Proceedings of the Institute of Acoustics

 

 

Acoustical privacy in the built environment: past, present, and future

 

Ethan Bourdeau¹, International WELL Building Institute, New York, USA

Viken Koukounian², KR Moeller Associates, Burlington, Canada

 

ABSTRACT

 

Although the need to consider acoustics in the built environment is well-accepted, an understanding of ‘what’ constitutes ‘good acoustics’ is not. The authors present posteriori—reviews of historical events and of empirical evidence—to develop an understanding of the current landscape of Standards, Guidelines and Codes. More specifically, special attention is devoted to the development of speech privacy metrics, as well as that of auditory and non-auditory effects of noise and sound on health. With a refreshed understanding of theory, a priori—i.e., an acoustical framework for design and/or performance verification with a special interest in speech privacy—is renewed and its impacts on the ongoing development of Standards, Guidelines and Codes is discussed. The presentation concludes with a case study highlighting the importance of an acoustical framework, which can be, systematically, relied upon to improve expectations of privacy in open-plan and enclosed spaces.

 

1. INTRODUCTION

 

As a subdiscipline of physics, the study of acoustics has received contributions from some of the most recognized physicists and mathematicians in history. However, the application of those theories for architectural use is, comparatively, in its beginnings. The study of sound in the built environment, also referred to as the field of architectural acoustics, is largely credited to Wallace C. Sabine for determining the quantitative relationship between the reverberation of sound and its environment. Given the circumstances of his work (to improve the acoustics of the Fogg Lecture Hall at Harvard), his discovery in the early 1900s became the foundation upon which new rhetoric for ‘good acoustics’ was developed. To be careful with our commentary, this new perspective did not challenge the general understanding of the benefits of sound insulation (sometimes also referred to as ‘sound isolation’) in providing freedom from noise but supplanted its importance. The then rise of awareness for the need for occupational health and safety (OHS) policies–including the topic of “noise exposure”–did provoke consideration about the order of priority. However, the eventual enactment of OHS policies into law created a need to develop and standardize a barrage of acoustic metrics to assess acoustic conditions (e.g., background noise, reverberation time) and ‘product related’ performances (e.g., attenuation of sound, impact noise attenuation). Naturally, the ability to characterize features of the acoustical environment created a demand for strategies that could be used for the purposes of design.

 

Acousticians met the needs of the Architecture and Design (A&D) community by developing design schemes–the most common of which are the ‘acceptable level,’ ‘categorization’ and ‘optimum reverberation time’ schemes–which have since made their way into nearly every guideline, standard, and code around the world. Unfortunately, these design tools were never intended to be used prescriptively because they were known to be insufficient at assuring the acoustical performance of a space. One such instance evidencing this claim is the publication, Speech Privacy in Buildings, by Cavanaugh in 1962 which reports the weak correlation between acoustical satisfaction and the sound insulating performance (i.e., transmission loss) of room assemblies (e.g., walls, floors, ceilings), and also explicitly declares “Do not use for design!” in the caption of Table I [categorization scheme] and Table II [acceptable level scheme] [].

 

Ahead of presenting more evidence–via posteriori–we propose to continue developing our thesis– via priori–by revisiting first principles to better understand ‘why’ the traditional design schemes are insufficient at assuring the acoustical performance of the designed space.

 

2. FIRST PRINCIPLES

 

Although ‘acoustics’ is generally understood to be in relation to the person, the scope of its study is only truly concerned with the transmission of mechanical energy (i.e., sound) through various media (e.g., air, water, ground). In contrast, the associated perception of sound and the associated physiological effects falls within the interdisciplinary field of psychoacoustics. With this understanding, the disconnect between physics and the person ought to be more apparent.

 

And while these nuanced understandings have significant implications, there are reasons why they are often dismissed–that is, a persuasive rhetoric. It is no longer sufficient to have a scientifically correct understanding of theory and its applications. This is especially true for the acoustical industry, which is evidenced by the widespread, yet incorrect, belief that ‘good acoustics’ is believed to be an environment of ‘low noise.’

 

For this reason, we review noise exposure history to identify and explain nuances that have carried significant implications. More specifically, at the turn of the 1950s, we had started to learn that sudden or sufficiently prolonged exposure to elevated levels of sound could cause physiological damage to the auditory anatomy. Empirical evidence (i.e., measurement data) was used to develop numerical models to predict the relationship which were used to establish safe limits for noise exposure to prevent the identified auditory health effects (i.e., hearing loss). And while experts understood the complementary relationship between the duration of exposure and the sound pressure level of a noise event, much greater emphasis was placed on the exposure level of noise than its duration. For instance, hearing loss may be instantaneous when exposed to a burst of noise greater than 120 dB. Alternatively, exposure to 100 dB and 80 dB may be considered safe if limited to 15 minutes and 8 hours, respectively. These values are reported by NIOSH, and calculated with reference to an 8-hour workday []. Later, other conditions, such as tinnitus and aging, were added to this categorization of health.

 

Given the nonlinearity of the relationship between level and duration of exposure, it is possible to calculate, using the same calculation methodologies, an equivalent ‘threshold’ sound pressure level value for exposure of continuous sound for a period exceeding the length of a day, which would be approximately 70 dB. This is to be interpreted to mean that for lower levels of sound, the conditions which risk hearing loss are essentially not present.

 

It is at this point that we choose to begin refining our use of acoustical vernacular. By way of example, the term ‘acoustic’ is used when associated with a term carrying ‘quantitative’ meaning (e.g., acoustic wave, acoustic pressure), while ‘acoustical’ is used when associated with a term carrying ‘qualitative’ meaning (e.g., acoustical engineer, acoustical comfort) [F. V. Hunt, "Acoustic vs Acoustical," The Journal of the Acoustical Society of America, vol. 27, pp. 975-976, 1955.]. Admittedly, the previous example is so nuanced and specific that its misuse would go unnoticed, as it does. In contrast, the same cannot be said about ‘sound’ and ‘noise.’ More precisely, “sound” refers to all acoustic waves, while “noise” is sound that is generally perceived as being distracting, disturbing, or otherwise having some negative impact [possible WHO reference]. The distinction is critical because we are constantly surrounded by acoustic energy, which supports everyday life. To further defend the need to differentiate between the two terms, we direct the reader’s attention to the circumstances and nature of noise exposure research. The vast majority ofstudiesinvestigate the impacts on health due to sound or noise that is perceptible. The authors suggest that these studies could be categorized as belonging to one of two groups: (1) surveys of populations that are in proximity of a sound sources(e.g., airports, railways, highways) or (2) investigations that invite participants to a research facility and subject them to stimuli (e.g., white noise, music). To reiterate, in both these types of studies, an assessment about the person is made based on ‘exposure.’

 

However, there is an alternative perspective which suggests that sound is not always a stimulus. To respect the merits of this statement, we consider–what may be the most important principle tying acoustics to the occupant–the Masking Effect. Documented since the 1950s, the Masking Effect describes our ability to perceive, discern or identify a sound [United States Department of Commerce - National Bureau of Standards, "Sound insulation of wall and floor constructions (Building Materials and Structures Report 144)," U.S. Government Printing Office, Washington 25, D.C., 1955.] To summarize generally, the principle explains the loss in intelligibility or audibility of a sound by a listener as it becomes increasingly masked by the ambient acoustic environment. Although documented in the aforementioned reference, it also has roots in speech intelligibility (and the development of the telephone by the Bell Telephone Labs in the early 1900s), which was adapted to provide indications of speech privacy between two positions in the built environment in the late 1940s and early 1950s.

 

3. TRICHOTOMY OF ACOUSTICS IN SPACES

 

The following three categories represent a past, present, and future set of pathologies whereby acoustical satisfaction has been defined so far throughout the history of acoustical engineering in the built environment.

 

3.1 Acoustical Design

 

Historically, acoustical satisfaction has been presented by way of minimum thresholds and criteria associated with the design-specific interventions (i.e., specifications, predictions, lab data assessment) to demonstrate future acoustical satisfaction is enough to suffice requirements found in building codes, elective rating systems, and best practice guidelines. Examples of this include thresholds for the minimum square footage of absorptive materials, thresholds for steady-state or instantaneous noise levels, minimum requirements for sound transmission class and impact isolation class between floors. These parameters can be predicted and calculated using references, data sets, and predictive software for the purposes of predicting acoustical performance.

 

The presentation of acoustical satisfaction through design-based criteria has allowed experts in the field of acoustical design to readily convey parameters to design teams with a focus on documentation, specification, and general accountability throughout all phases of design. However, the authors of this paper aim to make the assertion that design-based criteria alone are not sufficient in the process of designing to meet performance outcomes.

 

3.2 Acoustical Performance

 

In recent years, performance-based acoustical design, whereby parameters that can only be measured on-site by a trained expert in field-testing, have started to be included in a combination of proprietary workplace standards, elective rating systems, and compliance with municipal and regional code language. Performance-based analysis is intended to confirm the extent to which design-based parameters have been achieved in-situ. In most cases, on-site verification of acoustical performance relies on several other elements and systems already being in place. For example, in the case of commissioning acoustical metrics within an open office (e.g., speech transmission index, radius of distraction, spatial decay of sound) several factors, including spatial layout, background sound from HVAC systems, and surface finish treatments play a fundamental role with the outcomes of performance verification.

 

The presentation of acoustical satisfaction through performance-based criteria adds a layer of accountability on top of the design-based approach, acting almost as an audit for design teams seeking to demonstrate a higher degree of indoor environmental quality. In many cases, however, relying solely on performance-based data will not guarantee that occupants operating in the assessed spaces will ultimately be satisfied.

 

3.3 Acoustical Experience

 

With the topic of returning to the workplace and hybrid working being central to the future of design, experience-based design thinking has shed light on areas where acoustical design and performance verification fall short of adequately assessing occupant satisfaction, performance, and well-being. Experience-based acoustical design is only in its infancy but is already focusing on ways to better target not just occupants but the vast array of occupant types that make up any workplace. Recent research out of Tarkett and HOK have focused specifically on designing for neurotypical and non-neurotypical populations, stressing first and foremost that noise is a leading factor in designing for neurodiversity. Likewise, acoustical design and performance requirements fail to meet the growing demands for acoustically accessible spaces with only a small handful of codes and regulations focusing on intelligibility for individuals who are deaf, hard of hearing, or otherwise auditorily impaired.

 

The authors of this paper assert that experience-based acoustical design will encompass the next horizon of best practice design thinking with respect to sound and occupant satisfaction. Unlike the previous two methods, which rely on technical understanding of the physics of sound, acoustics, noise control, on-site assessment, and use of instrumentation, experiential design requires a deeper understanding of psychoacoustics, populations, demographics, target audiences, and true occupant centric outcomes. What acoustic satisfaction means for one group, let alone one person, may change drastically throughout a team, an office, an organization, and globally. Using experiential-design to design better-sounding, people-first places will require as concrete a definition for terms like “acoustical satisfaction” as possible.

 

4. ACOUSTICAL HIERARCHY

 

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4. CONCLUSIONS

 

This is the conclusion section.

 

5. ACKNOWLEDGEMENTS

 

We gratefully acknowledge the authors for submitting their work to INTER-NOISE 2022 GLASGOW.

 

6. REFERENCES

 

1. Poulsen, T. Influence of session length on judged annoyance. Journal of Sound and Vibration, 145(2), 217–224 (1991).

2. Ryherd, E. E. & Wang, L. M. Effects of exposure duration and type of task on subjective performance and perception in noise. Noise Control Engineering Journal, 55(3), 334–347 (2007).

3. Tang, Q., Liu, S. & Zeng, F. G. Loudness adaptation in acoustic and electric hearing. Journal of the Association for Research in Otolaryngology, 7(1), 59–70 (2006).

4. May, P., Davies, P. & Bolton, J.S., Correlations between objective and subjective evaluations of refrigerator noise. Proceedings of INTER-NOISE 96, pp. 2257-2260. Liverpool, U.K., July - August1996.

5. Zwicker E. & Fastl, H. Psychoacoustics: Facts and Models, 3rd Edition, Springer, 2006.

6. American National Standards Institute/Acoustical Society of America (ANSI/ASA), Procedure for the Computation of Loudness of Steady Sounds, S3.4 2007 Edition.

 

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