by Niklas Moeller — Sound masking systems are a common component of today’s interiors, from their original use in commercial offices to newer applications such as patient rooms in hospitals.
This technology distributes an engineered sound throughout a facility, raising its ambient level in a controlled fashion. The principle is simple: any noises below the new level are covered up, while the impact of those still above it is lessened because the degree of change between baseline and peak volumes is smaller. Similarly, conversations are either completely masked or their intelligibility is reduced, improving privacy and concentration.
There are many familiar examples of this type of effect: the drone of an airplane engine, the murmur of a crowd in a busy restaurant, or even the rustling of leaves in the wind. All have the potential to mask sounds a listener would otherwise hear. Of course, when introducing a sound to a workplace, it is vital to ensure that it is effective, comfortable and unobtrusive. Otherwise, it can become irritating, as was the case with the original masking systems developed in the 1960s, which used white noise generators. Though the term is still used interchangeably with ‘sound masking,’ modern masking systems do not emit a ‘white noise’ signal.
A sound masking spectrum
A sound masking spectrum—or curve— is engineered to balance acoustic control and comfort. It is typically defined in third-octave bands, ranging from at least 100 to 5,000 hertz (Hz), but sometimes as high as 10,000 Hz. It is usually provided by an acoustician or an independent party such as the National Research Council (NRC), rather than by a masking vendor.
The curve defines what the sound masking system’s measured output should be within the space. Regardless of how the system is designed, its ‘out-of-the-box’ settings, the size of its zones, or the orientation of its loudspeakers (i.e. upward- or downward-facing, sometimes called ‘direct-field’), the sound changes as it interacts with elements of the workplace interior, such as the layout, furnishings, and other variables. In order for the sound to meet the desired curve, the system’s volume and frequency settings have to be adjusted. In other words, it must be tuned for the particular environment in which it is installed.
Tuning is handled by a qualified technician after the ceilings and all furnishings are in place, and with mechanical systems operating at normal daytime levels. Because conversations and activities can prevent accurate measurement, it is done prior to occupation or after hours. Basically, the technician uses a sound level meter to measure the masking sound at ear height. They analyze the results and adjust the system’s volume and equalizer controls accordingly. They repeat this process as often as needed until they meet the curve at each tuning location.
Achieving spatial uniformity
Most people compare the sound of a professionally-tuned masking system to that of softly blowing air. However, there is much more significance to the tuning process than providing a pleasant listening experience.
A sound masking system’s effectiveness is directly related to its ability to closely match the specified curve throughout the space. Because variations in the sound can profoundly impact performance, a sound masking specification not only provides a target curve, but also a tolerance that indicates by how much the sound is allowed to deviate from that curve. Consistency is also important for comfort.
Historically, tolerance has often been set to 2 dBA (i.e. plus or minus two A-weighted decibels), giving an overall range of 4 dBA. However, these swings allow occupants to understand up to 43% more of a conversation in some areas than they can in others. Advances in masking technology—particularly over the last decade—allow tolerance to be set as low as 0.5 dBA, or an overall range of just 1 dBA, ensuring consistent coverage.
Centralized sound masking
The need to improve tuning capabilities has been the driving force behind improvements in sound masking technology since it was first introduced in the 1960s.
The earliest systems used a centralized architecture. The name derives from the fact that the electronic components used for sound generation, 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—sometimes as many as hundreds.
While most offer limited analog volume control at each loudspeaker (usually 4 to 5 settings, in 3 dBA steps), their centralized design means that large areas of the facility are nonetheless served by a single set of output settings with little option for local adjustment. Technicians have to set each large zone to a level that is best ‘on average.’
Due to variations in the acoustic conditions across a space and the impact of interior elements, the masking sound is too low to be effective in some areas and too high for comfort in others. This pattern repeats at unpredictable points across the space, which is why central system specifications typically allow tolerances of 2 to 3 dBA, giving an overall range of 4 to 6 dBA. Users can typically expect a 10 percent reduction in performance for each decibel below the target volume. In addition, a centralized architecture only provides a global frequency control for each large zone, further impacting performance.
Decentralized sound masking
Decentralized architecture emerged in the mid-1970s in order to address the problem with large zone sizes. The electronics required for sound generation, volume and contour control are integrated into ‘master’ loudspeakers, which are distributed throughout the facility—hence the ‘decentralized’ name.
Each ‘master’ is connected to up to two ‘satellite’ loudspeakers that repeat its settings. Therefore, a decentralized system’s zones are one to three loudspeakers in size (i.e. 225 to 675 ft2). Because each small zone offers fine volume control, local acoustic variations can addressed, allowing more consistent masking levels to be achieved across a facility. However, there are still limits to the adjustments that can be made with respect to frequency.
Furthermore, a technician must 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 future adjustments challenging. 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 or to occupancy. One cannot take a ‘set-it-and-forget-it’ approach.
Networked sound masking
The first networked sound masking system was introduced over a decade ago. This architecture leverages the benefits of decentralized electronics, but networks the system’s components together throughout the facility—or across multiple facilities—in order to provide centralized control of all functions via a control panel and/or software. Changes can be made quickly and easily, maintaining masking performance 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. Some networked sound masking systems can even be automatically tuned using a computer, which rapidly and precisely adjusts the masking output to match the specified curve.
Today, there are a number of different product offerings within the centralized, decentralized and networked categories. Some vendors utilize a hybrid design, implementing a decentralized architecture in closed rooms (e.g. private offices) and a centralized architecture in the open plan. While this strategy may reduce initial costs, the centralized architecture presents significant tuning challenges in exactly the type of area where occupants are likely to rely on masking the most for speech privacy and noise control.
Updating performance standards
Indeed, sound masking is a critical design element for which one does not want to leave a lot of room for error. Without a performance standard, one may not achieve the expected level of speech privacy, noise control and occupant comfort. An ASTM subcommittee specializing in speech privacy is currently working on such a standard: WK47433, Performance Specification of Electronic Sound Masking When Used in Building Spaces.
In the meantime, a minimum performance guideline is to require the masking sound to be measured in each 1000 ft2 (90 m2) open area and each closed room, at a height between 1.2 to 1.4 meters (4 to 4.7 feet) from the floor (i.e. at ear height rather than directly below a loudspeaker). 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 5,000 Hz (or as high as 10,000 Hz). 2 dB variation in each frequency band is a reasonable expectation.
The vendor should adjust the masking sound within each area and/or closed room as needs dictate and provide a final report verifying the final results. The report should also indicate areas where the masking sound is outside the specified tolerance and why (e.g. noise from HVAC).
In all but extreme cases, it is impossible to subjectively tell whether a sound is providing the expected level of masking. In order to make this determination, one has to establish whether the desired curve has been met throughout the facility. Clients can be assured of their sound masking system’s performance by requesting a detailed report of the tuning results from their acoustician or vendor. This document should demonstrate that the desired curve is consistently provided throughout the space, representing that the system’s benefits can be enjoyed by all occupants.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network sound masking system (www.logison.com). He has over 25 years experience in the sound masking field and also writes an acoustics blog at www.soundmaskingblog.com.