Evolution of sound masking in closed rooms

April 27, 2018

Photo © iStockphoto/lovro77

by Niklas Moeller
Typing the word “privacy” into a search engine yields a lengthy stream of entries describing the many ways in which privacy can be violated, including reports of hackers acquiring credit card information, law enforcement agencies mining social networking sites, and voice-activated electronics with the ability to eavesdrop on their owners.

The preoccupation with vulnerabilities exposed by the Internet and electronic products is understandable given their rapid spread into almost all aspects of everyday life. However, privacy can also be violated in “traditional” ways, and even by those who do not intend to infringe upon it. For example, people can be exposed to sensitive information simply by being within audible range of a conversation—an issue relevant to building design and construction, particularly for facilities where medical, financial, legal, or other confidential matters are discussed.

Constructing barriers
When attempting to create speech privacy for closed offices, organizations may specify walls with high sound transmission class (STC) ratings. However, these ratings are lab-tested and frequently overstate real-world performance by five to 10 points. Site-tested apparent STC (ASTC)—which takes into account all leakage paths, as well as the wall’s performance—or noise isolation class (NIC) ratings are better gauges, but unfortunately only testable after the fact.

Another common tactic is to construct full-height walls extending from the concrete floor and all the way to the deck. While this approach can increase isolation, it can also raise costs and reduces flexibility. Vigilance must still be maintained during design, construction, maintenance, and renovation to ensure penetrations in the walls’ structure are controlled, because even minor ones can substantially reduce acoustic performance. This level of care can be difficult to sustain over the life of the space.

In any case, modern design and construction standards do not always allow for a high level of physical containment. To preserve flexibility, walls are often built to the suspended ceiling or using demountable partitions. Walls may include substantial windows or even be built in glass from floor to ceiling. Budget can also limit options.

These challenges raise the question as to whether there are preferable and more reliable methods of achieving speech privacy for closed rooms, such as by incorporating sound masking technology into the design.

Speech privacy often proves elusive because of the many ways in which sound can transfer from one space to another. If the background sound level is lower than the sounds passing through a wall or via flanking paths, conversations can be overheard.
Photo © iStockphoto/MarcusPhoto1

Overcoming preconceptions
Over the last decade, there have been great advancements in the sound masking field, increasing performance, expanding functionality, and opening the door to new applications. Nevertheless, outdated design practices persist, often to the detriment of both speech privacy and acoustic comfort. One such custom is the exclusion of sound masking from closed 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 acoustic challenges encountered in an ever-growing number of open plans. This initial application led some to conclude masking was only intended for these types of areas.

This opinion was reinforced by a significant technical impediment. Early sound masking systems typically 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 enclosed rooms, with little opportunity to adjust the level within each space and no control over frequency. The resulting inconsistencies in the masking performance led vendors and dissatisfied customers to conclude the technology could not be applied in closed spaces.

Modern networked masking architecture addresses these historical objections by providing fine control over both level and frequency within small zones (i.e. one zone per closed office, and adjustment zones no larger than three loudspeakers, or 62.7 m2 [675 sf], within open plan), but some still argue closed rooms do not require sound masking because they are afforded sufficient speech privacy and noise control via physical isolation.

Any penetrations in the wall’s armor (e.g. outlets) can impact sound attenuation. An open door virtually eliminates the wall’s sound-attenuating effect.
Photo © iStockphoto/powerofforever

Cracks in the armor
By the same token, when a closed room fails to provide these attributes for its occupants, the failure is typically blamed on deficiencies in its design, construction, and/or maintenance.

While they might be a contributing factor, this failure cannot solely be attributed to cracks in the walls’ armor, because speech privacy is not determined by isolation alone. A person’s ability to clearly understand a conversation is dependent on two variables: the level of the speaker’s voice and the background sound level in the listener’s location. 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. If a solution has not been implemented to control the minimum background sound level in adjoining areas and it is lower than the sounds passing through the wall or via various flanking paths, conversations and noises will still be heard and potentially be intelligible. (Of course, a room’s Achilles’ heel is the door. When open, the barrier provided by the wall is compromised. For example, an STC 40 wall with an open door representing 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 0.91-m [3-ft] door in a 3 x 3-m [10 x 10-ft] wall—then the effective STC drops to around seven.)

Regardless, unless a sound masking system is implemented—as well as professionally tuned, and verified for performance post-installation—the minimum background sound level is not a known quantity. HVAC and other mechanical systems are sometimes thought to provide masking, but one cannot reasonably expect this type of equipment to deliver a consistent level over time/space or to even generate a spectrum conducive to speech privacy.

Accordingly, ASTM E1374, Standard Guide for Open Office Acoustics and Applicable ASTM Standards, was recently revised. The discussion of HVAC noise in the newly released ASTM E1374-18, Standard Guide for Office Acoustics and Applicable ASTM Standards, pertains only to limiting maximum noise levels rather than using this equipment for masking. Further, a sound masking system is identified as the only viable source of a continuous minimum background sound level. As the title change suggests, this standard’s scope has also been broadened; it now applies to private offices and conference rooms, not only to open plan.

Though masking technology distributes a sound often compared to softly blowing air, unlike HVAC noise, it is continuous and precisely controllable. Using it—even to apply a level as low as the 30 dBA on which STC ratings and, hence, wall choices are based—allows the expected degree of speech privacy to be more reliably achieved. After being professionally tuned post-installation, the masking system’s measured output meets a particular spectrum or “curve” specifically engineered to balance acoustic control and comfort.

Building professionals can use this predictable background sound level as the foundation for the remainder of their acoustical plan, allowing more accurate specification of the blocking and absorptive elements—and providing a means of reducing the specifications for the room’s physical shell, while still achieving the desired level of speech privacy. (For more information about using sound masking as the starting point for interior planning, see this author’s “A New Approach to Acoustics: Using sound masking as a design platform” in the January 2018 issue of The Construction Specifier.)

Applying sound masking technology throughout the facility allows organizations to more accurately specify the blocking and absorptive elements employed in their design and also prevents noticeable changes in the ambient level between the open and closed areas.
Photo © BigStockPhoto.com

Calculating the benefits
For example, sound masking can be used in combination with walls (or demountable partitions) built to a suitably ceiling attenuation class- (CAC) rated suspended ceiling to provide a cost-effective and more flexible alternative to deck-to-deck construction. Budget wise, sound masking may represent one to two dollars 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 can also increase the ease and cost-effectiveness of relocating them to suit future needs. However, 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 sound masking on intelligibility. This exercise can be done using the Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index (ASTM E1130-16) for calculating articulation index (AI), which is a metric of speech intelligibility and takes both factors into account. While this ASTM standard references open plan spaces, it is generally agreed this method can also be applied to closed spaces, with slight modification to the test equipment.

Calculation of articulation index is based on several measurements taken in the space in question, as well as a standardized normal voice level. Onsite testing determines the amount by which voice level reduces between the source room and the listener location. The difference between the voice level and the background in each of the third-octave frequency bands (200 to 5000 Hz) provides the signal-to-noise ratio in the listener location. The AI method assigns a specific weighting formula to determine an AI contribution within each frequency band, and 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 masking level, allowing comparison of the two strategies. Obviously, as wall attenuation increases, for each decibel reduction 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 for determining 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 level is raised by one decibel.

Masking typically adds approximately five to 12 dBA of ambient sound to closed rooms, which is why one sometimes hears the general statement it “adds 10 STC points” to walls.

The relationship between articulation index (AI) and intelligibility is not linear. For example, a value of 0.5 means a listener can clearly understand approximately 95 percent of a conversation, not 50 percent. A very low AI value is required for true privacy.
Image courtesy KR Moeller Associates Ltd.

Designing, tuning, and reporting
This type of integrated acoustic design is only viable when the minimum background level is consistently delivered by the sound masking system. Therefore, it is incumbent on those responsible for acoustic planning to ensure it is properly designed and implemented.

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 4.5-m (15-ft) spacing. Including closed rooms in this pattern reduces 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 it is fed by a dedicated masking sound generator and 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 closed room.

Third, each control zone must have precise output adjustments for level and equalization. Third-octave equalization over the specified range of the masking spectrum is necessary, which is typically from 100 to 5000 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 tuning, the masking spectrum in closed rooms should be identical to the one used in open plans. However, the overall volume level is typically several decibels lower. This provides a good degree of consistency between the open and private spaces, but addresses occupant’s expectation the ambient levels in smaller rooms are lower than in large open venues. Overall masking levels in private offices usually range from 42 to 45 dBA, but as stated above, benefits can be realized even at lower volumes.

A well-designed and professionally-tuned system is able to keep variations in masking level to ±0.5 dBA and those in frequency to ±2 dB per third octave, providing dependable coverage throughout treated areas.

Installation of sound masking manages the background sound level and increases speech privacy while maintaining the flexibility of floor-to-ceiling partitions.
Image courtesy Screen Solutions UK

The newly released ASTM E1573-18, The Measurement and Reporting of Masking Sound Levels Using A-Weighted and One-Third-Octave-Band-Sound Pressure Levels, will predominantly be used by acousticians tasked with verifying the performance of an installed and calibrated sound masking system against a specification outlining such target levels and tolerances. However, sound masking vendors should also follow this standard to ensure they provide good data for the verification process. As with ASTM E1374-18, this standard’s title has also been updated to reflect the fact sound masking is used in closed rooms as well as open plans.

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 reliable ambient level, allowing organizations to save on wall construction by reducing the STC ratings of walls and/or 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 closed spaces, and also prevent the noticeable voids in the background sound level, which are created when masking is only applied to open plans.

Offering keypad control allows users to adjust the masking level in private offices and meeting rooms, if desired. The masking system can also be programmed to prevent users from setting it below the minimum on which design decisions were made.
Photo © Zahid Ghafoor

Providing occupant control
Often, it is practical to include an in-room control permitting occupants in private offices and meeting rooms to regulate the masking level, as well as paging and background music. While such individual control is undesirable in shared, open plan areas, closed rooms should afford a measure of personal control. (It is important to note the masking level in a private office does not interfere with communication inside the room itself. The level 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.)

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 the masking level. If an occupant is allowed to mute or lower the volume beyond a certain limit, others’ speech privacy suffers. Also, frequency control should not be included because the user has neither the tools nor the training required to make informed adjustments to the masking spectrum.

If occupants are given control in closed 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 levels to default settings. (If the loudspeakers in a conference room are assigned to their own control zone and the masking is tuned to approximately 42 dBA, it provides a measure of acoustic control while not conflicting with 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], installing an in-room control allows users to adjust the level to a low enough volume so that voice clarity is restored and overall sound quality is maintained.)

Success stories
In practice, organizations designing with ceiling-height walls and sound masking have realized both speech privacy and cost savings goals.

For example, the University of Southern California was struggling with how to achieve privacy between medical exam rooms within a healthcare consultation center. With an open plenum, they 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.”

Unlike sound masking, HVAC equipment neither has any means of controlling the minimum background sound level nor does it generate a spectrum designed for speech privacy.
Photo © iStockphoto/Alexjey

After testing mock-up facilities with ceiling-height walls and sound masking, a major American healthcare provider also changed its construction standards for medical office buildings. They reported cost savings of hundreds of thousands of dollars for a project of just over 2787 m2 (30,000 sf), while achieving as good or better speech privacy as deck-to-deck construction. (There are some cases where one might want to implement both deck-to-deck construction and sound masking. For example, spaces where raised voices or high-volume media will be used, as well as areas with high security needs. Also, if the facility features an open ceiling, full height walls are recommended to ensure some degree of inter-zone isolation.)

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. Speech privacy levels fluctuate from wall assembly to wall assembly, depending on their performance in the frequencies used to calculate STC, as well as the inconsistent noise levels and spectrum generated by HVAC—not to mention sound leakages through various flanking paths such as gaps along the window mullions, ceiling and floors, as well as through the plenum, ductwork, return air grills and, of course, open doors. Each crack in a wall’s armor facilitates the transmission of sound to and from neighboring spaces. Ultimately, the lack of sufficient background sound allows conversations to be overheard.

Combining physical barriers with sound masking can ensure effective results while helping to control the cost of initial construction and future changes. In most situations, sound masking provides not only cost and flexibility advantages, but also as good or better speech privacy as deck-to-deck construction. Further, with sound masking, one has the opportunity to increase the background sound level in the event a partition does not perform to spec and remedial action would be cumbersome and/or costly—a flexibility uniquely afforded by this technology.

Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network (logison.com) and MODIO Guestroom Acoustic Control (modio.audio). He has more than 25 years of experience in the sound masking field. Moeller can be reached at nmoeller@logison.com[9].

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