In pursuit of acoustical equity

Figure 3: A simplified model showing how sound propagates as it moves away from a source, across—or, rather, throughout—a space. Each notch along the horizontal axis represents 0.3 m (1 ft), with ‘0’ marking the origin of the noise.
Figure 3: A simplified model showing how sound propagates as it moves away from a source, across—or, rather, throughout—a space. Each notch along the horizontal axis represents 0.3 m (1 ft), with ‘0’ marking the origin of the noise.

The need to offer a supportive environment highlights the importance of providing beneficial acoustical conditions throughout the workplace. While occupants can be impacted by acoustical design in myriad ways, it is important to continue with the example of speech privacy. Some might consider it a niche application only relevant to particular offices (e.g. law firms), healthcare and military environments, but surveys such as those conducted by the Center for the Built Environment (CBE) show lack of speech privacy is the top workplace complaint, indicating it is a broadly applicable concern (see K.L. Jensen’s ‘Acoustical quality in office workstations, as assessed by occupant surveys,’ presented at Indoor Air 2005, as well as D. Artan, E. Ergen and I. Tekce’s ‘Acoustical Comfort in Office Buildings’ from the proceedings of the 7th Annual International Conference – ACE 2019 Architecture and Civil Engineering). Further, this deficiency is not only relevant to the occupants of private offices, but to those working within open plans. Although individuals within the latter group are more likely to characterize lowering speech intelligibility as “reducing distractions” rather than “improving speech privacy,” taking measures to achieve this goal means they will have an easier time concentrating on tasks, make fewer errors, and also suffer less stress and fatigue (for more on the subject of acoustical privacy, read Niklas Moeller’s ‘Corporate Confidential: Understanding acoustic privacy within the built environment‘ in the June 2015 issue of Construction Canada).

The need for control

Equity involves ensuring the design provides beneficial acoustical conditions throughout the workplace to allow all occupants to function at the highest possible level, in accordance with the goals the space(s) is/are designed to meet and help fulfill. While acoustical privacy is not the only objective, it is a highly sought-after quality with widespread relevance that can serve as the foundation for an acoustical plan within many types of spaces. Any deviations from (e.g. to improve intelligibility in a large training room) or additions to (e.g. biophilic sounds or music in particular spaces) the acoustical conditions required to achieve it must be intentional (i.e. designed to meet a particular goal or occupant need), not unintentional. Essentially, there is a need for control of the acoustic environment and, specifically, background sound.

Although categorization and acceptable-level schemes endeavor to minimize occupants’ negative reaction to the sound experienced within a space, they do not control the actual levels emitted by various noise sources (e.g. building systems), nor do they actively address the background—or ambient—sound that actually exists in the space, which experts maintain is “probably the most important room variable affecting speech privacy,” (read J. Keranen and V. Hongisto’s paper ‘Prediction of the spatial decay of speech in open-plan offices,’ Applied Acoustics, vol. 74, 2013. Also, consult W.J. Cavanaugh, W.R. Farrell, P.W. Hirtle, and B.G. Watters’ ‘Speech privacy in buildings,’ in The Journal of the Acoustical Society of America, vol. 34, no. 4).

Figure 4: A simplified model showing how masking reduces the distance over which the noise shown in Figure 3 can be heard. The effect is noticeable in terms of where the propagating signal reaches and falls below the level of masking sound (gray shaded area).
Figure 4: A simplified model showing how masking reduces the distance over which the noise shown in Figure 3 can be heard. The effect is noticeable in terms of where the propagating signal reaches and falls below the level of masking sound (gray shaded area).

If one only implements maximum thresholds, one leaves this key variable up to ‘whatever is left’ or ‘whatever happens.’ Since the ability to discern the intrusion of speech depends on the level and spectrum of background sound “which actually exists (not the background noise criterion) in the listening space,” setting minimum—not maximum—levels for background sound is critical to attaining speech privacy (read J. Keranen and V. Hongisto’s paper ‘Prediction of the spatial decay of speech in open-plan offices,’ Applied Acoustics, vol. 74, 2013. Also, consult W.J. Cavanaugh, W.R. Farrell, P.W. Hirtle, and B.G. Watters’ ‘Speech privacy in buildings,’ in The Journal of the Acoustical Society of America, vol. 34, no. 4). While maximum limits mitigate the impact of ‘unwanted sound’ from noise sources (e.g. building systems), minimum levels call for ‘wanted sound’ from dependable sources. These two criteria are exclusive of each other, because wanted sound is needed to mask that which is unwanted.

A minimum background sound level can only be reliably achieved through the application of the ‘C’ in the ‘ABC Rule.’ While ‘A’ stands for ‘absorb’ and ‘B’ for ‘block,’ ‘C’ stands for ‘cover’—or, more accurately, ‘control’—which requires the
use of a sound-masking system. While ‘C’ is the final letter in the rule, it is only because the abbreviation is meant to be memorable and is, therefore, in alphabetic sequence. It is not intended to assign priority level to the acoustical strategies involved or indicate the extent of the role each plays in the outcome. Rather, the rule reinforces the fact a holistic approach is required for the best results.

It is important to note, the interrelationship—and interdependency—of the acoustical features of a built environment is not a wholly occupant-centric consideration. Taking a holistic approach to the execution of an acoustical plan also allows one to gain ‘system-level’ efficiencies that help manage construction-related costs (e.g. lowers STC requirements, permits walls to be built to the ceiling instead of up to the deck), allow for more effective and efficient operation of building-related systems, and avoid post-completion noise mitigation efforts (breaking out of our entrenched ways requires a coordinated effort, not only of building professionals, but of the tools available at their disposal. There is growing realization that improvement at a ‘component level’ is reaching practical limits, promoting new interest in gaining ‘system-level’ efficiencies through a more holistic approach to acoustical design. To learn more about the project savings engendered by a holistic approach, read Niklas Moeller’s ‘A New Approach to Acoustics: Using sound masking as a design platform’ in the January 2018 issue of The Construction Specifier. Additionally, although the evaluation of all the contributing sound sources is complex, if engineers are able to align their specifications with acoustical expectations of the built environment, one can argue it is even possible to avoid circumstances where overly stringent noise criteria force building systems to comply to unnecessarily low criteria).

Looking beyond level

The role ‘C’ plays in providing beneficial acoustical conditions becomes even clearer when one considers there is more to human experience of sound within the built environment than overall level—or, more colloquially, “volume”—particularly at the lower decibels established by minimum and maximum limits. At these levels, the psychoacoustical impacts have less to do with the magnitude of sound (i.e. in the sense the mechanisms that cause temporary or permanent hearing loss due to sudden or prolonged exposure to sufficiently elevated sound levels are entirely absent) and more to do with its temporal, spectral, and spatial qualities.

These qualities are not as well understood by those that are outside the acoustical community and, hence, not typically as well-considered when designing a space. If the sound that actually exists within a particular space is left to various noise sources (e.g. building systems), these qualities are also inherently variable—and will remain so, despite efforts to mitigate, absorb, and block noise—unless ‘C’ is implemented.

Temporal

The temporal component of sound refers to the variation in the level of sound as a function of time; in other words, from one moment to the next.

Neither HVAC nor mechanical, electrical, and plumbing (MEP) systems can be relied on to provide continuous and constant (i.e. unchanging) control—and nor should they, for reasons relating to the spectral characteristics of these noise sources. Figure 2 illustrates the issue. While the receiver experiences a moment of privacy (highlighted in blue), they are not free from distraction the remainder of the time because the signal-to-noise ratio is positive. When ‘C’ is applied, it not only improves speech privacy, but also increases occupants’ perception of acoustical consistency by reducing the frequency and severity of the intermittent changes in sound levels (i.e. dynamic range) caused by speech and noise, over time.

Spectral

The spectral component of sound is a more nuanced topic. Just as visible light comprises a range of wavelengths, sound, as one hears it, is the result of a combination of frequencies.

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