September 13, 2022
Controlling temporal, spectral, and spatial properties of sound
By Viken Koukounian, Ph.D., P.Eng., and Niklas Moeller
The increased focus on safety and work/life balance prompted by the pandemic has kicked the “people first” mindset extolled by standards such as WELL and Fitwel into high gear. As organizations start to bring, or attempt to draw, work-from-home (WFH) staff back into the office, there is strong agreement about workplaces being designed with deep commitment not only to occupant satisfaction, but to health, well-being, and social connection.
At the same time, there is growing conviction amongst the architectural and design community that these goals must be achieved through concern with “equity”—and applied to “real-world” needs—rather than offering amenities such as pool tables, private chefs, and other perks; in other words, they are a matter of how employees are treated rather than what they are treated to.1
The experiences of those forced into WFH arrangements by stay-at-home orders (i.e. rather than making the choice in response to their own needs) proved anything but equitable. Residential environments are not specifically designed to support information workers. Although some were able to carve out independent and well-equipped workspaces within their homes, others struggled with less-than-ideal conditions (e.g. sharing a workspace with children and other family members) which negatively impacted their productivity and engagement.
Employees whose homes are conducive to focus might be surprised to learn one of the primary reasons respondents to the 2020 Gensler Work from Home Survey cited for wanting to return to the office was the need for a quiet, distraction-free environment; however, this desire makes sense when placed in the context of the results of surveys, such as one run by worldwide employment website Indeed, which indicated 48 per cent of those working from home are distracted by outside noise in their neighbourhoods, and apartment dwellers are 51 per cent more likely to be distracted by their neighbours than employees living in single-family homes.
Whether an organization wants their office to be occupied full-time post-pandemic or to serve as a critical part of a hybrid working model, it has the potential to act as a “great equalizer”—a shared facility specifically designed to support all its occupants. It is clear acoustics matters—to those hesitant to return because they find it easier to concentrate at home, as well as to those wanting to return because they need a space conducive to focus—and is, therefore, vital to ensuring employees not only enjoy equal access to the facility itself, but to a key Indoor Environmental Quality (IEQ) parameter needed to work comfortably and effectively.
But what is “acoustical equity”? And how does one achieve it?
These are significant questions, particularly considering the fact the Center for the Built Environment’s (CBE’s) post-occupancy survey dataset—one of the largest in the world—shows poor acoustics remains the most significant source of dissatisfaction among workplace occupants.
The sound which actually exists
En route to answering them, one must first consider the traditional approach to acoustics, which relies on “categorization” and “acceptable-level” schemes prevalent throughout building standards and codes. The former specifies sound-rating values (e.g. sound transmission class [STC], noise isolation class [NIC], impact isolation class [IIC], ceiling attenuation class [CAC]) for the boundaries of a room or building envelope, while the latter uses noise-rating values (e.g. noise criteria [NC], noise rating [NR], room criteria [RC]) to set maximum limits for noise, such as those generated by building systems, services, and utilities. However, neither offer insight into the “actual acoustics” (i.e. the sound actually present) within a space or occupant experience of it.
To improve results—a goal one can call “better acoustics”—and fulfil the objective of designing with occupants in mind, one must focus on the sound actually present in a space and look at it through the lens of both architectural acoustics (i.e. the study of sound and its behaviour in and due to a space) and psychoacoustics (i.e. the study of the psychological and physiological effects of sound and its perception). Either one cannot be separated from the other, as psychoacoustical evaluation of a space considers the outcome of the combined performance of all acoustical features.
Acoustical privacy is key
The reactions of building occupants are captured using psychoacoustic metrics, some of which are subjective (e.g. surveys evaluating comfort, distraction, perceived productivity) and others that are objective (e.g. intelligibility, audibility).
Research shows one’s overall acoustical satisfaction is strongly correlated with acoustical privacy, a concept with clear ties to the workplace, but is also relevant to other environments. Although people tend to equate acoustical privacy with speech privacy, the former is not limited to the intrusion of speech content; it also considers the audibility of unintelligible speech and other types of noise. For example, surveys of multi-unit residences demonstrate links between acoustical privacy and annoyance, fatigue, and sleeping problems (e.g. due to noise from traffic and neighbours).2
It is challenging to use acoustical privacy as a starting point for a conversation about acoustical equity. The science around acoustical privacy is not sufficiently nuanced; it is not yet addressed by a standardized metric or even a proposed methodology.
Speech privacy, on the other hand, is both well-defined and measurable (e.g. using articulation index or speech privacy class). Therefore, it is a psychoacoustic metric which can be used in both theoretical (i.e. to illustrate the concept of acoustical equity) and practical ways (i.e. to set expectations during design and estimate occupants’ subjective impression of the built space). In this case, evaluation of acoustical privacy is effectively a review of the signal-to-noise ratio; it considers an intruding “signal” (speech) and its level relative to the background “noise” (or, rather, sound) in the receiving space.
By way of example, see the rooms and occupants in Figure 1:
If one were to assume the orange and blue signals are people speaking, the orange talker’s voice carries into Room 1; however, it is masked by the background sound. The listener in the room cannot identify and/or understand speech and the orange talker enjoys speech privacy. The blue talker’s voice is carried into Room 2; however, it is not masked by the background sound and the listener can identify and/or understand speech. The blue talker does not have speech privacy.
There are impacts beyond the one-way speech privacy. It is understood the orange talker has speech privacy because the background sound in the adjoining room masks the received level of their voice. However, the orange talker’s “perception of privacy” is violated because they can hear the blue talker. This discrepancy can cause reactive behavioural changes on the part of the orange talker (e.g. lowering of voice, avoiding confidential topics). It is also accepted the blue talker does not have speech privacy because the background sound in the adjoining room does not mask the received level of their voice. However, the blue talker has a false perception of privacy engendered by the fact they are unable to hear the orange talker. This discrepancy can result in breaches of confidentiality, the implications of which can run the gamut—or gauntlet, depending on the consequences—from embarrassment to legal proceedings.
Understanding acoustical equity
One can appraise this situation using the basic dictionary definition of “equity” (i.e. fairness or justice in the way people are treated) and conclude the occupants do not have acoustical equity simply by virtue of the fact 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, director of race equity and inclusion at the Annie E. Casey Foundation, thinks of it as each of us reaching our full potential.” Since design impacts one’s well-being and level of functioning, it is one of the factors in life that—in the words of built environment strategist Esther Greenhouse—has the “power to disable or enable.” Greenhouse maintains if there is a “poor fit between a person and their environment, the environment acts as a stressor, pressing down on their abilities, pushing them to an artificially low level of functioning.”
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 CBE show lack of speech privacy is the top workplace complaint, indicating it is a broadly applicable concern.3 Further, this deficiency is not only relevant to 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”4 taking measures to achieve this goal means they will have an easier time concentrating on tasks, make fewer errors, and suffer less stress and fatigue.
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 is designed to meet and help fulfil. While acoustical privacy is not the only objective, it is a highly sought-after quality with widespread relevance serving 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) and not unintentional. There is a need for control of the acoustic environment and specifically, background sound.
Although categorization and acceptable-level schemes endeavour 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.”5, 6
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,”10 setting minimum—not maximum—levels for background sound is critical to attaining speech privacy. 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 which sound is unwanted.
A minimum background sound level can only be reliably achieved through the application of the “C” in the “ABC rule.” “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. “C” is the final letter in the rule 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”7 efficiencies which 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.
Looking beyond level
The role “C” plays in providing beneficial acoustical conditions becomes even clearer when one considers there is more to the 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 causing 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 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.
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 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.
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.
Singular—or discrete—frequency values are called “tones,” and the human ear can hear between approximately 20 and 20,000 hertz (Hz). To simplify reporting data for the nearly 19,980 individual frequencies, it is common practice to divide this range into sections called “fractional octave bands.” The customary fractions are full octave bands (also referred to as “1/1”) and one-third octave bands (or “1/3”). Between 20 and 20,000 Hz, there are 29 one-third octave bands. The combination of all audible frequencies of a sound sum to its overall level.
It is possible for two sounds equal in overall level to be perceptibly different. Borrowing descriptors from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), one can generally state a sound that has too much low-frequency content is “too rumbly,” while a sound that has too much high-frequency content is “too hissy,” and sound that has too much mid-frequency content has a strong “hum” or “buzzing” quality.
If empowered with the ability to adjust the frequency content for a fixed level of sound (e.g. 45 dBA), there exists a favourable combination of frequencies that is “most comfortable” or balanced. This “shape of sound” is documented in literature by Beranek (and BBN) and Warnock—and, more recently and precisely, by the National Research Council of Canada (NRC)—and forms the basis for the synthesis of masking sound.8 When professionally tuned to meet this “shape” (typically called a “spectrum” or “curve”) for the majority of the audible frequency range (100 to 10,000 Hz), background sound resides in the “Goldilocks zone.” Occupants’ perception of the final product may be described as “quiet”—free from rumble, hiss, or buzz, and absent of hum or buzzing; further, the overall level is neither too high to disturb occupant comfort, nor too low to compromise acoustical privacy.
The spatial component of sound is no less complex. It refers to the variability of the level—also, inherently, that of the spectra—of sound, in space. These variations are a function of many parameters, including not only the source and location from where the sound originates (e.g. building systems, occupants, appliances, and even oneself), but also the space’s architecture (i.e. size, shape, geometry) and fit out (i.e. finishings, fixtures, furnishings).
As sound from a source is generated, it propagates with its level decaying as a function of distance, and by the number of times it is reflected (loses energy) from other surfaces or at room boundaries. While its energy continually dissipates, its eventual inaudibility is not because the level is attenuated below one’s auditory threshold, but because it drops below the background sound in one’s environment—the background sound that actually exists. This phenomenon is known as the “masking effect,” where the background sound covers the propagating noise. Figures 3 and 4 (page 36) provide simplified modelling of this effect. Not only does masking sound reduce the distance over which a noise can be heard (sometimes referred to as the “radius of distraction”), it creates a more consistent—and equitable—acoustical experience for occupants, both in their individual work areas and as they move throughout the space.
Control versus cover
While many still associate the “C” in the “ABC rule” with “cover,” “control” is a more accurate term for several reasons.
Use of the word “cover” can unintentionally reinforce the view this crucial element of architectural acoustics simply involves placing any sound overtop of others—like a blanket—strengthening the historical misperception where only level matters; in other words, a sound only needs to be “louder” than other sounds to provide the masking effect and, hence, meet the requirements of “C.” This misperception opens the door to commoditization of sound-masking systems—the notion the effect will simply be provided by the product, rather than in tandem with a service that ensures the sound actually meets the specified masking spectrum.
The study of architectural acoustics demonstrates the physics of the behaviour of sound within the built environment is exceedingly complex—and this is true for any sound, even those introduced via a sound-masking system. Regardless of the sophistication of the technology, the system’s layout or loudspeaker orientation (e.g. upward-facing within the plenum or downward-facing using cut-throughs), the masking effect can only be achieved through skilled field commissioning—or “tuning”—which adapts the sound actually produced in the room/space by accounting for its architecture and fit out. Small zones (i.e. no larger than one to three loudspeakers in size) offering fine volume (in 0.5 dBA steps) and frequency (1/3-octave) adjustment capabilities provide the technician with frequent and precise control points across the environment, helping to consistently achieve the masking effect throughout the space and, hence, a better outcome for the occupants.
Post-installation tuning and performance verification are crucial to ensuring the sound-masking system is, in fact, effectively controlling the spectrum and level of the sound that actually exists within the built environment—and, hence, dependably providing the masking effect throughout the space. It is only under these assured conditions—temporally, spectrally, and spatially consistent acoustics—that occupants can appreciate acoustical privacy.
In 1962, William Cavanaugh et al., authors of “Speech Privacy in Buildings,” affirmed acoustical satisfaction could not be assured by any single parameter, forming the foundation for the “ABC rule” of architectural acoustics. However, until recently, building codes, standards, and certification programs largely focused on “A” and “B,” while “C” often succumbed to a historical preoccupation with limiting the “loudness” of sound and corresponding belief that the goal is to make spaces as silent as possible. Undoubtedly, architectural acoustics are amid a paradigm shift.
In the pursuit to better understand how one can be psychologically and physiologically supported by the spaces they inhabit, the important role played by “C” becomes apparent. Sound will always remain within the built environment, and the impact of such low-level background sound—which actually exists in the space—cannot be separated from acoustical satisfaction and its equitable delivery. Therefore, controlling it is as important as controlling the “signals.”
As Greenhouse states, the built environment “impacts us whether designed well or poorly, so why not design well?” If one is to reliably design buildings to function acoustically for their users (e.g. provide adequate speech privacy, freedom from distraction, reduced annoyance, a good night’s sleep, and so on), one needs to establish a known level of spectrally neutral (or balanced) background sound, rather than leaving it—and the end result—in question.
1 Talitha Liu and Lexi Tsien in “The Office as We Knew It No Longer Exists,” Azure, September 2020.
2B. Rasmussen and O. Ekholm, “Is noise annoyance from neighbours in multi-storey housing associated with fatigue and sleeping problems?” in Proceedings of the 23rd International Congress on Acoustics (ICA), Aachen, Germany, 2019.
3 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.
4J. Keranen and V. Hongisto, “Prediction of the spatial decay of speech in open-plan offices,” Applied Acoustics, vol. 74, 2013.
5, 6 W.J. Cavanaugh, W.R. Farrell, P.W. Hirtle, and B.G. Watters, “Speech privacy in buildings,” The Journal of the Acoustical Society of America, vol. 34, no.
7 Breaking out of our entrenched ways requires a co-ordinated 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. Although the evaluation of all contributing sound sources is complex, if engineers can 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. To learn more about the project savings engendered by a holistic approach, read Niklas Moeller’s “Placing sound masking on the front line of acoustic design,” in the July 2017 issue of Construction Canada. To read the article, visit www.constructioncanada.net/placing-sound-masking-on-the-front-line-of-acoustic-design.
8 Although they are still often referred to as ‘white noise’ systems, modern sound-masking technologies synthesize the spectrum and level of the sound that actually exists within the space.
Viken Koukounian, Ph.D., P.Eng., is an acoustical engineer at K.R. Moeller Associates Ltd. He is an active and participating member of many international standardization organizations, such as the Acoustical Society of America (ASA), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the American Society of Testing and Materials (ASTM), the Green Building Initiative (GBI), and the International WELL Building Institute (IWBI), the Standards Council of Canada (SCC), and also represents Canada at International Organization of Standardization (ISO) meetings. He completed his doctorate at Queen’s University in Kingston, Ontario, with foci in experimental and computational acoustics and vibration. Koukounian can be reached via email at firstname.lastname@example.org.
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’ experience in the sound-masking industry. Moeller can be reached via email at email@example.com.
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