As interest lies in conditions conducive to acoustical comfort rather than discomfort, the reader is now invited to add a level of background sound to the imaginary room, such as that produced by HVAC. Technically, this sound corresponds with the typical maximum limits (30 to 55 dBA) defined in ASHRAE Handbook – HVAC Applications. Since frequency plays a role in determining comfort, with low, mid, or high frequencies potentially causing discomfort due to their rumbling, buzzing, or hissing content, the reader should also apply an appropriate spectrum—a concept at the heart of most noise-rating systems, which depend on a reference contour to assess whether the ambient acoustic conditions are spectrally “neutral.”
However, because building-related systems, services, and utilities are designed to support other functional needs such as (e.g. thermal, air quality, water, lighting) and sound is simply a byproduct, it varies (temporally) according to the optimal performance and efficiency conditions of the various components. At the same time, the positioning and arrangement of these sources, which encompass elements such as ductwork and piping, exert a significant influence on how acoustic energy is distributed within a facility, particularly in terms of its spatial transmission. In other words, in practice, the background sound (or noise) produced by such equipment is not temporally or spatially consistent, or spectrally neutral—especially collectively.14 Building professionals can only strive to ensure overall levels do not exceed acceptable maximum limits.
To dependably manage the overall level and spectral distribution of background sound within the built environment—thus ensuring the minimum limit needed for speech privacy, as well as the frequency range required to effectively mask speech and a wide range of noises, while maintaining occupant comfort—one must employ a sound-masking system: an acoustical solution intended to manage the ambient acoustic conditions in a space.15
Masking technology has come a long way since its inception in the 1960s; with the introduction of small control zones and precise computer tuning, appropriately trained technicians are now able to control its output with precision. When handled correctly, the sound achieved within the space not only minimizes the disruptive impact of noise and protects the privacy of conversation, but it also does it consistently and unobtrusively. In this case, sound has a “damping” effect, rather than acting as a stimulus, as it either entirely masks noise or diminishes its disruptive impact. This reduction in the dynamic range, which denotes the variation in sound levels over time or the difference between the loudest and quietest sounds measured over a period, plays a crucial role in mitigating distraction and discomfort. Consequently, it helps in averting the activation of the sympathetic nervous system responsible for the “fight or flight” response.
Implementation of masking sound
The importance of managing beneficial background sound within interiors is increasingly recognized in standards, guidelines, and codes worldwide, but obstacles remain in understanding how it should be handled. Despite its role as one of the three pillars of effective architectural acoustical design (i.e. the “C” in the “ABC Rule,” which stands for “cover” or, more accurately, “control”), sound masking remains the most poorly understood. In the absence of industry standards pertaining to design and performance, notably significant variations also exist among the available systems and implementation methods.
While the concept is straightforward, the benefits of the Masking Effect are not achieved via a “plug and play” electronic system. As the generated sound is affected by the facility’s interior layout, furnishings, and finishings, effective application requires not only diligent design through small control zones, technical expertise in sound masking and general acoustics, and specialized equipment (ANSI Type 1 one-third octave analyzers and Class 1 calibrators), but also precise field tuning aligned with ASTM E1573-22, Standard Test Method for Measurement and Reporting of Masking Sound Levels, Using A-Weighted and One-Third Octave-Band Sound Pressure Levels). Further, to be accountable to the specification and, ultimately, to the client, the contractor must properly measure and report the tuning technician’s results.
It is worth noting sound masking’s current location (27 51 19) within MasterFormat, Division 27 – Communications means it is often managed from a communications (A/V) perspective rather than an acoustical one. Although audio and masking systems use similar components (e.g. electronics, cable, loudspeakers), their purpose is fundamentally different: the former distributes communication, while the latter is intended to obscure it. Masking also fulfils other goals outside the scope of communication systems such as acoustical comfort. When there is lack of industry standards, project teams are heavily reliant on experts and contractors to provide guidance and masking delivery. Signaling that masking is an acoustical technology rather than an audio system would afford them the opportunity to carefully consider what parties are best suited to provide advice and handle masking implementation.
