January 8, 2018
by Niklas Moeller
With mounting recognition of the need to support focused work and promote wellness, many organizations are looking to provide building occupants with improved speech privacy, noise control, and acoustic comfort.
Background sound is key to achieving these goals. Indeed, all acoustic designs consider this factor when determining sound transmission class (STC), articulation index, or signal-to-noise ratio. However, building professionals often neglect to use the only accurate means of controlling the minimum background level—a sound masking system—as a design tool.
By turning the traditional three-tiered approach of absorb, block, and cover—collectively known as the ‘ABC Rule’—on its head and using sound masking as the starting point for interior planning, building professionals can set the base level of background sound throughout a facility. They can then more accurately specify the blocking and absorptive elements used in their design, thereby allowing it to be delivered in a more cost-effective manner, and with greater assurance of achieving the intended results.
The variability of HVAC
Both ASTM E1130, Test Method for Objective Measurement of Speech Privacy in Open-plan Spaces Using Articulation Index, and ASTM E2638, Standard Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room, consider background sound when calculating speech privacy. (ASTM E2638 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.” Similarly, when calculating speech privacy, ASTM E1130 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. Many acousticians use E1130 for both open and closed spaces.)
However, ASTM E2638 also reminds readers speech privacy class is only valid at the time it is measured because the background level is presumed to be provided by HVAC. Even if well-designed, this equipment’s output is only governed in that it is not to exceed maximums defined by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) in the 2013 ASHRAE Handbook—Fundamentals. It cannot control the minimum background sound level.
HVAC output often varies by 15 dBA or more, according to zone, time of day, and season as well as the type of equipment used. Whenever and wherever the background level falls below the 30 dBA on which STC ratings—and, hence, wall choices—are based, occupants can no longer rely on the partition assembly for speech privacy. Further, HVAC does not generate a spectrum conducive to speech privacy.
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 generated by HVAC—not to mention sound leakages through various flanking paths. If privacy is achieved, it is largely due to good luck or overbuilding. If not, a sound masking vendor is contacted. In this scenario, the technology is consigned to band-aid status.
A sound masking system uses a series of electronic components and loudspeakers to distribute a sound most people compare to softly blowing air. However, unlike HVAC, the sound is not only continuous, but also precisely controllable.
Though this technology is often referred by the term ‘white noise,’ modern systems do not utilize a particular color of sound. (Most people are not thinking of the technical implications when they say ‘white noise.’ However, this particular spectrum of sound has not been used by masking systems since the 1970s. White noise is a random ‘broadband’ sound—meaning it includes a wide range of frequencies—typically spanning the audible range of 20 to 20,000 hertz [Hz]. While it is an effective masker, it is also irritating. Most people describe it as ‘static’ due to its uncomfortable, hissing quality.) Rather, they are engineered so their output can be tuned post-occupancy in order to provide a spectrum or ‘curve’ specifically designed to balance acoustic control and comfort.
The advent of localized computer tuning—whereby software adjusts the system’s output in order to meet the masking curve throughout all treated areas—means a minimum background sound level is now a readily deliverable component of architectural acoustic design. Building professionals can use this predictable, controlled level as the foundation for the remainder of their acoustical plan.
Formula for success
When 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 developed a formula providing a predictive model for this approach. Basically, 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. Some refer to this formula as speech privacy potential (SPP). (It is important to note this formula is based on the composite STC [STCc] of the partition assembly rather than the wall rating alone—a distinction often lost on those untrained in acoustic design. STCc includes the negative impact on acoustic performance when elements such as doors and windows are added to the partition. For example, an STC-50 partition can degrade by as much as 23 points when un-gasketed door covering 20 percent of the wall is installed.)
As dBA is assumed to be 30, STCc must be at least 45 to achieve the combined total of 75. Using 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 professionally tuned masking system is also precise. Therefore, the speech privacy it provides is greater than the typically erratic spectrum produced by HVAC, even at the same volume.
While the need for speech privacy is obvious to organizations consistently dealing with sensitive information—such as hospitals and law offices—most people expect conversations occurring within closed rooms to remain private, making SPP broadly applicable. Even if an organization decides it is more motivated by the need for a high-performance workplace than speech privacy, taking the steps required to lower speech intelligibility allows them to reap both rewards. (For the same reasons, masking should also be applied within the open plan, where it is typically set to 46 to 48 dBA.)
In the above scenario, the masking sound is set to a level far below the one used in traditional applications. Despite being barely audible, it still provides the minimum necessary to accurately plan the remaining design elements. However, 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.
If speech privacy equals 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 dBA to 35 dBA, for instance, construction costs for partition types would start to drop significantly because the STCc could 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 between 40 and 43 dBA in such spaces. Depending on various factors, including occupant comfort, they may be set higher. Therefore, although 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. (The recommended maximum masking levels are still so low as to not to affect face-to-face communication or hearing. Since the decibel scale is logarithmic, the difference between 48 dBA [the typical maximum for open plan] and 80 dBA [the point at which hearing concerns begin, with prolonged exposure] is dramatic. Expressed in terms of distance, if 80 dBA is equivalent to 1.6 km [1 mi], 48 dBA is only 1 m [39 in.]. Also, conversation generally falls into the range of 50 to 60 dBA. In other words, sound masking is also set to a far lower level than a lot of sounds one regularly encounters in everyday life.)
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. In one example, a major U.S. healthcare provider changed its construction standards for medical office buildings away from deck-to-deck construction. After significant testing of mockup facilities, the company determined they 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 2787 m2 (30,000 sf).
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. Once constructed, the acoustical properties of walls and ceilings cannot be easily changed, and when engineered and installed, neither can the sound masking system’s architecture.
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 percent 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.
To maximize control over the sound, each closed room should be provided with its own loudspeaker(s) allocated to its own control zone, and each zone within open plan should not exceed three loudspeakers or 63 m2 (675 sf). They 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). Following installation, the vendor should also tune each zone 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 CBA: cover, block, and absorb.
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 to compensate for deficiencies in partition assemblies—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 and MODIO Guestroom Acoustic Control. He has more than 25 years of experience in the sound masking field. Moeller can be reached via e-mail at firstname.lastname@example.org.
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