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).
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.
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.
