Understanding Acoustical Equity

One can appraise this situation using the basic dictionary definition of equity (i.e., fairness or justice in the way that people are treated, per Merriam-Webster) and conclude that the occupants do not have acoustical equity simply by virtue of the fact that they do not enjoy equal levels of speech privacy, or even perceived privacy. However, there is more to the concept of equity.

According to conversations occurring in philanthropic circles, equity is also “about each of us getting what we need to survive or succeed—access to opportunity, networks, resources, and supports—based on where we are and where we want to go. Nonet Sykes, the director of race equity and inclusion at the Annie E. Casey Foundation, thinks of it as each of us reaching our full potential.”⁶ Because design impacts our well-being and level of functioning, it is one of the factors in our lives that—in the words of built environment strategist Esther Greenhouse—has the “power to disable or enable.”⁷ Greenhouse also maintains that if there is a “poor fit between a person and his or her environment, the environment acts as a stressor, pressing down on the person’s abilities, pushing him or her to an artificially low level of functioning.”⁷

The need to provide a supportive environment highlights the importance of providing beneficial acoustical conditions throughout the workplace. While occupants can be impacted by acoustical design in a myriad of ways, let us continue with the example of speech privacy. Some might consider it a niche application only relevant to particular offices (e.g., law firms), health care, and military environments, but surveys such as those conducted by the Center for the Built Environment show that lack of speech privacy is the top workplace complaint, indicating that it is a broadly applicable concern.⁸ Furthermore, this deficiency is relevant not only to occupants of private offices but also 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.

The Need for Control

Equity involves ensuring that 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 that 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 that are 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.”^{9, 10}

If one only implements maximum thresholds, one leaves this key variable up to whatever is left or whatever happens. Because our 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.¹¹ 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 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 that each plays in the outcome. Rather, the rule reinforces the fact that a holistic approach is required for the best results.

It is important to note that 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.^{12, 13}

Looking Beyond Level

The role that C plays in providing beneficial acoustical conditions becomes even clearer when one considers that 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 that 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 outside of the acoustical community and hence not typically as well considered when designing a space. If the sound that actually exists within a 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 MEP systems can be relied upon 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.

Image courtesy of K.R. Moeller Associates Ltd.

Sounds that are equal in overall level can be perceptibly different, depending on their frequency content. Differing spectrums also impact speech privacy. Here, a masking system is tuned with varying degrees of precision. Despite the fact that the resulting sounds are at exactly the same overall level (i.e., 47 dBA), note the impact on comprehension (i.e., privacy) when the frequencies defined by the National Research Council (NRC) masking spectrum are not met.