Tuning Out Noise: Providing acoustic comfort with sound masking

Networked sound masking
The first networked sound masking system was introduced a little over 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—to provide centralized control of all functions via a control panel and/or software. Zoning (i.e. for paging, timer functions, and in-room occupant control) is also digital rather than hardwired. Therefore, changes can quickly be made following renovations or moving furniture or personnel, maintaining masking performance within the space 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. For improved efficiency, 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.

Guidelines and reporting
Due to these advancements in the field of sound masking and the essential role it plays in achieving effective acoustics in today’s facilities, ASTM Subcommittee E33.02 on Speech Privacy—part of ASTM Committee E33 on Building and Environmental Acoustics—is currently working to update the related performance standards through WK47433, Performance Specification of Electronic Sound Masking When Used in Building Spaces. The group is 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; and
  • ASTM E2638, Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room.

In the meantime, a minimum-performance guideline involves requiring the masking sound be measured in each 90-m2 (1000-sf) open area and each closed room, at a height between 1.2 to 1.4 m (4 to 4.7 ft) 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 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 5000 Hz (or even as high as 10,000 Hz). Having ±2 dB variation in each frequency band—this tolerance is different from that set for volume—is a reasonable expectation.

The technician should adjust the masking sound within that area as needs dictate and provide the client with a detailed final report demonstrating the desired curve is consistently provided throughout the space. If there are any areas where the masking sound is outside the tolerance, this document should clearly identify the location and reason (e.g. noise from mechanical equipment
or HVAC).

Tuning can be a time-consuming process, but it is essential if the client is to derive the full benefit from their investment in sound masking technology. In this way, they can be confident the system is providing the intended effects and they are equally enjoyed by all occupants across their facility.


A sound masking system’s role is to control the acoustic conditions throughout a facility in the same way as temperature and lighting. One does not want cold or dark areas and, similarly, one should strive to achieve a consistent acoustic environment—not have a low ambient volume in one area and an effective one in others.

Intentionally omitting sound masking from particular areas runs contrary to the goal of ensuring this technology is as effective and unobtrusive as possible. Occupants will walk in and out of treated areas that differ in ambient volume (sometimes by as much as 10 to 12 dBA), calling their attention to the sound and, if the loudspeakers are visible, also reveal its source. The same can be said of attempting to spot-treat an area where a more obvious acoustical issue exists, such as within an open plan or outside a boardroom.

However, many people continue to exclude sound masking from private offices and meeting rooms, primarily in the belief closed spaces are afforded sufficient speech privacy and noise control via physical isolation.

Modern construction does not always allow for a high level of physical containment. To preserve flexibility, walls are often built to below the suspended ceiling or using demountable partitions, and may be largely composed of glass. Construction budgets can also limit wall options. In any case, even if walls are built deck-to-deck, voices find their way from one room to another through a variety of pathways. An open door is the biggest Achilles’ heel, but other common channels include passing through the plenum, return air grilles, and ductwork, gaps along the window mullions, ceiling, and floor—and even the walls themselves.

In order to use floor-to-ceiling walls with lower sound transmission class (STC) ratings and still achieve the acoustic control occupants expect in closed rooms, it is best to include sound masking in their design. If a wall decreases the intrusion of voice into the room by a decibel, then the signal-to-noise ratio (SNR) drops by a decibel. An identical drop occurs when the masking volume is raised by one decibel. Sound masking typically adds 5 to 12 dBA of ambient volume to closed rooms, which is why one sometimes hears that sound masking ‘adds 10 STC points’ to walls.

Budget-wise, the sound masking may represent $10 to $20/m2 ($1 to $2/sf) of space, but it offsets much more than that in terms of construction above the ceiling. The ability to provide private rooms with walls to the ceiling also increases the ease and cost-effectiveness of relocating them to suit future needs.

An exception to this guideline might be a large training room, where speech intelligibility is vital and, therefore, sound masking is omitted. Such rooms should be well-isolated using deck-to-deck construction with higher STC walls.

Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., the manufacturer of the LogiSon Acoustic Network sound masking system. He also writes an acoustics blog at soundmaskingblog.com. Moeller can be reached via e-mail at nmoeller@logsion.com.

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