July 13, 2017
By Niklas Moeller
Background sound is critical to the levels of speech privacy, noise control, and overall comfort occupants experience within a facility. Indeed, all acoustic design considers this factor when determining sound transmission class (STC) or speech privacy class (SPC), as well as when calculating articulation index (AI) or the signal-to-noise ratio (SNR).
In other words, background sound is always part of the acoustic equation. However, the only accurate means of controlling it—a sound masking system—has often been relegated to the last step in the design process. Unfortunately, this technology is often used solely to cover up remaining noises after implementing absorption and blocking strategies, or as a post-construction Band-Aid when occupants discover their speech privacy levels are not what they expected.
While sound masking is increasingly at the forefront of open-plan design, the advantages of employing it in closed rooms remain largely overlooked, despite the fact STC ratings—and, hence, wall choices—are based on an assumed level of 30 A-weighted decibels (dBA). (The human ear does not perceive all frequencies equally. For example, a 125-Hz tone sounds 16 dB lower to one’s ear than a 1000-Hz tone of the same volume. Therefore, a weighting method is used to modify dB levels so they more closely approximate how people hear them. Expressed as dBA, this A-weighting method de-emphasizes higher and particularly lower frequencies.) Rather than using controlled minimum background sound as a design tool, many design professionals continue to plan for the worst-case scenario, which leads to over-specification of physical construction, increased costs, reduced flexibility, and unpredictable results.
The variability of HVAC
Even in closed rooms, speech privacy depends on the background sound at the listener’s position being higher than the residual voice level penetrating the wall. This point is highlighted in ASTM E2638, Standard Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room, which defines speech privacy class as “an objective rating of the speech privacy provided by a closed room, calculated as a sum of factors related to sound isolation provided by the room, and the background noise at the receiving point.” (Many acousticians prefer to use ASTM E1130, Test Method for Objective Measurement of Speech Privacy in Open-plan Spaces Using Articulation Index, for both open and closed spaces. When calculating the level of speech privacy, E1130 also considers the combined effects of transmission loss due to the partition assembly and reduced signal-to-noise ratio [SNR] due to the background sound level.)
However, ASTM E2638 also frequently reminds readers SPC is only valid at the time it is measured, because the background noise is presumed to be derived from the HVAC system and is, therefore, highly variable. Even when well-designed, this equipment’s output is only governed insofar as it is not to exceed certain maximums defined by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) in the 2013 ASHRAE Handbook−Fundamentals. It has no means of controlling the minimum.
Levels can vary by zone and at certain times of day or year by 15 dBA or more. In some cases, different heating and cooling strategies are employed in the exterior and core, resulting in even more variable acoustic conditions across the space. If chilled beam systems are used, the overall level will be dramatically lower than traditional HVAC. Whenever and wherever the background sound falls below the 30 dBA on which STC ratings are based, occupants can no longer rely on the partition assembly for speech privacy.
Further, HVAC systems do not generate a sound spectrum conducive to speech privacy. Instead, it is largely arbitrary and varies considerably from space to space, as well as over time. Similarly, despite having the same STC ratings, walls isolate different frequencies according to design and assembly.
In consequence, 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 level and spectrum the HVAC system generates—not to mention sound leakages through various flanking paths. If privacy is achieved, it is largely due to either good luck or overbuilding. If not, a sound masking vendor is typically contacted.
Alternatively, when used as the starting point for interior planning, sound masking lets building professionals set the base level of background sound throughout the facility and, hence, more accurately specify the blocking and absorptive elements used in their design. In other words, this controlled minimum level becomes the canvas on which the rest of the acoustic plan is painted, allowing it to be delivered in a more cost-effective manner and with greater assurance of achieving the intended results.
A sound masking system—often mistakenly referred to by the term ‘white noise’—consists of a series of electronic components and loudspeakers integrated in a grid-like pattern above the ceiling, as well as a method of controlling their zoning and output. The loudspeakers distribute a sound similar to softly blowing air, causing many occupants to presume HVAC is its source. However, unlike HVAC, this sound is continuous and professionally tuned to meet a particular spectrum—or ‘curve’—engineered to balance acoustic control and occupant comfort.
Considering sound masking has been available since the late 1960s, one might wonder why the building community has yet to embrace it as the foundation for interior planning. To understand this delay, one has to consider the technology’s history.
Sound masking was first adopted to help with the obvious acoustic challenges encountered in an ever-growing number of open-plan spaces. This initial application led some to conclude it was only intended for these areas—an opinion reinforced by a significant technical impediment. Early systems used a centralized architecture, which is very limited in terms of its ability to offer local control over the masking sound. Zones containing large numbers of loudspeakers spanned numerous private offices and other closed rooms, with little opportunity to adjust the volume within each space (i.e. simply via 3-dBA transformer taps), and none for frequency. The resulting inconsistencies in volume and spectrum impacted the sound’s performance and occupant comfort, leading both vendors and dissatisfied users to conclude it could not be applied to closed spaces.
Advances made in sound masking technology—particularly decentralization of sound generation, volume, and frequency control, as well as the introduction of computer auto-tuning—mean minimum background sound level is now a readily deliverable component of architectural acoustic design.
ASTM E2638 expresses SPC as the sum of LD(avg) + Lb(avg), where LD is average reduction in source level at the listening position (i.e. transmission loss) and Lb is the average background sound level at the listening position. While preparing “Sound & Vibration 2.0: Design Guidelines for Health Care Facilities”—the companion document to the Facility Guidelines Institute’s (FGI’s) 2014 Guidelines for Design and Construction of Hospitals and Outpatient Facilities—acousticians simplified this formula, declaring to “achieve confidential speech privacy the sum of the composite STC and the A-weighted background noise level shall be at least 75,” or STCc + dBA ≥ 75. The composite STC (STCc) metric includes the negative impact on acoustic performance when elements such as doors and windows are added to the partition.
Some refer to this revised method as speech privacy potential (SPP) and, indeed, it provides a basic predictive model for achieving speech privacy in closed rooms. As dBA is assumed to be 30, STCc must be at least 45 to achieve the combined total of 75. Using sound masking to apply a continuous level of 30 dBA eliminates the variability of the source, and speech privacy is more reliably achieved with the stated STCc. The curve generated by a well-designed and professionally tuned masking system is also precise; therefore, the speech privacy it provides is greater than the typically erratic spectrum produced by HVAC equipment, even at the same volume.
However, in this scenario, it is important to note the sound is set to a level far below that used in traditional masking applications. Leaving a range of adjustment ‘on the table’ provides two additional advantages—cost savings and flexibility.
If speech privacy = STCc + 30 dBA ≥ 75, then for every 1-dBA increase in the background sound level, it is possible to reduce STCc by one point and achieve the equivalent level of speech privacy. Were the background sound to be increased from 30 to 35 dBA, for instance, construction costs for partition types would start to drop significantly because the STCc can be reduced by five points.
Again, 30 dBA—and, indeed, even 35 dBA—is well below typical masking levels in closed rooms. Usually, they are set to between 40 and 43 dBA in these spaces. Depending on various factors, including occupant comfort, they may be set higher. Therefore, while 30 dBA can be used as a design benchmark, the lowest STCc rating possible to achieve an SPP of 75 is actually determined by the highest comfortable level of continuous minimum background sound.
While the established maximum levels for HVAC can form the basis for the controlled minimum background sound provided by the sound masking system, there are significant opportunities for further value engineering because the predictable overall volume and spectrum allows one to reduce the specifications for the room’s physical shell.
With a suitable design of sound masking, walls, and ceilings, it is also possible to achieve privacy with walls built to the suspended ceiling rather than to the structure, affording additional cost savings and flexibility. (For more information about this topic, see this author’s “Mind the Gap: Using Sound Masking in Closed Spaces” in the October 2012 issue of Construction Canada.)
When sound masking is incorporated into the facility design, one also has the opportunity to increase the background sound level if the partition construction fails to live up to its rated level—for example, as a consequence of common deficiencies such as flanking paths—and remedial action would be cumbersome or costly. While the minimum planned level should be at least 30 dBA, as noted, the level traditionally recommended in most closed rooms is 40 to 43 dBA, leaving a range of adjustment at the facility manager’s disposal.
Of course, this rationale can also be applied to existing spaces not performing as expected. However, by waiting to install masking post-occupancy, an organization forgoes the opportunities to reduce construction costs and the specifications for other acoustic treatments.
System design and tuning
It is important to note this type of integrated acoustic design is only viable when the minimum background level is precisely generated and consistently delivered by the sound masking system.
ASTM E1111, Standard Test Method for Measuring the Interzone Attenuation of Open Office Components, acknowledges variations as small as 2 dBA can significantly influence speech privacy, while other studies indicate even a single dBA affects comprehension by up to 10 per cent and, in almost every situation, impacts articulation index by 0.0333. (See this author’s “Exploring the Impacts of Consistency in Sound Masking” in Canadian Acoustics, 42, 2014.)
Variations in spectral quality can have similarly negative effects. Therefore, it is incumbent on those responsible for acoustic planning to ensure the sound masking system is designed and implemented with due consideration for these stringent requirements. A poorly designed or improperly tuned system can allow as much as 4- to 6-dBA variation, meaning the system’s effectiveness is halved in unpredictable areas within the facility.
For example, having a large zone (i.e. more than three loudspeakers or 58 m2 [625 sf]) connected to one set of controls limits one’s ability to adjust the system’s output to meet the specified curve in each closed room and ensure the same masking spectrum is applied throughout the facility. If large zones span multiple private offices and/or meeting rooms, it also prevents one from adjusting the level to suit occupant preferences or needs.
To maximize control over the sound, each closed room should be provided with its own loudspeaker(s) allocated to its own control zone. Each zone should offer precise output adjustments for both volume (i.e. 0.5-dBA increments) and equalization (i.e. third-octave over the specified masking spectrum, which is typically from 100 to 5000 Hz or higher). Lastly, while occupants can be given control over the masking volume within closed rooms—for example, using a programmable keypad—the system should prevent them from setting it lower than the minimum level established for speech privacy within the facility.
Following installation, the vendor should tune all treated areas at ear height (i.e. where occupants experience the masking effects) and provide a detailed report of the results. Although outdated specifications still in circulation might allow for a wide tolerance (e.g. up to 4 dBA), a well-designed and professionally tuned system is able to keep variations in volume to ±0.5 dBA and those in frequency to ±2 dB per third octave, providing dependable coverage throughout an installation.
While acoustic professionals have always advocated the ABC Rule of absorbing, blocking, and covering unwanted noise, listing ‘C’ last reinforces the notion it is a final consideration and perpetuates the misplaced emphasis on isolation and absorption strategies when designing for speech privacy. Instead, the approach should be CAB: cover, absorb, block. By using sound masking to define and, therefore, know exactly what the background sound level will be anywhere in a facility, one can more accurately specify the remaining materials. Further, the volume can be increased at a later date if more acoustic control is needed or desired—a flexibility uniquely afforded by this technology.
Building professionals should not hesitate to take advantage of this value-engineering opportunity by employing a judicious balance of controlled minimum background sound and isolation in all facilities where speech privacy and noise control are priorities.
|THE ROLE OF BACKGROUND SOUND|
Many people use the words ‘noise’ and ‘sound’ interchangeably. However, not all sound is noise. Rather, one can define ‘noise’ as any unwanted sound. Similarly, ‘silent’ and ‘quiet’ have different meanings. A silent space is one with no sound at all, whereas a quiet one has no unwanted sound.
Understanding these seemingly subtle differences is critical to comprehending the role sound itself plays in creating an effective acoustic environment. All too often, noise control strategies are mistakenly pursued with the intention of making a facility as silent as possible. However, the more silent one tries to make a space, the noisier it can seem to occupants. This phenomenon can be attributed to the fact an effective acoustic environment partially relies on an appropriate level of continuous background sound.
Due to improvements in construction materials, as well as quieter office and mechanical equipment, ambient levels in most facilities are already too low, leaving employees in library-like environments. These pin-drop conditions allow them to easily hear conversations occurring from a distance and even from within closed rooms. Though occupants typically describe such a workplace as ‘noisy,’ the root of the problem is they are, in fact, too silent. Put another way, the absence of sound makes noises easier to hear.
A sound masking system is the only acoustic treatment that can accurately control the background sound level within a facility. This technology basically consists of a series of loudspeakers installed above the ceiling or within an open ceiling, which distribute a sound most people compare to softly blowing air. The premise behind this solution is simple—any noises and conversations below the new background sound level are covered up, while the disruptive impact of those above it is lessened due to the reduction in the degree of change between the baseline and these volume spikes. Consequently, occupants perceive treated spaces as quieter.
There are many everyday examples of this effect, including running water, rustling leaves, or the murmur inside a busy restaurant. However, when introducing a sound to a workplace, it is vital to ensure it is also as unobtrusive as possible. While ‘white’ and ‘pink’ noise were utilized by early masking systems,* modern technologies are designed to produce a spectrum specifically engineered to balance acoustic control and occupant comfort. As no masking system can produce this spectrum ‘out of the box,’ post-installation tuning of the sound is an essential part of the commissioning process within each facility. If this step is skipped, the equipment is unlikely to provide the desired effects throughout the space.
* For more on the ‘colours’ of sound, see this author’s Construction Canada article at www.constructioncanada.net/if-you-need-sound-masking-ask-for-it-by-name.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network sound masking system (logison.com). He has more than 25 years of experience in the sound masking field. Moeller also writes an acoustics blog at soundmaskingblog.com. Moeller can be reached via e-mail at firstname.lastname@example.org.
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