Tuning Out Noise: Providing acoustic comfort with sound masking

April 19, 2016

All images courtesy K.R. Moeller Associates Ltd.
All images courtesy K.R. Moeller Associates Ltd.

By Niklas Moeller
Open-plan space, modular walls, and reflective surfaces such as glass, concrete, and metal are just a few of the design trends making today’s interiors even more dependent on sound masking for speech privacy and noise control. Since a sound masking system’s ability to provide these benefits largely depends on meeting the specified spectrum—or ‘curve’—throughout the facility, post-installation tuning is an essential part of the commissioning process. When handled poorly (or skipped altogether), the tuning of sound masking can greatly affect speech intelligibility, as well as occupants’ concentration and their overall workplace satisfaction.

The curve defines what the sound masking system’s measured output should be within the facility where it is installed. This target should be set by the client’s acoustician or a third party such as the National Research Council (NRC), rather than by the sound masking system’s manufacturer or vendor. The typical range is between 100 to 5000 hertz (Hz), but can go as high as 10,000 Hz. Unlike white or pink noise—terms often mistakenly substituted for ‘sound masking’—the volume of these frequencies follows a non-linear curve specifically engineered to balance acoustic control and occupant comfort. Successful sound masking implementation involves achieving both goals in equal measure. (For a deeper exploration of the ‘colours’ of noise, see this author’s article, “If You Need Sound Masking, Ask for it By Name,” for Construction Canada Online. Visit www.constructioncanada.net/if-you-need-sound-masking-ask-for-it-by-name[1]).

Regardless of how the sound masking system has been designed (i.e. the out-of-the-box settings, placement, and orientation of loudspeakers), the sound it distributes changes across the facility as it interacts with various interior elements, such as the layout and furnishings. To meet the specified curve, the client’s acoustician or the sound masking vendor’s technician must adjust the system’s volume and frequency settings. In other words, the masking sound has to be tuned for the particular environment in which it is installed.

This process should occur after the ceilings and furnishings are in place, and with mechanical systems operating at daytime levels. As activity and conversation prevent accurate measurement, it should also be done before the facility is occupied or after hours. The exact method will vary by product, but generally the acoustician or technician uses a sound level meter to measure the masking sound at ear height (i.e. the level at which occupants experience its effects), analyzes the results, and adjusts the volume and frequency settings accordingly. He or she repeats these steps until the curve is met at each tuning location.

Understanding tolerance
Some degree of variation from the curve is expected because it is impossible to achieve perfection in every tuning location. However, variations have an impact on the masking sound’s performance and can draw occupants’ attention to it. For that reason, the specified curve is usually accompanied by a ‘tolerance’ limiting the amount by which the sound is permitted to deviate from the goal across the client’s space.

Historically, this value was often set to ±2 dBA (i.e. plus or minus two A-weighted decibels), giving an overall range of 4 dBA—however, such wide swings in volume have a profound impact on speech intelligibility. Site tests are required for absolute Articulation Index (AI) or comprehension levels, but one can generally state each decibel decrease in overall masking volume reduces performance by 10 per cent. Therefore, a tolerance of ±2 dBA can allow occupants to understand up to 40 per cent more of a conversation in some areas than they can in others. (For more, see this author’s article, “Exploring the Impacts of Consistency in Sound Masking,” in a 2014 issue of Canadian Acoustics (vol. 42, no. 3), the journal of the Canadian Acoustical Association).

Specifications allowing ±2 dBA or even ±3 dBA are still in circulation, but they are a remnant of the capability of legacy technologies. When properly designed and tuned, newer sound masking systems can achieve ±0.5 dBA, giving an overall range of 1 dBA.

The role of masking architecture
The importance of achieving tight tuning tolerances throughout a sound masking installation is emphasized by how the ‘architecture’ used by this technology has evolved since first introduced in the 1960s. To improve both the accuracy of the tuning process and the efficiency with which it is done, industry engineers have sought to reduce zone size (i.e. individually controllable groups of loudspeakers) and devise new control methods.

Centralized sound masking
The earliest sound masking systems used a centralized architecture. In this configuration, the electronic equipment for generating and amplifying the masking sound, as well as providing volume and frequency control, are located within an equipment room or closet. The settings established at this central point are broadcast over a large number of loudspeakers. A global frequency control is provided for each of these large zones. Though most offer analog volume control at each loudspeaker within a large zone, it is limited to four to five settings, typically in 3-dBA steps.

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Since acousticians or technicians cannot make precise volume changes in specific areas, they have to set each large zone to a level that is best on average. Due to variations in the acoustic conditions across the space and the impact of interior elements, the masking sound is too low in some areas and too high in others. If they raise the volume to address a performance deficiency in one area, the sheer size of the zone means they simultaneously increase it in others, reducing occupant comfort, or vice versa. This pattern repeats at unpredictable points across the facility, which is why centralized system specifications typically set tolerance to ±2 to 3 dBA, giving an overall range of 4 to 6 dBA.

Decentralized sound masking
Decentralized architecture emerged in the mid-1970s to address a major deficiency in the ability to tune centralized systems—large zone size. Rather than locating sound generation, volume, and frequency control in a central location, the electronics required for these functions are integrated into ‘master’ loudspeakers, which are distributed throughout the facility—hence the ‘decentralized’ name.

Each ‘master’ connects up to two ‘satellite’ loudspeakers, which repeat their settings. Therefore, a decentralized system’s zones are only one to three loudspeakers in size (i.e. 20 to 62 m2[225 to 675 sf]). As each small zone offers fine volume control, local variations can be addressed, allowing more consistent and effective masking levels across a facility. However, there are still limits to the adjustments with respect to frequency. Further, the technician must enter the ceiling to make changes directly at each ‘master’ loudspeaker, using either a screwdriver (i.e. with analog controls) or an infrared remote (i.e. with digital controls), making adjustments time-consuming.

It is advisable to measure performance and modify a sound masking system’s settings when changes are made to the physical characteristics of the space (e.g. furnishings, partitions, ceiling, flooring) or to occupancy (e.g. relocating a call centre or human resource functions into an area formerly occupied by accounting staff). The likelihood these types of change will occur during a sound masking system’s 10- to 20-year lifespan is almost certain, so one simply cannot take a ‘set-it-and-forget-it’ approach. Sound masking engineers needed to develop a more practical way of adjusting the sound.

Networked sound masking
The first networked sound masking system was introduced a little over a decade ago. This technology leverages the benefits of decentralized electronics, but networks the system’s components together throughout the facility—or across multiple facilities—to provide centralized control of all functions via a control panel and/or software. Zoning (i.e. for paging, timer functions, and in-room occupant control) is also digital rather than hardwired. Therefore, changes can quickly be made following renovations or moving furniture or personnel, maintaining masking performance within the space without disrupting operations.

When designed with small zones of one to three loudspeakers offering fine volume (i.e. 0.5 dBA) and frequency (i.e. 1/3 octave) control, networked architecture can provide consistency in the overall masking volume not exceeding ±0.5 dBA, as well as highly consistent masking spectrums, yielding much better tuning results than possible with previous architectures. For improved efficiency, some networked sound masking systems can also be automatically tuned using a computer, which first measures the sound and then rapidly adjusts the masking output to match the specified curve.

Guidelines and reporting
Due to these advancements in the field of sound masking and the essential role it plays in achieving effective acoustics in today’s facilities, ASTM Subcommittee E33.02 on Speech Privacy—part of ASTM Committee E33 on Building and Environmental Acoustics—is currently working to update the related performance standards through WK47433, Performance Specification of Electronic Sound Masking When Used in Building Spaces. The group is also in the process of updating:

In the meantime, a minimum-performance guideline involves requiring the masking sound be measured in each 90-m2 (1000-sf) open area and each closed room, at a height between 1.2 to 1.4 m (4 to 4.7 ft) from the floor (i.e. at ear height rather than directly below a loudspeaker), and adjusted within that area as needs dictate. Some systems can adjust for smaller areas, but this is an acceptable baseline.

Masking volume is typically set to between 40 and 48 dBA, and the results should be consistent within a range of ±0.5 dBA or less. The curve should be defined in third-octave bands and range from 100 to 5000 Hz (or even as high as 10,000 Hz). Having ±2 dB variation in each frequency band—this tolerance is different from that set for volume—is a reasonable expectation.

The technician should adjust the masking sound within that area as needs dictate and provide the client with a detailed final report demonstrating the desired curve is consistently provided throughout the space. If there are any areas where the masking sound is outside the tolerance, this document should clearly identify the location and reason (e.g. noise from mechanical equipment
or HVAC).

Tuning can be a time-consuming process, but it is essential if the client is to derive the full benefit from their investment in sound masking technology. In this way, they can be confident the system is providing the intended effects and they are equally enjoyed by all occupants across their facility.

SOUND MASKING AND WALL CONSTRUCTION 

A sound masking system’s role is to control the acoustic conditions throughout a facility in the same way as temperature and lighting. One does not want cold or dark areas and, similarly, one should strive to achieve a consistent acoustic environment—not have a low ambient volume in one area and an effective one in others.

Intentionally omitting sound masking from particular areas runs contrary to the goal of ensuring this technology is as effective and unobtrusive as possible. Occupants will walk in and out of treated areas that differ in ambient volume (sometimes by as much as 10 to 12 dBA), calling their attention to the sound and, if the loudspeakers are visible, also reveal its source. The same can be said of attempting to spot-treat an area where a more obvious acoustical issue exists, such as within an open plan or outside a boardroom.

However, many people continue to exclude sound masking from private offices and meeting rooms, primarily in the belief closed spaces are afforded sufficient speech privacy and noise control via physical isolation.

Modern construction does not always allow for a high level of physical containment. To preserve flexibility, walls are often built to below the suspended ceiling or using demountable partitions, and may be largely composed of glass. Construction budgets can also limit wall options. In any case, even if walls are built deck-to-deck, voices find their way from one room to another through a variety of pathways. An open door is the biggest Achilles’ heel, but other common channels include passing through the plenum, return air grilles, and ductwork, gaps along the window mullions, ceiling, and floor—and even the walls themselves.

In order to use floor-to-ceiling walls with lower sound transmission class (STC) ratings and still achieve the acoustic control occupants expect in closed rooms, it is best to include sound masking in their design. If a wall decreases the intrusion of voice into the room by a decibel, then the signal-to-noise ratio (SNR) drops by a decibel. An identical drop occurs when the masking volume is raised by one decibel. Sound masking typically adds 5 to 12 dBA of ambient volume to closed rooms, which is why one sometimes hears that sound masking ‘adds 10 STC points’ to walls.

Budget-wise, the sound masking may represent $10 to $20/m2 ($1 to $2/sf) of space, but it offsets much more than that in terms of construction above the ceiling. The ability to provide private rooms with walls to the ceiling also increases the ease and cost-effectiveness of relocating them to suit future needs.

An exception to this guideline might be a large training room, where speech intelligibility is vital and, therefore, sound masking is omitted. Such rooms should be well-isolated using deck-to-deck construction with higher STC walls.

Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., the manufacturer of the LogiSon Acoustic Network sound masking system. He also writes an acoustics blog at soundmaskingblog.com[2]. Moeller can be reached via e-mail at nmoeller@logsion.com[3].

Endnotes:
  1. www.constructioncanada.net/if-you-need-sound-masking-ask-for-it-by-name: http://www.constructioncanada.net/if-you-need-sound-masking-ask-for-it-by-name
  2. soundmaskingblog.com: http://soundmaskingblog.com
  3. nmoeller@logsion.com: mailto:nmoeller@logsion.com

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