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.