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 (0.5dB 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.
The 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 48dBA within open plans, closed rooms are typically set to 40dBA to 45dBA. Providing each room with its own zone not only allows the technician to set the volume to an appropriate level, but to tune the sound in order to provide a spectrum identical to that used within the open plan.
If a keypad is installed, this zoning pattern also allows occupants to adjust the masking volume and paging settings according to their need or preference.
Although the reasons for not allowing a control zone to cross open and closed areas are obvious, it is also necessary to avoid creating one that covers large areas of open plan.
Although 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 225ft2 to 675ft2. This size still allows the tuning technician to adjust the sound’s volume and frequency to address local variations and, hence, to 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. Although this decision might further 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 in which larger zones impact performance:
In some 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, one can address this problem, at least to some degree, by adding audio transformers to the loudspeakers. However, doing so adds cost and diminishes the system’s flexibility.
One size affects how efficiently and cost-effectively changes can be made. The likelihood that 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 need to be made not only to the masking sound, but also to paging, music, timer, keypad and security settings.
Most importantly: If the technicians cannot adjust the sound in small local areas, any changes they make to try to improve performance or comfort will affect large areas. Although they might be able to resolve a problem in one part of a zone, its sheer size means that 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.
As shown in Figure 1, the larger the zone, the greater number of people potentially affected by these variations. Although 225ft2 of space typically accommodates only 1.3 occupants and 675ft2 holds 3.9, if you design a sound masking system using 8, 25, 50 or 100 loudspeakers in a zone, you are 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 typically can expect a 10% reduction in performance for each decibel drop in the masking volume. So, although occupants might only be able to understand 30% of a conversation with the masking set to 48dBA, they might comprehend more than 70% at 44dBA. The typical specification for larger zoned systems is ±2dBA (i.e. plus or minus two A-weighted decibels), giving a range of 4dBA 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 ±3dBA variation, or 6dBA overall.
In order to be consistently effective, masking volume should vary no more than ±0.5dBA 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 technician tunes the zones individually.
Can the performance deficiencies of large zones be overcome by, for example, installing the loudspeakers facing downward? If a technician takes “near-field” measurements a couple inches below each loudspeaker, they might conclude that they have achieved consistent results. However, if he measures the masking sound 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 3dBA to 4dBA differences at ear height between two offices, whereas the output at the loudspeaker was nearly identical.
At the end of the day, the client is paying for the masking effect, not the equipment. Although expanding zone size reduces the cost of the system, there comes a point where it does so at the expense of performance. Unfortunately, some clients learn this lesson the hard way, as did the City of Mississauga when, due to budget pressures, they elected to implement a lower-cost design using eight-speaker zones. Up to that point, the organization had always used zones no larger than three loudspeakers in its properties. Raj Sheth, Director, Facilities & Property Management, stated that they “have since come to appreciate the substantial reduction in performance caused by this seemingly small difference.” He added, “Rather than the consistent masking we had in other spaces, we ended up with a patchwork of masking levels.”
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.