Design Tips
Sound masking can reduce the requirement for other acoustic treatments. However, as with all acoustic treatments, the benefits of sound masking systems are maximized when used in conjunction with other noise control methods.
The workplace should provide occupants with speech privacy, acoustic comfort and freedom from noise problems, and enable them to work without disrupting others. The creation of a space optimized for speech privacy and noise control should be cost-effective, while maintaining the flexibility required to accommodate changes.
The formula many professionals use to achieve these results is the ABC Rule: Absorb, Block and Cover Up. In recognition of the fact that a combination of these three elements is required to provide proper acoustic control, the guideline is also known as the Rule of Threes.
While it acknowledges the role of absorption, physical barriers and sound masking, the rule ignores one valid and common method of addressing workplace noise: reduction at the source.
This strategy requires the identification and reduction or elimination of unnecessary workplace noise. The task can be accomplished by modifying employees’ noise-producing behaviors and by replacing office noise-generating equipment with quieter technologies.
Despite its exclusion from the ABC Rule, workplace noise source reduction has been one of the defining influences for developing acoustic control in modern offices. Building mechanicals such as heating, ventilating and air-conditioning equipment have become progressively more silent. Office equipment has followed the same path, progressing from typewriters and noisy copy machines to early printers and finally, to modern keyboards, laser printers and photocopiers. However, some new technologies such as speakerphones can actually increase workplace noise levels, and their use should be carefully considered.
Changing behaviours can also significantly reduce the level of office noise. Reasonable office etiquette should be enforced, but it must be recognized that some workplace noise is unavoidably created while employees perform their tasks. One of the goals of good acoustic control is to provide employees with a comfortable working environment that lets them perform their tasks without feeling as though they are disrupting or irritating others.
Reducing office noise at the source has practical limitations. Once this method has been exhausted, any remaining workplace noise is there by necessity or because it is infeasible to eliminate. These noise problems must be controlled in other ways.
Adding absorptive wall materials, ceiling tiles and flooring reduces the energy and, therefore, the volume of sounds reflected off their surfaces back into the office space.
Because the ceiling is usually the largest unbroken surface in a facility, a good absorptive tile helps lessen the distance over which workplace noise and conversation can be heard. Offices should invest in the best tile they can afford and ensure consistent coverage throughout the facility. Any partial acoustic treatments in a space will decrease the possibility for complete acoustic control.
Ceiling absorption is often rated using Noise Reduction Criteria (NRC), which ranges from 0 (0% absorption) to 1.00 (100% absorption). The higher the NRC, the better. In decreasing order of acoustic performance, ceilings typically rank as follows: fiberglass tile, mineral tile, perforated metal tile, no dropped ceiling, drywall and solid metal tile. The last four types usually exhibit a significant decline in acoustic comfort; however, there are mineral and perforated metal products that demonstrate better-than-average NRC ratings.
The thickness of a mineral or fiberglass tile affects its acoustic performance. Generally, the thinner the tile, the more transparent and less absorptive it is. Foil backing on a fiberglass tile helps contain noise within closed offices. Foil also increases the dispersion of a sound masking signal, ensuring greater consistency in sound masking levels throughout the space. Placing fiberglass insulation above the ceiling tiles affords only marginal acoustic control and hinders access to the ceiling.
Lighting components can increase the acoustic reflectivity of the ceiling because they replace these absorptive ceiling materials with hard surfaces. In order to limit the lighting system’s impact on the absorptive performance of the ceiling, select lighting that incorporates a minimum number of ceiling fixtures while still meeting the specified lighting requirements. From an acoustic control perspective, indirect lighting systems are best because they are suspended from the ceiling and maintain the maximum surface area of the acoustic ceiling tiles. When it is not possible to install an indirect system, consider using a deep parabolic lens instead of the traditional solid plastic lens. Standard acrylic lenses are highly reflective and, because they can take up to 20% of the ceiling, they have an obvious effect on sound transmission.
Though they are used less frequently than acoustical ceilings, absorptive wall materials can also play a significant role toward achieving office privacy. Absorptive panels are effective when applied to large vertical surfaces and to key reflective locations - such as atrium walls or walls that reflect office noise from the foyer up into the office space. They can also be used in areas where the ceiling treatment is not absorptive.
Workstation partitions can also perform an absorptive function. To reduce sound paths, minimize openings between and around the workstation panels, as well as underneath them if carpeting has not been used. Also minimize the number and size of reflective surfaces - such as glass, metal and drywall components - in the workstation because they increase the reflection of workplace noise and conversation, causing them to be heard over greater distances.
Absorption and the reduction of footfall noise are the main acoustic control considerations when selecting flooring. Hard flooring is highly reflective and results in a more reverberant environment. Carpeting greatly reduces footfall noise, but typically provides only minimal absorption of frequencies in the range of human speech. Flooring will have a greater absorptive effect when special under-padding is used.
While the inclusion of absorptive materials in the office is necessary, their use lowers the ambient or background sound level, actually making the environment sound noisier and reducing speech privacy. Conversations will be more distinguishable and intelligible. In other words, absorption addresses some acoustic problems while worsening others.
Another method of controlling office noise is to block sound transmission. Closed plan designs achieve the majority of noise control in this manner, but blocking is also a relevant consideration in the open-concept office. There are several strategies to review.
The most basic barrier is a wall, though efforts to increase the flexibility of offices and reduce construction costs have reduced their use in most facilities. However, walls should still be used in areas where a very high degree of speech privacy is required. Ensure doors are well-sealed and that there are no gaps between the walls and the suspended ceiling. When walls are built along the building’s perimeter, do not allow spaces to remain between the wall and the window mullion, because they provide a clear path for the transmission of sounds from one office to another. If not properly designed or treated, HVAC components can also provide a path for workplace noise and conversations to travel through the walls.
Plenum barriers are used to block sound transmission over walls that extend only to the suspended ceiling. They can be expensive and somewhat difficult to properly install. Breaks – either from initial installation or from subsequent damage – significantly reduce the effectiveness of lead, drywall or rigid fiberglass barriers. Furthermore, the use of plenum barriers can require the installation of expensive acoustic air return ducts in order to limit the transmission of sound through the ductwork while maintaining airflow.
In open-plan environments, the office layout can be used to maximize blocking of workplace noise to prevent it interfering with employees’ ability to concentrate. Locate noisy office machines and areas with high activity and office noise levels - such as call centers - in remote or isolated areas. Try to maximize the distance between employees because the volume of noises and conversations will decrease over distance.
Blocking in open-plan areas can also be achieved through the use of physical barriers such as workstation partitions. Minimize direct paths of sound transmission from one person to another by seating employees facing away from each other on either side of partitions. The height of the partitions is also significant to acoustic control. Partitions lower than 50 inches (1.25 meters) essentially provide an office with the same acoustic comfort from one workstation to another as a no-partition system. Generally speaking, 64 inches (1.60 meters) is effective because it extends beyond seated head height. Partitions higher than 70 inches (1.75 meters) offer decreasing acoustic comfort relative to their cost; however, using slightly higher partitions in high-traffic areas can be beneficial.
In the end, however, an over-reliance on physical barriers can raise costs and render a workplace relatively inflexible, while still failing to satisfy all occupants’ needs for acoustic control.
Though sound masking systems have been used for over forty years and in thousands of facilities worldwide, they remain novel to many people; therefore, it is useful to provide a brief description of this technology.
Basically, sound masking systems consist of a series of speakers that distribute an electronically generated background sound within a facility. Many people refer to sound masking systems as white noise systems; however, this is a misnomer. The term white noise describes a very specific type of sound used in early sound masking systems developed in the 1970s. These systems were unsuccessful due to their inflexibility and the irritating hissing quality of the sound they produced, but the term white noise became widely adopted. Newer sound masking systems do not use a white noise signal; rather, they offer an engineered sound that is much more comfortable, unobtrusive and effective.
Sound masking addresses the lack of sufficient background sound that is characteristic of most office spaces. Among acoustic treatments, it is the only one that reduces noise disruptions and speech intelligibility by increasing the noise floor. All other acoustic treatments cause a reduction in the level of the noise itself.
Sound masking systems work because the ear cannot perceive simultaneous sounds of similar volume and frequency. Sound masking systems add a constant background sound across a wide frequency range in order to reduce intelligibility, decrease the dynamic range, provide an acceptable background sound level, and minimize the differences in the quality and level of sound across the facility. The result is that unwanted noises are more difficult, or impossible, to hear or comprehend.
As with all acoustic treatments, the benefits of sound masking systems are maximized when used in conjunction with other office noise control methods. Since sounds decay over distance, sound masking requires some distance to become effective, and this distance is minimized when sufficient physical barriers and absorptive materials are used in the design of the office. Furthermore, if absorption is increased, the sound masking system’s volume can be reduced without affecting its performance level.
Use of sound masking systems can reduce costs by eliminating the need for additional insulation, extra layers of drywall, plenum barriers, high-spec walls, or permanent walls around private offices. In this way, sound masking also maintains the flexibility of the office space for future renovations and changes. In open plan spaces, sound masking can help maintain a level of acoustic control as density increases and workstation partitions lower.
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