April 3, 2019
by Niklas Moeller
Sound masking is an electronic acoustic treatment used to control a facility’s background sound level. Though the sound—distributed via a network of loudspeakers—is often compared to softly blowing air, it is professionally tuned to meet a particular spectrum or ‘curve.’ The sound either completely covers up noises or reduces their disruptive impact by decreasing the amount of change between the baseline level and any volume peaks within the space. Similarly, conversations are either entirely masked or their intelligibility is reduced, improving privacy.
When designing a sound masking solution, it is vital to limit the size of its control zones so the technician or acoustician can precisely tune the sound and, hence, deliver the specified masking curve to the client.
Within this context, a control zone is a group of loudspeakers fed by a dedicated masking sound generator for which one can establish individual volume (0.5 dB steps) and frequency (1/3-octave) settings. Basically, smaller zones give the technician more control over the sound, allowing them to achieve a consistent masking effect throughout the installation.
Zoning closed rooms
Best practice for closed rooms, such as private offices and meeting rooms, is to provide a control zone for each one. Whereas masking can be set as high as 48 dBA within open plans, closed rooms are typically set between 40 and 45 dBA. Providing each room with its own zone not only allows the technician to set the volume to an appropriate level, but also to tune the sound to provide a spectrum identical to the one used within the open plan.
If a keypad is installed, this zoning pattern also allows the occupant to adjust the masking volume and paging settings according to their need or preference.
Addressing the open plan
While the reasons for not allowing a control zone to cross open and closed areas are obvious, it is also necessary to avoid creating one covering large areas of open plan.
Though these spaces appear homogenous with their rows of bench seating or workstations, their acoustic conditions are usually anything but consistent. The unmasked ambient level varies across the floor plate—in some cases significantly, and even over short distances. The sound introduced by the masking system also fluctuates as it interacts with the interior’s layout, furnishings, and other materials. To achieve the desired sound masking curve, at the required volume, the technician must be able to address these local issues where they arise.
Using single-speaker zones throughout the design is ideal from the perspective of maximizing the technician’s control over the masking sound, and modern technologies can easily be implemented this way. However, if the client’s budget requires a modest concession, it is acceptable to have up to three loudspeakers per zone across open plans, expanding coverage from 21 to 63 m2 (225 to 675 sf). This size still allows the tuning technician to adjust the sound’s volume and frequency to address local variations and achieve a consistent—and therefore consistently comfortable and effective—masking sound throughout the installation.
Venturing beyond three loudspeakers introduces ever-escalating, tuning, and commissioning challenges. Though this decision might reduce the initial purchase price of the sound masking equipment, it carries a steep cost in terms of effectiveness, comfort, and flexibility.
There are three key ways larger loudspeaker zones impact performance.
In some sound masking systems using six- to eight-speaker zones, the output difference between the first and last loudspeaker in the zone far exceeds the allowable volume variation in common performance specifications. Technically, acousticians can address this problem—at least to some degree—by adding audio-transformers to the loudspeakers, but doing so adds cost and diminishes the system’s flexibility.
Zone size affects how efficiently and cost-effectively changes can be made. The likelihood the client will need to make adjustments during a sound masking system’s 10- to 20-year lifespan is almost certain. Large zones limit their ability to reconfigure the system without first altering its design, moving loudspeakers, and/or rewiring. If changes are made to the physical characteristics of the space or to occupancy, modifications might not only need to be made to the masking sound, but also to paging, music, timer, keypad, and security settings.
Lastly—but most importantly—if the technician cannot adjust the sound in small local areas, any changes they make to try to improve performance or comfort will affect large areas. While they might be able to resolve a problem in one part of a zone, its sheer size means they are likely to intensify it in another area or create a new issue altogether—and always at unpredictable points across the client’s space. For example, if they need to raise the masking volume to improve effectiveness in one location, it might become too loud in another, decreasing satisfaction. Similarly, if they raise a band level to address a deficiency in one area, it will rise throughout the zone, bringing the sound out of spec in locations where the adjustment was not required initially.
As shown in Figure 1, the larger the zone, the greater the number of people potentially affected by these sound variations. While 21 m2 of space typically accommodates only 1.3 occupants and 63 m2 holds 3.9, if one designs a sound masking system using eight, 25, 50, or 100 loudspeakers in a zone, one is covering 10.3, 32.1, 64.3, or 128.6 people, respectively.
One might try to brush off concerns about seemingly minor volume changes, but their impact is rather significant, even without taking frequency into consideration. Users can typically expect a 10 per cent reduction in performance for each decibel drop in the masking volume. So, while an occupant might only be able to understand 30 per cent of a conversation with the masking set to 48 dBA, they might comprehend more than 70 per cent at 44 dBA. The typical specification for larger zoned systems is ±2 dBA (i.e. plus or minus two A-weighted decibels), giving a range of 4 dBA overall, despite a technician’s best efforts to accurately tune the sound. An even more poorly designed masking system can allow as much as a ±3 dBA variation, or 6 dBA overall.
To be consistently effective, masking volume should vary no more than ±0.5 dBA at test points across the entire installation, except in circumstances well beyond the technician’s ability to control (e.g. HVAC). A sound masking system designed with zones no larger than three loudspeakers—each offering fine control over volume and frequency—can achieve these results, providing the zones are tuned individually.
Can the performance deficiencies of large zones be overcome by, for example, installing the loudspeakers facing downwards? If ‘near-field’ measurements are taken a couple of inches below each loudspeaker, a technician might conclude they have achieved consistent results. However, if masking sound is measured at ear height—where occupants actually experience its effects—the findings will be different. For example, one recent acoustic study of an installation using downward-facing or ‘direct-field’ loudspeakers found 3 to 4 dBA differences at ear height between two offices, whereas the output at the loudspeaker was nearly identical.
|THE PERFORMANCE THRESHOLD|
While expanding zone size can reduce the initial purchase price of a sound masking system, there comes a point where it does so at the expense of performance and occupant comfort.
Unfortunately, some organizations, such as the City of Mississauga, have experienced this trade-off firsthand. Due to budget pressures, they elected to implement a lower-cost design using eight-speaker zones. The city is an excellent case study because, up until then, it had always used zones no larger than three loudspeakers in its properties, including multiple offices, call centres, and City Hall. Therefore, they could easily tell their masking was not performing as it should.
When describing occupants’ experience within this space, Raj Sheth, director, facilities and property management, noted, “In contrast to our other spaces, we had to call for service repeatedly, either to make adjustments to suit our needs or to address staff feedback. Each time we attempted to fix a problem in one location, we affected a large area and often created a new problem in another. Over time, this trend pushed the masking volume lower and lower, to the point where it was no longer effective in some areas.”
“Rather than the consistent masking we had in other spaces, we ended up with a patchwork of masking levels,” he added. “It became increasingly clear the inability to achieve the results we desired was due to the inherent inflexibility of the eight-speaker design.”
The city came to appreciate the substantial reduction in masking performance caused by what at first glance appears to be a small difference in system design. Modifying this installation to conform to the three-speaker zone limit resulted in a more uniformly effective and comfortable masking sound. They since asked the vendor to modify this installation to conform to the three-speaker zone limit.
At the end of the day, the client is paying for the effect, not the sound masking equipment. To deliver value rather than simply introducing a random amount of background noise to a facility, professional integrators need to accurately achieve the specified masking spectrum throughout the installation. The smaller the zones, the more test and adjustment points the design offers the technician. The more precise the masking sound, the better the outcome for the client.
Niklas Moeller is vice-president of KR Moeller Associates, manufacturer of LogiSon Acoustic Network and Modio Guestroom Acoustic Control. He has more than 25 years of experience in the sound masking field. He can be reached at firstname.lastname@example.org.
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