The need for control
Given both the scientific and human factors, one can readily see there are advantages to controlling background sound, rather than accepting large variations in its level and spectra.
The consequences of neglecting this principal parameter of architectural acoustical design is an environment that is perceived to be ‘noisy,’ as presented in Figure 1. The alternative—to add sound to reduce the perception of a noisy environment—might seem counterintuitive, but consider Figure 2. By precisely controlling the spectrum and level of sound (in this case, to a target overall sound pressure level of 47 dBA), one can make the space sound more comfortable.
The difference between one’s experience in a space with a very low ambient level and one with a higher ambient level suggests just as there is a need to differentiate between ‘sound’ and ‘noise,’ there is value in distinguishing between a ‘silent’ space and a ‘quiet’ one. Whereas ‘silent’ infers the absence of sound, a ‘quiet’ space can be characterized by a constant ambient sound that is comfortable and not readily noticeable by its occupants. Spaces such as these are perceived to be less ‘noisy’ and more comfortable—or ‘quiet.’
Evolution of sound masking
Advanced technologies—called sound masking systems—have been developed to control the level and properties of ambient sound within commercial spaces. These systems consist of a series of loudspeakers installed in a grid-like pattern in an open ceiling or above the ceiling treatment. The loudspeakers’ output is controlled using additional equipment. The first instances of their installation followed the publication of Cavanaugh’s book. However, at the time, there were several obstacles to the adoption of sound masking as an effective acoustical design strategy.
Firstly, these early systems were considered failures due to technological limitations and a lack of understanding and application of acoustical theory, which affected both their design and commissioning (e.g. large zones, limited control over volume and frequency settings). Ultimately, they failed to deliver a consistent, comfortable sound.
Secondly, their deployment coincided with the sudden awareness and aggressive regulation of ‘noise exposure.’ Although earlier efforts were made in other countries, the most comprehensive attempt to combat noise nuisance came in the form of the United States of America’s Noise Control Act of 1972. Many other governments and organizations have since used this document as the basis for their own regulations regarding occupational health and safety, environmental noise, transportation noise, and built environments (e.g. HVAC, building services), the primary focus of which is to limit the sudden or prolonged exposure to high ‘noise’ levels that would cause hearing loss (in the current discussion of appropriate noise exposure limits for activity-based spaces [e.g. for learning, recovery, sleep]—such as those set by the World Health Organization [WHO]—the focus continues to be on sound level, with little consideration of the other factors playing into human evaluations of acoustics. There is little appreciation of psychoacoustical parameters, which would consider the existing level and spectrum of background sound in review of the intruding sound from the noise source. By way of example, lacking understanding of the existing level and spectrum of sound in space, it is impossible to conclude sleep disruption can occur as a result of intruding traffic noise at any defined value [e.g. 30 dBA]).
However, research shows the definition of ‘noise’ should also include ‘unwanted sound’ (e.g. that which interferes with one’s ability to concentrate on the task at hand or get a good night’s rest). These noises do not meet the same criteria as those damaging physical structures, but their impact is nonetheless undesirable. Background sound can have a positive and mitigating effect here and, hence, the need to make the distinction. Rather than trying to create a silent, library-like space in which there is little to no sound at all (i.e. a ‘silent’ space), the goal is to create a ‘quiet’ space—one in which there is little to no unwanted sound.
Lastly, developing the masking spectrum was an iterative process spanning several decades. With the development of methodology to assess the acoustical privacy of spaces—namely, the articulation index (AI)—discussion turned toward specification of reasonable targets for acoustical privacy and renewed interest in determining what type of background sound would work best and how it could be delivered. After all, if the intention is to improve privacy, one not only needs to control the level of background sound, but also ensure the sound has specific qualities. In the 2000s, the National Research Council Canada (NRC) refined the spectrum, based on tests measuring both comfort and effectiveness, resulting in the cost-effective open-plan environment (COPE) masking spectrum (Figure 3).
It is important to note delivery of effective ‘masking’ is not a product of the sound generating and control equipment (i.e. the electrical signal), but rather the ability of the sound masking system to adapt the generated sound that is actually delivered to the space and which is dependent on the space’s architecture—its layout, furnishings, and finishings. To achieve the desired effect, the sound produced within the space must be adjusted to a specific spectrum through a post-installation process known as tuning.
Ensuring effective performance also requires verification. ASTM E1573-18, Standard Test Method for Evaluating Masking Sound in Open Offices Using A-Weighted and One-Third Octave Band Sound Pressure Levels, offers guidance and instruction on the measurement procedure to evaluate the performance of a commissioned masking system. Measurements are performed in every 93 m2 (1000 sf) of open space and a representative number of closed rooms to review effectiveness of the tuning process against performance targets and tolerances, and to provide an indication of the spatial uniformity of the masking sound.