Sound absorption in the standards, guidelines, and rating systems
ANSI/GBI 01-2019 requires minimum ceiling absorption of NRC 0.90 in open offices, patient and eldercare areas, medication safety zones in healthcare facilities, and exam and treatment rooms in medical office buildings. The General Services Administration (GSA) PBS-P100, Facilities Standards for The Public Buildings Service, requires ceilings over open offices areas to be NRC 0.90 or higher for 100 percent of the space (Referenced from Table 3.1 on page 101 of the 2018 version of GSA PBS-P100. This rating is for when sound masking is not used in the space.). According to Sound Matters: How to achieve acoustic comfort in the contemporary office, a related GSA document, published in December 2011:
Open workspaces require acoustical treatment on a significant portion of the surfaces in the space to absorb noise from people and equipment. The more absorptive the material added to the open space and the higher the acoustical performance rating of the material, the more acoustically comfortable the environment will be. Two surfaces are key contributors to absorption: high quality acoustic ceiling material is typically the most significant contributor to sound absorption. Similarly, walls may be treated with acoustic material, either applied to a surface or integral with the wall finish.
The WELL Building Standard requires the ceiling over an open office space is NRC 0.90 or higher for the entire surface area exclusive of light fixtures and air devices (Details can be found in the Comfort section 80 on page 130 of version one of the WELL Building Standard. While the second version pilot has been released, it is still to undergo a public review process, and as such, is not yet finalized). Complying with this criterion improves the functioning of the cardiovascular, endocrine, and nervous systems of the building’s occupants.
Section 5.3 of ANSI/Acoustical Society of America (ASA) S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, requires core learning spaces to be designed in a way their RTs can be readily adapted to achieve RTs not greater than 0.30 seconds. To achieve this level of performance, the ceiling of an average-sized classroom needs to be a minimum of NRC 0.90, possibly even higher, and some wall or floor absorption is required as well.
Why NRC 0.90?
While it is known good acoustic design requires high-performing sound absorption of NRC 0.90 or higher, the question of ‘why’ might still linger in the minds of some architects and specifiers. An abbreviated answer to this question is because exhaustive and conclusive research has shown the benefits of it.
In the early 2000s, the National Research Council Canada (NRCC) methodically isolated and tested a set of a dozen physical features of open offices relative to speech privacy (Read the National Research Council Canada (NRCC) reports on “Acoustic Design Guide for Open Offices,” “Measurements of Sound Propagation between Mock-up Workstations,” “Acoustical Design of Conventional Open Plan Offices,” “A Renewed Look at Open Office Acoustical Design,” and “Acoustical Design for Open-plan Offices” for more information.). Those features included ceiling absorption and height, screen wall absorption and height, light fixtures, workstation size, furnishings, etc. Some studies used mockups of actual cubicles in open spaces. Other studies used sophisticated acoustical analysis software based on the image sources technique. These studies used the acoustic metric sound absorption average (SAA), which is very similar to NRC.
Of the design features studied, it was found ceiling absorption, screen wall height, and workstation plan size have the largest effects on speech privacy. The most significant noise paths are those that reflect sound from the ceiling and diffract sound over the separating screen wall. However, only a limited range of these parameters will lead to acceptable speech privacy.
The ceiling is a critical element. There are no obstacles to prevent sound from reaching the ceiling and being reflected down to other areas. The absorptive properties of the ceiling can have a large effect, but speech privacy values are only substantially reduced for highly absorbing ceiling tiles. For a wide range of medium- and low-absorption ceiling tiles (NRC 0.50 to 0.80), acceptable speech privacy is unachievable and not influenced much by the ceiling absorption because too much sound is still reflected off these low-performing panels (Figure 1).
The main conclusion about ceiling absorption, after years of intense investigation, is a minimum ceiling absorption for achieving acceptable speech privacy is an NRC/SAA of 0.90. In practice, it would be better to have an even more absorptive ceiling than this to compensate for any limitations or absence of the other important design parameters.
