Creating acoustical equity

These background sound level measurements were taken in 26 locations within an open plan. Without masking (inset), the occupants experienced varying acoustical conditions across the space. Since masking sound was applied and tuned to reliably meet the National Research Council (NRC) masking spectrum within each small zone, occupants experience a far more consistent level of acoustical privacy and comfort throughout the space. Photo courtesy


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 conclusion

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

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

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

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