Mind the Gap

Using sound masking in open and closed spaces

By Niklas Moeller

The majority of employees spend over half their time on individual focus work and another large percentage on the telephone. Their environment should support these activities, but instead, workplace design trends are steadily eliminating many methods of controlling acoustics. While the proportion of open plan and occupant densities grow, partitions are lowered or omitted, absorptive finishes are forgone in favor of exposed surfaces, and closed rooms are built using demountable walls, reducing room-to-room isolation.

Whether such decisions are made for the sake of aesthetics, sustainability, or short-term budget goals, they all reduce acoustic performance. The situation is compounded by improvements in construction materials, mechanical, and office equipment, which have lowered the ambient—or background—sound level. The resulting ‘pin-drop’ environment allows conversations and noise to easily be heard and understood, even from a distance. What ambient sound remains does not exhibit the correct mix of frequencies needed for speech privacy, noise control, and comfort.

Sound-masking technology is used to distribute an engineered sound throughout a facility, raising its ambient level in a controlled fashion. While adding more sound to a space might seem to contradict the goal of achieving effective acoustics, the premise behind this solution is simple: it covers up noises that are lower in volume and reduces the disruptive impact of those that are higher by decreasing the magnitude of change between the baseline and any peaks in the space. Conversations are also either entirely covered or their intelligibility is reduced. Hence, occupants perceive treated spaces as quieter and more private.

Most people have experienced this type of effect—for example, when washing dishes at the kitchen sink while trying to talk to someone in the next room. The listener can tell the other person is speaking, but it is difficult to understand exactly what is being said because the running water has raised the ambient level in their area. In fact, everyday examples are virtually endless: the drone of an airplane engine, the murmur of a crowd in a busy restaurant, or even the rustling of leaves in the wind. They all have the potential to mask sounds the listener would otherwise hear.

Of course, when introducing a sound to a workplace, it is vital to ensure that it is as comfortable and unobtrusive as possible. Though most compare the sound of a well-designed and professionally tuned masking system to that of softly blowing air, it actually follows a nonlinear spectrum or ‘curve,’ which is defined in third-octave bands and typically ranges from 100 to 5,000 hertz (Hz), or as high as 10,000 Hz. Unlike ‘white noise’ or ‘pink noise’—terms often, but incorrectly, used in this context—this curve is specifically designed to balance acoustic control and occupant comfort. A successful implementation involves achieving both goals, in equal measure.

Myths and Misconceptions

Over the past decade, there have been tremendous advancements in sound-masking technology, increasing performance, expanding functionality, and opening the door to new applications. Yet, certain outdated design practices persist, often to the detriment of speech privacy, noise control, and overall acoustic comfort. One such custom is the exclusion of sound masking from closed rooms, such as private offices, meeting, and audio/video conferencing rooms.

Several reasons are used to justify this type of design. The first is an historical remnant from the days when sound masking was first adopted to help with the obvious acoustic challenges encountered in an ever-growing number of open plans. This initial application led some to conclude that masking was only intended for these areas.

However, this opinion was also reinforced by a significant technical impediment. Early sound-masking systems exclusively used a centralized architecture, which is very limited in terms of its ability to offer local control over the masking sound. Large zones spanned numerous private offices and other closed rooms, with little opportunity to adjust the volume within each space (i.e., only via transformer taps located on the loudspeakers) and no control over frequency. The resulting inconsistencies in volume and spectrum impacted masking performance and occupant comfort, leading both vendors and dissatisfied customers to conclude that the technology simply could not be applied in closed spaces.

Despite the fact that modern networked sound-masking architectures address these deficiencies by providing fine control over both volume and frequency within small zones (i.e., of one to three loudspeakers), some still argue that masking is best left to open plans.

Beyond its historical foundations, there are a few other reasons consistently cited for excluding closed rooms from sound-masking designs. One objection is the idea that these spaces do not require masking because they are afforded sufficient speech privacy and noise control via physical isolation. Another is the concern that sound masking will interfere with communication, either within the room itself or over audio/video conferencing equipment.

In fact, if handled properly, there are numerous advantages to including sound-masking coverage for closed rooms: greater consistency of acoustical characteristics across the facility, higher occupant satisfaction, increased speech privacy and protection from noise disruptions, construction savings, and the preservation of flexibility for future renovations.

Wall Construction

Closed offices and meeting rooms are built with the intention of providing occupants with both visual and acoustic privacy. While the first goal can easily be achieved, the latter often proves elusive because of the many ways in which sound can transfer from one space to another.

In the attempt to create sufficient speech privacy, one might specify walls with high sound transmission class (STC) ratings. However, STC ratings are lab tested and frequently overstate real-world performance by 5 to 10, or more, points. Site-tested field STC or noise isolation class ratings are better gauges, but unfortunately, only testable after the fact.

A common tactic used to improve speech privacy in a closed space is to construct full-height walls that extend from the concrete floor all the way to the deck above (i.e., deck-to-deck or slab-to-slab construction). The aim is to completely seal the room. While this approach does increase effectiveness, it also raises costs and reduces flexibility. Vigilance must be maintained during design, construction, maintenance, and renovation to ensure that penetrations in the wall’s structure are controlled. Even minor ones can substantially reduce acoustic performance.

These challenges raise the question as to whether there are alternate and preferable methods of achieving high levels of speech privacy in closed spaces, such as adding sound-masking technology to closed rooms with walls built only to the suspended ceiling.

Cracks in the Armor

Each crack in a wall’s armor facilitates the transmission of sound to and from neighboring spaces.

For example, wall performance is very sensitive to gaps along the perimeter, such as those that can occur along window mullions or the floor. If light can pass through, so can sound, and often well enough to substantially reduce the wall’s impact. Other ‘imperfections’ also weaken the wall’s sound-isolating performance; for instance, HVAC elements that pass between closed spaces, and even back-to-back electrical switches and outlets. Interior windows may also contribute to sound transfer.

In the case of full-height walls, the seal between the top of the wall and the deck must be maintained, which can be quite difficult if the surface is irregular, such as with a corrugated steel deck. The sound isolation performance of the barrier above the ceiling may also be compromised by penetrations. Openings can exist from the day of construction or be introduced during servicing or upgrades to the facility. Any gaps due to building structure, pipes, conduit, cables, and raceways must be carefully managed to ensure the integrity of the wall. This level of care can be challenging to sustain throughout the life of the space.

Moreover, a closed space only provides acoustic isolation when the door is closed. Most occupants leave their door open for the majority of the time—exposing a closed room’s biggest Achilles’ heel. For example, an STC 40-rated wall with an open door that represents 10 percent of the wall’s area reduces its effective STC to 10. The same is true for STC 45 and 50 walls. If the door represents 20 percent of the wall area—which is the case for a standard 3-foot door in a 10-by-10-foot wall—then the effective STC drops to around 7.

To avoid making the door the weak link even when it is closed, it must at least match the wall’s STC rating. Any improper seals will provide a convenient route for sound to escape or enter the room.

Cost and Flexibility

Full-height walls also present financial challenges. Compared to a wall built from the floor to the suspended ceiling, the additional costs of materials and labor are obvious. However, there are other ways that deck-to-deck construction can substantially add to the initial budget. Each time a wall is built above the suspended ceiling, the ceiling grid must be restarted—a time-consuming process. The separated plenum space requires separate return air ducts and may necessitate additional HVAC control zones. Return ducts must be treated to prevent the transfer of sound along their length from one location to another.

It is also more difficult and costly to renovate because moving such a wall requires changes to the ceiling grid, tiles, and HVAC returns.

Constructing and moving floor-to-ceiling walls is a much simpler and less costly exercise. Modular wall systems permit even more rapid relocation. However, both open up a further pathway for the transmission of sound. Typically, the acoustical tile has a lower attenuation rating than the wall. Sounds pass through it, reflect from the deck above, and down into the neighboring space. In this case, a tile with a ceiling attenuation class (CAC) of 35 to 40 is recommended. It lessens the flanking of sound through the ceiling and plenum, but the room is still subject to the aforementioned acoustic leakages.

The Speech Privacy Equation

At the end of the day, whether built from floor to ceiling or deck to deck, walls only address part of the speech privacy equation. A person’s ability to clearly understand a conversation is actually dependent on two variables: the volume of the speaker’s voice and the volume of the background sound level in the space. The relationship between the two is called the signal-to-noise ratio.

Traditional closed-room construction attempts to provide privacy by simply reducing the signal. Even if a deck-to-deck wall is well-designed and constructed (i.e., all penetrations are addressed), it still may not provide the sought-after level of speech privacy. If the background sound level in the adjoining space is lower than the sound passing through the wall, noises and conversations will still be heard and potentially intelligible. With today’s building standards, this is often the case.

Calculating the Benefits

Sound-masking technology can be used in combination with walls built to the suspended ceiling or demountable partitions in order to provide a cost-effective and more flexible alternative to deck-to-deck construction.

Budget wise, sound masking may represent $1 to $2 of cost per square foot 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.

But is an equal or greater level of privacy achievable using this alternative?

The most objective method to resolve the speech privacy question is to quantify the effects of increased attenuation and increased sound masking on intelligibility. This exercise can be done using the ASTM standard method (ASTM E1130-08: Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index) for calculating articulation index (AI), which is a metric of speech intelligibility and takes both factors into account. While the ASTM standard references open-plan spaces, it is generally agreed that this method can also be applied to closed spaces, with slight modification to the test equipment used.

Calculation of the AI is based on several measurements taken in the space in question, as well as a standardized normal voice level. On-site testing determines the amount by which voice level reduces between the source room and the listener location. This amount is subtracted from the standardized voice level to give the volume in the listener location. That volume is compared to the background sound level in that location. The difference between the volume of the voice relative to the volume of the background in each of the third-octave frequency bands (200 to 5,000 Hz) provides the signal-to-noise ratio in the listener location. The articulation index method assigns a specific weighting formula to determine an AI contribution within each frequency band. These are summed to arrive at the AI value.

Using this method, one can quantify the impact of increasing the attenuation of the wall and that of increasing the masking level, allowing one to compare the two strategies. Obviously, as wall attenuation increases, for each decibel there is an increase in speech privacy levels. Mathematically, the same can be achieved by raising the background sound level by a decibel. To understand why, one need only look to the step in the above AI calculation that determines the signal-to-noise ratio. If a wall decreases the intrusion of voice into the room by a decibel, then the signal-to-noise ratio drops by a decibel. An identical drop occurs when the masking volume is raised by one decibel.

Depending on a variety of factors—including mechanical system noise—the background sound level in closed rooms without sound masking usually ranges from the low 30s to 40 dBA. Levels for sound masking in closed rooms range from approximately 40 to 45 dBA, depending on the size of the room and other conditions. In other words, sound masking typically adds approximately 5 to 12 dBA of ambient volume, which is why one sometimes hears that sound masking ‘adds 10 STC points’ to walls.

Given that the ceiling tile in closed spaces already attenuates sound transmission to neighboring spaces, in most cases, it is unlikely that extending the wall above the ceiling will produce a greater increase in attenuation. This is particularly true if the space’s design follows the recommendation for tiles with a ceiling attenuation class (CAC) of 35 to 40. Of course, if wall and ceiling specifications are low, then benefits may be found by upgrading those elements, while also controlling the background sound levels.

Noise Control

Noise control is yet another consideration for private offices. If sound masking is only used in open areas, the lower ambient level within closed rooms exposes occupants to the disruptions caused by conversations and activities occurring outside their space. These interruptions force them to either endure the noise or close the door, which can be interpreted as anti-social.

Because sound masking works ‘at the ear of the listener,’ it is effective against noises or conversations regardless of how they find their way into the room and may, therefore, eliminate or reduce the need to address other acoustical pathways between spaces (e.g., sealing gaps between the walls and window mullions). This quality also makes sound masking a potentially effective tool against noises originating from outside the building. Whereas the shell of the building may not completely block the noise of traffic or passing aircraft, a masking system often easily covers these sounds or lessens their effect on occupants.

Acoustic Consistency

Achieving acoustic consistency throughout the facility is another reason that sound masking should not be limited to open plans. The system’s role is to control the acoustic conditions in the same way that one controls 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.

If sound masking is excluded from closed rooms, there are dramatic differences between the ambient levels in these spaces and those of the open plan. The overall background sound level can differ greatly, often by as much as 10–12 dBA. The spectrum also varies. These changes call occupants’ attention to the masking system as they enter and exit treated areas.

In other words, intentionally omitting masking from particular areas of the facility runs contrary to a key design goal: ensuring the masking sound is as unobtrusive as possible. Occupants of masked spaces are supposed to forget that it is present. If sound masking is installed in all occupied areas of a facility, a more uniform sound level is maintained that, by virtue of consistency, is not noticed by occupants.

Designing Masking for Closed Rooms

As noted earlier, not all system architectures can provide effective and comfortable masking sound in the fragmented and individual environments presented by private offices and other closed rooms. So, what is current best practice in these areas?

First, each room should be provided with its own loudspeaker. In open plans, loudspeakers are typically located according to a standard grid at 15-foot spacing. Including closed rooms in this pattern leads to reduced localized control because one loudspeaker may span more than one room.

Second, the loudspeaker should be allocated to its own control zone. Ideally, this means that it is fed by a dedicated masking sound generator and that it also has dedicated volume control and third-octave equalization. Having a number of loudspeakers connected to a shared set of controls inherently limits the system’s ability to meet the specified masking curve in each office. It also limits the ability to adjust the loudspeaker’s output (e.g., masking and paging volume) according to the occupant’s preference.

Third, each zone must have precise output adjustments for volume and equalization. Third-octave equalization over the specified range of the masking spectrum is necessary, which is typically from 100 to 5,000 Hz, or as high as 10,000 Hz. Precise volume control is also needed. Modern masking technologies provide fine steps (e.g., 0.5 dBA increments) for individual zones.

In terms of system commissioning in private offices, the masking spectrum should be identical to that used in open plans; however, the overall volume level will typically be several decibels lower. This provides a good degree of consistency between the open and private spaces, but addresses occupant’s expectation that the ambient volumes in smaller rooms are lower than in large, open venues. Overall masking volumes in private offices usually range from 40 to 45 dBA, whereas they max out at 48 dBA in open-plan workspaces.

This best practice system design ensures that masking levels are more consistent between offices with respect to volume and spectrum, leading to uniform performance and greater occupant comfort.

That said, regardless of the sound-masking system’s design, where its loudspeakers are located, or whether they face upward or downward, the sound broadcasted by the loudspeakers changes across the interior as it interacts with various elements, such as the layout and furnishings. In order to meet the specified sound-masking curve, the system must be tuned.

Tuning 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 of the facility or after hours. The exact method varies by product, but basically, 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. They repeat these steps until they meet the specified curve at each tuning location.

Though this process can be time consuming, it is essential to ensuring the expected benefits are equally enjoyed by all occupants across the facility.

Providing Occupant Control

Often, it is also practical to include an in-room control that permits occupants in private offices and meeting rooms to regulate the masking volume, as well as paging and background music. While such individual control is undesirable in shared open-plan areas, these rooms should afford a measure of personal control.

In-room control can be provided via hardware, such as a programmable keypad or rotary volume control, a software application, or integration with third-party equipment. However, when such controls are offered, there are additional functional considerations. For instance, the user should not be given unfettered control over masking volume settings. If the user is allowed to mute or lower the volume beyond a certain limit, speech privacy will suffer. Also, frequency control should not be included because the room occupant has neither the tools nor the training required to make informed adjustments to the masking spectrum.

If occupants are given control over the settings in rooms that are shared, such as meeting or conference rooms, then it may also be desirable to have those user adjustments reset automatically at certain times, restoring masking, and paging volumes to default levels.

Impact on Communication

The fact that masking levels in private offices and other closed rooms are typically lower than in open areas helps address the two remaining objections.

First, the masking level in a private office will not interfere with communication inside the room itself. The volume of a typical voice is 55 to 65 dBA at conversational levels. The distance between two people talking in a private office is not sufficient for the masking to interfere with intelligibility.

Second, if the loudspeakers in a conference room are assigned to their own control zone and the masking is tuned to approximately 42 dBA, it will provide a measure of acoustic control while not conflicting with meeting room occupants’ communication requirements or the signal-to-noise ratio required for good microphone response during video or teleconferencing.

If a meeting or training room is actually large enough to allow the masking sound to impact occupants’ ability to communicate (i.e., over long distances), an in-room control also allows users to adjust the volume to a low enough level that voice clarity is restored, but overall sound quality is maintained. Though such occurrences are rare, they are not out of the question.

Success Stories

In practice, organizations designing with ceiling-height walls and sound masking have realized both their speech privacy, noise control, and cost savings.

In one example, the University of Southern California was struggling with how to achieve privacy between medical exam rooms in a health-care consultation center. With an open plenum, it attempted a number of successive design interventions to improve speech privacy. The addition of plenum barriers—effectively extending the walls to the deck above—did little to address the problem. According to Curtis Williams, vice president of Capital Construction, it was the addition of masking that “greatly reduced the intelligibility of conversations between the exam rooms, allowing patients and doctors to talk with peace-of-mind knowing that their discussions could not be understood in adjacent rooms.”

A major American health-care provider also recently changed its construction standards for medical office buildings away from deck-to-deck construction for similar reasons. After significant testing of mockup facilities, the company determined that it achieved as good or better speech privacy with ceiling height walls and sound masking. They reported cost savings of hundreds of thousands of dollars for a project of just over 30,000 square feet.

Special Considerations

There are cases where one may want to implement both deck-to-deck construction and sound masking; for example, in spaces where raised voices or high volume media will be used, as well as in areas with high security needs (i.e., that require confidential speech privacy). Also, if the facility features an open ceiling, full-height walls are recommended to ensure some degree of inter-zone isolation.

Of course, eavesdropping can also be intentional and handled in a much more sophisticated manner than leaning one’s ear against a glass and putting it up to the wall. Without the proper treatment, windows, doors, ducts, pipes, floors, ceilings, and walls also present opportunities for electronic forms of eavesdropping. Speech causes vibrations on these structures, which can be picked up by probes or microphones and translated into intelligible speech. These types of listening devices are difficult to detect because they can be used at a considerable distance from the target facility.

If an organization suspects that it might be subject to such a threat, a sound-masking system can be connected to transducers, which transfer the masking sound to the aforementioned physical structures, impeding the use of audio surveillance equipment. In this case, it is key to ensure that the system produces a truly random masking sound (i.e., rather than on a loop) so that it cannot be filtered out of recordings.

Retrofit Installation

If an organization moves into its facility and finds that the initial design or construction has failed acoustically, implementing sound masking might not be the only improvement necessary, but the only feasible choice.

First, budget pricing for a sound-masking system is low, particularly relative to other acoustical treatment (typically $1 to $2 per square foot, depending on project conditions). A separate paging and background music system is not required because most masking systems provide these functions over the same set of loudspeakers. Contemporary systems require minimal space for below-ceiling equipment. The additional electrical load and cost of operation are also negligible. Owners can relocate the system to future facilities, extending its useful life for the organization.

Second, sound masking is typically less disruptive to apply to an already-occupied workplace than other treatments. For example, installing a suspended ceiling in an occupied space involves not only the cost of the tile, but modifications to the lighting, HVAC, sprinkler system, and more. Furthermore, certain acoustical interventions may not be possible to implement in some facilities due to the structural changes involved (e.g., historic properties) or the noise and operational disruption that accompanies their installation (e.g., hospitals).

By comparison, sound-masking components are small and the installation process tidy. They can typically be installed without conduit in most jurisdictions. The work can be handled discretely after hours or with only minor temporary local disruption to occupants during regular hours. It usually proceeds quickly as well, further minimizing the impact on an organization’s operations. There are few requirements for power tools, making the work relatively quiet, and unless the ceiling is unusually high, only ladders are needed to gain access.

Third, as outlined above, sound masking can not only be used to improve acoustics in open plans, but to increase privacy for closed spaces, such as private offices and meeting rooms. Unlike closing the ceiling or extending walls to the deck, masking has no impact on other building systems. It also continues to function when the room’s door is open and the acoustical isolation it provided virtually disappears.

There are some implications of retrofitting a sound-masking system rather than including it in the original design. The cost to install may be slightly higher than in new construction due to the increased labour requirements. More importantly, by waiting to install masking post-occupancy, an organization may forgo opportunities to reduce construction costs or the requirements for other acoustical treatments. As discussed, high-spec walls and plenum barriers can be replaced by a combination of floor-to-ceiling walls and sound masking, achieving the same or better privacy, while reducing the cost of initial construction and future changes.

In retrofit situations, it is also essential to select a masking system that offers a ramp-up feature. It slowly introduces the sound beginning at a level near the existing ambient volume, allowing occupants to gradually acclimatize to their new acoustical conditions. Over a short period of time, the sound becomes a natural part of their environment. In fact, if handled after hours, occupants are often unaware that the system has been installed. Full effectiveness is achieved once the masking sound has reached its final level.

In Conclusion

Building cost-effective and flexible closed spaces for true speech privacy can be challenging. However, combining physical barriers with sound masking can ensure effective results, while helping to control the cost of initial construction and future changes.

Indeed, the debate over whether sound masking should be included in closed rooms should be put to rest. In almost all situations, it is better to combine a reasonable amount of isolation with a raised ambient level, allowing companies to save on wall construction by reducing the STC ratings of walls and using floor-to-ceiling rather than deck-to-deck construction.

As long as the system is properly engineered for this type of environment, it is possible to provide the client with a suite of acoustic benefits that could not otherwise be achieved in private offices and other closed spaces, and also prevent the noticeable voids in the background sound level that are created when masking is only applied to open plans.

Niklas Moeller is vice president of KR Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network. He has more than 25 years of experience in the sound-masking field.

Today’s interiors are even more dependent on sound masking for speech privacy and noise control. The LogiSon Acoustic Network is tuned using TARGET, an application that accurately adjusts each small zone to the specified spectrum, maximizing benefits and occupant comfort. Worldwide distributors provide turnkey services and support. www.logison.com