Audio

Achieving An Effective Sound Masking Curve

Recent design trends make today’s facilities ever more dependent on sound masking for speech privacy and noise control. Because a sound masking system’s ability to provide these benefits largely depends on meeting the target spectrum throughout the installation, measurements and adjustments a technician makes are an essential part of the commissioning process.

Understanding The Curve

The target masking spectrum, or “curve,” should be provided by the project’s acoustical engineer: an independent party, such as the National Research Council (NRC), rather than by the system’s manufacturer. The range is typically between 100Hz to 5000Hz, or as high as 10,000Hz. Unlike white or pink noise, terms often incorrectly substituted for “sound masking,” the volume of these frequencies is not equal, nor do they decrease at a constant rate as frequency increases. Rather, they follow a non-linear curve engineered to balance acoustic control and comfort (Figure 1).

This curve defines what that system’s measured output should be within the space. To meet it, the technician must adjust the system’s volume and frequency settings, because (regardless of how the system is designed, its out-of-the-box settings, the placement or orientation of its loudspeakers) the sound it distributes changes as it interacts with various interior elements within the facility, such as the layout and furnishings. In other words, the system must be tuned to the particular environment in which it is installed.

Figure 1. A sound masking spectrum or “curve” should be specified by an acoustician or supplied by an independent third party, such as the National Research Council.
Figure 1. A sound masking spectrum or “curve” should be specified by an acoustician or supplied by an independent third party, such as the National Research Council. Courtesy NRC.

After Everything’s In Place

This process should be handled after the ceilings and furnishings are in place, and with mechanical systems operating at daytime levels. Because activity and conversation prevent accurate measurement, it should also be done prior to occupation or after hours. Though the exact method varies from system to system, basically, the technicians use a sound level meter to measure the masking sound at ear height, analyze the results, and then adjust the system’s volume and equalizer controls accordingly. They repeat these steps as often as required to meet the desired curve at each tuning location.

Some degree of variation from the target curve is expected; it is impossible to achieve perfection in every tuning location. However, because variations in the sound can profoundly impact masking performance, the specification should also provide a “tolerance” that indicates by how much the sound is allowed to deviate from the target curve across the space. Achieving consistency is also important for comfort: A uniform sound fades into the background more easily and occupants come to consider it a natural part of their space.

Tuning can be a time-consuming process, but is essential if the client is to derive the full benefit from the investment in this technology.

What Is The Value?

Indeed, a sound masking system’s ability to provide the intended effect is tolerance directly related to the ability to closely match the target curve throughout the space. Historically, this was often set to ±2dBA (i.e., plus or minus two A-weighted decibels), giving an overall range of 4dBA. However, such wide swings in overall volume across the space can allow occupants to understand much more of a conversation in some areas than they can in others, as illustrated by the Articulation Index (AI) tests conducted between two workstations.

Figure 2. Articulation Index (AI) tests were conducted between these two workstations to illustrate the impact of tuning tolerances on sound masking’s ability to improve speech privacy and, hence, the importance of closely and consistently meeting the specified sound masking spectrum or curve throughout the facility.
Figure 2. Articulation Index (AI) tests were conducted between these two workstations to illustrate the impact of tuning tolerances on sound masking’s ability to improve speech privacy and, hence, the importance of closely and consistently meeting the specified sound masking spectrum or curve throughout the facility. Courtesy KR Moeller Associates Ltd.

In this case study, the occupants sit about 15.5 feet (4.7 meters) apart within an open plan (Figure 2). The partitions are 64 inches (1.6 meters) in height and the ceiling tile is highly absorptive. However, without sound masking, the ambient level is only 40.6dBA and the listener can understand 85% of the other person’s conversation. When masking is applied, comprehension quickly declines. In fact, for each decibel of increase in masking volume, comprehension drops by an average of 10% (Figure 3). With the masking set to 48dBA (i.e., the typical maximum level for comfort) with a narrow tolerance of ±0.5dBA, the listener can understand just 14% to 25%. When a broader tolerance of ±2dBA is applied, they can understand up to 59%, barely an improvement over the unmasked conditions.

Though this example focuses solely on volume, variations in frequencies can similarly impact masking performance.

Figure 3. Though a small amount of deviation is unavoidable, the Articulation Index tests show that too broad a tolerance (e.g. of ±2dBA) represents more than a 40% drop in sound masking performance. A tight tolerance of ±0.5dBA ensures that the masking effect is consistently experienced across the space.
Figure 3. Though a small amount of deviation is unavoidable, the Articulation Index tests show that too broad a tolerance (e.g. of ±2dBA) represents more than a 40% drop in sound masking performance. A tight tolerance of ±0.5dBA ensures that the masking effect is consistently experienced across the space. Courtesy KR Moeller Associates Ltd.

What Are The Risks?

Poor masking performance carries real risk, particularly in facilities where there is a perceived need for speech privacy or an expectation on the part of its users. In some industries, protecting verbal communication is even mandated by law. The Health Insurance Portability and Accountability Act (HIPAA) is a good example, requiring healthcare entities to take “reasonable safeguards” to ensure speech privacy during both in-person and telephone conversations.

Speech privacy is also vital to employees’ overall satisfaction with their workplace. A decade-long survey of 65,000 people, run by the Center for the Built Environment (CBE), University of California, Berkeley, found that lack of speech privacy is the number-one complaint in offices.

Studies also show that it has a significant impact on productivity. For instance, research conducted by Finland’s Institute of Occupational Health shows that unwilling listeners demonstrate a 5% to 10% decline in performance when undertaking tasks such as reading, writing and other forms of creative work. Though some organizations may dismiss the importance of speech privacy, particularly within an open plan, taking steps to lower speech intelligibility enhances concentration and productivity.

Architecture’s Role

The importance of achieving tight tuning tolerances throughout the space is also emphasized by the evolution of sound masking architecture. Since the technology was first introduced in the 1960s, numerous advancements have been made in order to make tuning a more precise and efficient exercise.

  • Centralized Sound Masking: The earliest sound masking systems used a centralized architecture. The name derives from the fact that the electronic components used to generate the masking sound, as well as to provide volume and frequency control and amplification, are all 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 (Figure 4). Although most offer limited analog volume control at each loudspeaker (usually four to five settings, in 3dBA steps), their centralized design means that large areas of the facility are nonetheless served by a single set of output settings with little or no option for local adjustment.Because the technicians cannot make precise volume changes in specific areas, they have to set each large zone (i.e., individually controllable groups of loudspeakers) 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 the technician raises the volume to address a performance deficiency in one area, the sheer size of the zone means it is increased simultaneously in others, affecting occupant comfort. If it is lowered to boost comfort, speech privacy and noise control are sacrificed. This pattern repeats at unpredictable points across the space, which is why central system specifications typically allow large variations in overall masking volume. Tolerance is typically 4 to 6 decibels (i.e., ±2 to 3dBA). Furthermore, centralized architecture only provides a global frequency control for each large zone.
Figure 4. A centralized masking architecture consists of a centrally located rack of electronic equipment that is used for sound generation, volume and frequency adjustment. This equipment is connected to a large number of loudspeakers (as few as eight or as many as hundreds) forming a single adjustment zone. Courtesy KR Moeller Associates Ltd.
Figure 4. A centralized masking architecture consists of a centrally located rack of electronic equipment that is used for sound generation, volume and frequency adjustment. This equipment is connected to a large number of loudspeakers (as few as eight or as many as hundreds) forming a single adjustment zone. Courtesy KR Moeller Associates Ltd.
  • Decentralized Sound Masking: Decentralized architecture emerged in the mid-1970s in order 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 (Figure 5).Each “master” is connected to 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., 225 to 675 square feet (30 to 62 square meters). This distributed design inherently controls phasing. In addition, because each small zone offers fine volume control, local variations can be addressed, allowing more consistent and, therefore, effective masking levels to be achieved across a facility. However, there are still limits to the adjustments that can be made with respect to frequency, which impacts performance.
Figure 5. A decentralized masking architecture uses “master” loudspeakers to house the electronics required for sound generation, volume and contour control. Adjustment zones are one to three loudspeakers in size. Local changes are made using a screwdriver or remote control.
Figure 5. A decentralized masking architecture uses “master” loudspeakers to house the electronics required for sound generation, volume and contour control. Adjustment zones are one to three loudspeakers in size. Local changes are made using a screwdriver or remote control. Courtesy KR Moeller Associates Ltd.

Changing Each ‘Master’

Furthermore, the 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 (e.g., furnishings, partitions, ceiling, flooring) or to occupancy (e.g., relocating a call center or human resource functions into an area formerly occupied by accounting staff). The likelihood that these types of changes will occur during a sound masking system’s 10- to 20-year lifespan is almost certain; therefore, one cannot take a “set-it-and-forget-it” approach. Engineers had to develop a more practical way of adjusting the masking sound.

  • Networked Sound Masking: The first networked sound masking system was introduced slightly more than 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, in order to provide centralized control of all functions via a control panel and/or software (Figure 6). Changes can also be made quickly following renovations, moving furniture or personnel, to maintain masking performance within the space without disrupting operations.When designed with small zones of one to three loudspeakers offering fine volume (i.e., 0.5dBA) and frequency (i.e., 1/3 octave) control, networked architecture can provide consistency in the overall masking volume not exceeding ±0.5dBA, as well as highly consistent masking spectrums, yielding much better tuning results than possible with previous architectures. 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.
Figure 6. A networked masking architecture uses “hubs” to house the electronics required for sound generation, volume and frequency control. Adjustment zones are one to three loudspeakers in size. All local and global changes, including those to zoning, are made from a central location, such as a small panel or software application.
Figure 6. A networked masking architecture uses “hubs” to house the electronics required for sound generation, volume and frequency control. Adjustment zones are one to three loudspeakers in size. All local and global changes, including those to zoning, are made from a central location, such as a small panel
or software
application. Courtesy KR Moeller Associates Ltd.

Guidelines & Reporting

Sound masking is a critical design element for which one does not want to leave a lot of room for error. ASTM Subcommittee E33.02 on Speech Privacy, part of ASTM Committee E33 on Building and Environmental Acoustics, is currently working to update the performance standards for sound masking, accordingly through WK47433, Performance Specification of Electronic Sound Masking When Used in Building Spaces. They are also in the process of updating:
—ASTM E1130, Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index
—ASTM E1374, Guide for Open Office Acoustics and Applicable ASTM Standards
—ASTM E1573, Test Method for Evaluating Masking Sound in Open Offices Using A-Weighted and One-Third Octave Band Sound Pressure Levels
—ASTM E2638, Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room.

In the meantime, a minimum performance guideline requires the masking sound to be measured in each 1000-square-foot (90-square-meter) 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), and adjusted within that area as needs dictate. Some systems can adjust for smaller areas, but this is an acceptable baseline. Masking volume typically is set to between 40 and 48dBA, and the results should be consistent within a range of ±0.5dBA or less. The curve should be defined in third-octave bands and range from 100 to 5000Hz (or as high as 10,000Hz). A reasonable expectation is ±2dB variation in each frequency band.

The technician should adjust the masking sound within that area as needs dictate and provide the client with a detailed final report demonstrating that the desired curve is consistently provided throughout the space; if there are any areas where the masking sound is outside tolerance, this document should clearly identify the location and reason (e.g., noise from mechanical equipment or HVAC). In this way, the client can be confident that the sound masking system’s benefits are enjoyed equally by all occupants in their facility.

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