Historically, background sound levels were (and continue to be) non-uniform in level and spectra, and highly variable over time and throughout the built environment. As a result, partition walls were often overbuilt in an effort to reduce the transmission of sound from source to receiver. Rather than employing effective controls for background sound, designers and engineers heavily overcompensated by using additional materials to provide greater isolation and absorption. This hyper-focused approach on objective components of acoustics consistently failed to appreciate the importance of human factors—namely, that a space can be perceived as quiet.
It is in the consideration of the pseudo-subjective evaluation of acoustical privacy—estimating acoustical privacy of a space using a combination of objective metrics (i.e. measure of isolation and background sound)—that we have the opportunities to realize cost savings at the design stage of a project.
Consider the following simplified scenario, which is intended to quickly illustrate the approach:
- an enclosed room conforming to a field-tested STC-45 rating requires a 30 dBA background sound level for speech privacy; and
- alternatively, if the background sound level is raised to a constant level of 35 dBA, the composite performance of the construction of the environment (walls, ceiling, floor) can be reduced to STC-40 rating.
A mere five STC points may not seem significant; however, the cost savings in terms of materials, labor, and time can be. Also, if one reduces the STC rating of the partition by five points and raises the controlled background sound levels by 10 dBA to a level of 40 dBA, acoustical privacy is more assured and the psychoacoustics of the space are improved.
A more assertive effort could pursue the reduction of the STC rating of the partitions from STC-45, which relies on 30 dBA of background sound, to an STC-35 with 40 dBA of background sound (this considers field-tested STC values reflecting composite acoustic performance of all sound transmission paths). Providing the background sound is precisely generated and consistently delivered by the masking system, this enhanced design process affords the opportunity to explore new options including:
- greater selection of materials (performance, cost);
- cost of labor associated with installation (difficulty, and the time required);
- building to the ceiling instead of to the deck, with appropriate selection of acoustical tile with isolation properties (ceiling attenuation class [CAC] criteria);
- reduction in quantity of materials and labor (installation time, difficulty, and complexity);
- ease of reconfiguration of a space due to demising walls;
- layout consideration and post-building adjustments (facility flexibility);
- the cost of continuing the partition above the ceiling to the deck is more significant and the difficulty in installation (height of plenum, penetrations, interferences); and
- reduction in material waste and environmental repercussions.
As mentioned earlier, using masking to control background sound levels also acts as an ‘insurance policy.’ Should the space underperform—which can occur for any number of reasons—there still remains the opportunity to raise the background sound level upward to 45 dBA for enclosed spaces and 48 dBA for open areas to improve the psychoacoustical measures
of the space.
Although sound is ubiquitous—a constant and inescapable experience—its positive role within the built environment is underappreciated, leading to ongoing debate about the control (or lack thereof) of background sound levels within various types of facilities. Reviewing the technical and popular use of various words allows one to gain appreciation of how they can lead to misunderstandings of what it takes to achieve an effective acoustic environment and, more specifically, the role played by background sound set to a controlled level and spectrum. The distinction between ‘noise’ and ‘sound’ is expansive and the implications are significant in terms of subjective and objective attributes of the built environment. It is by refining the definitions of those terms, as well as that of ‘silence’ and ‘quiet,’ that appreciable opportunities can be fostered to improve the design of the built environment and promote occupant well-being.