment. Sound is not always detrimental, and a complete lack of sound is usually not desirable. Although a certain variety in the type and rhythm of sounds is necessary to provide stimulation and to maintain alertness, office workers who are subjected to excess levels of noise report fatigue and irritability.
Considerations for Building Systems
Noise can be controlled by either removing the source or limiting the amount of sound transmitted. Sound-limiting strategies include increasing sound absorption and decreasing sound transmission through barriers. Background noise can also be raised (to a degree) to increase conversational privacy. Some of the most common acoustic control measures available for an office facility are listed below.
- Ceiling Systems: The ceiling system is the most significant sound-absorbing surface within the office environment. Good acoustic ceilings have an NRC (noise reduction coefficient) rating of approximately .70 or more, although the ratings for mineral wool fiberboard covered with fabric can approach 1.0.
- Air Distribution: The air-distribution system can contribute excessive noise if air is delivered through poorly designed supply diffusers, at too high a velocity, or both. Ductwork can also transmit noise if the connections to mechanical equipment are not tight or if acoustic liners are absent in long runs. In particularly critical noise-control situations, the inside surfaces of all ducts should be lined. It is especially important to remember that sound takes the path of least resistance; therefore, an overhead air duct with no lining or acoustic dampers will transmit sound between two offices, even if the walls and ceilings are stuffed and lined with fiberglass insulation.
- Sound Masking: Sound-masking systems increase speech privacy by raising background noise levels (white noise). These solid-state sound generators produce a continuous, broadband, random sound signal. A good system should blend in with background sound patterns of normal office activity; should not contain sound frequencies that produce disturbing, audible tones; and should produce sound through individually adjustable speakers. The source must be nondirectionalimpossible to locate by ear. Such systems have been found to be most successful when end users are unaware of their presence. Therefore, the best time to install such a system is during a floor renovation. Once installed, the system should never be adjusted while the space is occupied.
Considerations for Workstation Design
Acoustic control measures often conflict directly with pressures to create higher-density workspaces to reduce space rental and maintenance costs. Thus, acoustic control must be achieved by reducing sound at its source rather than by locating people and machines farther from each other.
Partitions and Panels Panels must absorb sound as well as block its transmission. In addition, panels should be evaluated according to their ability to absorb and block frequencies characteristic of human speech. Generally accepted guidelines are a minimum of NRC-.60 and CAC (ceiling attenuation class)-21. Panels should be approximately twice as wide as they are high to minimize the movement of sound around them. Every 12 inches of additional panel height cuts sound transmission levels by about five decibels, but this increased performance must be weighed against sharply increased panel costs and worker comfort in paneled space. This space is usually small and may feel claustrophobic if panels are too high; panels can also interfere with air distribution, creating hot spots.
Panels cannot absorb sound if they are blocked. For example, occupants sometimes cover fabric surfaces with papers, notes, and announcements. The papers reflect the sound and bounce it all over the workspace. The solution is to provide tackboards, preferably away from sound sources, and insist that papers be posted on them instead.
Acoustic wall covering and carpeting can also be used over fixed partitions and walls in areas where sound absorption is crucial, such as in noisy common areas or conference rooms. NRC ratings for these materials are very good, but be sure to evaluate the entire assembly, including the wall on which these materials are applied.
Furniture and Workspace Organization Perhaps even more important than the properties of the panel is the location of sound sourcestelephones or office equipmentclose to sound-absorbent panels. Some practical suggestions follow:
- Design workstations so that employees face acoustical panels, especially while on the telephone.
- Place side panels at 90-degree angles to primary workstation panels to contain sound.
- Limit the number of workstation openings.
- Use floor-to-ceiling partitions strategically, especially around such areas as copy rooms, fax machines, and break areas, where people tend to congregate and chat.
- Segregate noisy equipment, such as large copiers, in acoustically controlled rooms.
- Use a buffer zone of storage or filing space between quiet and noisy areas. Install sound-soak panels on the backs of metal file cabinets.
- Specify telephones with quiet rings. Lower the ring volume of existing telephones.
- Limit the use of speakerphones to enclosed offices.
Auditory Distractions
Many of the new sounds produced by office technologysignal beeps, clicks, and whirring noises, and an array of clever cell phone ring tonesare often generated at much higher frequencies than other office sounds, which are typically at levels and frequencies closer to background noise. As a result, workers often perceive technology-related sounds to be intrusive and much louder than they actually are.
Planners should improve localized sound absorption in panels, ceilings, walls, and floors as much as possible to reduce the loudness of technology-generated sounds. To do this successfully, disruptive sounds should be identified and their frequencies measured; they should then be matched with proper sound-absorbing materials. All absorbent materials should be kept as close to the sources of sounds as possible. Close attention to overall acoustic comfort is also important, especially in open-plan spaces where people are using computers between 30 and 60 percent of their time each workday. Be sure to review overall planning for office spaces to see whether the degree of existing enclosure for people and machines, the panel heights, or the locations of panels or other enclosures can be adjusted to further reduce acoustical distractions. Finally, stay abreast of the latest sound-cancellation technologies that may reduce or eliminate intrusive sounds of all types in the office environment.
Sound Masking Technology
Because sound absorption materials and techniques cannot completely remove unwanted noise, another option that has become common is electronic noise cancellation, also known as sound masking. Sound masking adds a barely perceptible sound to the environment that raises the ambient background sound level slightly. The background sound is a very quiet, subtle hushed sound similar to a gentle breeze. This reduces the ability of noise to travel freely through the environment, making it sound quieter. It also muffles conversation, making it unintelligible to others more than five to six feet away, which enhances privacy.
Sound masking systems consist of a sound-generating unit connected to speakers. While the speakers are usually mounted in the ceiling, they can be positioned almost anywhere. Ceiling-mounted units usually work better when used with paging or music speakers. The sound-generating unit is tuned to the specific characteristics of the room during setup, and it emits frequencies that are effective for cancelling the kinds of noises that are in the area.
The use of electronic sound masking has become especially common in open office areas where many cubicles are close to each other. Sound masking allows greater densities while preventing ambient noise from interfering with workers. It is especially useful in call centers where many people are speaking simultaneously.
This article is adapted from BOMI International’s course Technologies for Facilities Management. More information regarding this course is available by calling 1-800-235-2664. Visit BOMI International’s Web site. Give the gift of a BOMI International education to your entire facilities team this holiday season.