Controlling office noise starts with a sound-absorbing ceiling that carries an NRC 0.90 rating.
By Gary Madaras, Rockfon
Every week there seems to be another compelling report about how bad open offices are for people, their well being, and their productivity. “Very few companies have taken meaningful steps to address the problem: noise is an afterthought in office construction and executives overestimate the ability of employees to drown it out with the tools available to them (“When the walls come down: How smart companies are rewriting the rules of the open workplace,” Oxford Economics, oxfordeconomics.com).” Far less frequently, evidence emerges that open offices are good for team collaboration, but private areas must be available so people can work without interruptions.
To limit the amount of speech or noise that gets transmitted from one work station to another, there are proven design practices, including the size and layout of the space and the types of room surfaces and finishes. According to the “Acoustic Design Guide for Open Offices” (Warnock, A.C.C., National Research Council Canada report IRC-RR-163, March 2004) the following design approaches will minimize speech transmission between workstations:
• Use a sound-absorbing ceiling with a minimum noise-reduction coefficient (NRC) of 0.90.
• Place barriers between workstations with a minimum height of 65 in., minimum NRC of 0.75, and minimum sound-transmission class (STC) of 20.
• Install carpeting on the floor. Normal commercial-grade carpeting is acceptable.
• Include an electronic sound-masking system that has been tuned to approximately 45 a-weighted decibels (dBA) and the proper frequency spectrum.
• Lay out the office to prevent direct lines of sight and sound between employees.
Today, it is often difficult to tell if one is in a modern open office or a restaurant with digital devices. Trends in aesthetic design and sustainability have seemingly moved open-office design away from tall barriers between workstations, or workstations at all. With them went any occupant density control and any ability to block direct lines of sight and sound for visual and acoustic privacy. Carpeting on the floor has gone by the wayside as well.
The move away from at least three of the five main components of good open-office acoustic design places far greater emphasis on the necessity of the remaining two—overhead sound absorption and sound masking. People who fail to implement these two components in their designs typically find themselves scrambling soon after occupancy to add them in some manner or another.
Overhead Sound Absorption
Here is a short review of the fundamental terminology for the acoustic performance of sound-absorbing ceilings and other types of overhead systems.
• Noise-reduction coefficient: NRC is the most frequently used and specified metric to quantify the sound-absorption capabilities of a material or surface. It generally ranges from 0.0 to 1.0 in 0.05 increments. Higher values indicate more sound absorption. It can be categorized as good at 0.70+, better at 0.80+, and best at 0.90+. NRC is the average of the one-third-octave band absorption coefficients at 250, 500, 1,000, and 2,000 Hz. NRC is used when the acoustic concern is multiple sound waves reflecting around inside a room at random angles and potentially interfering with speech intelligibility and/or overall loudness and comfort. NRC does not apply to three-dimensional absorbers hung free in space, such as baffles, clouds, and islands.
• Sound-absorption average: SAA is very similar to NRC and is in a rather long process of replacing NRC. Whether or not SAA will eventually replace NRC completely is yet to be determined. The main differences include the use of all one-third-octave band absorption coefficients in the average as opposed to just the four included in NRC. This accounts for potential performance anomalies at specific frequencies that NRC does not include. SAA is also rounded to the nearest 0.01 as opposed to NRC’s 0.05 rounding. Performance differences near 10% can round to the same NRC rating, not so with SAA. Manufacturers do not yet typically report SAA, nor do architects specify it.
• Articulation class (AC) and NRC are highly correlated: The values generally range between 150 for hard, sound-reflecting surfaces and 210 for highly sound-absorptive surfaces. A goal value for open offices is 180 or higher, which correlates with an NRC of 0.90. The values do not carry any units, making it a bit of a challenge to relate to them. AC is used when the main acoustic concern is sound waves from a single, fixed source reflecting at a specific angle off the ceiling over a cubicle wall from one workstation to another and preventing speech privacy. Because AC is so correlated with NRC and the values are hard to relate to, architects generally prefer NRC in their specifications even though AC would technically be the better metric to specify. Some manufacturers report AC as well as NRC.
• Sabins: Sabins are the unit of sound absorption. When the NRC of a material is multiplied by the area of the material, the result is Sabins of absorption. For example, 10 sq. ft. of a material with an NRC rating of 0.90 provides 9 Sabins of absorption. NRC cannot be used for three-dimensional absorbers hung free in space, such as islands, clouds, and baffles. Sabins of absorption are used instead.
Some manufacturers incorrectly report NRC values for these types of absorbers. As a result, some architects specify them incorrectly as well. More technically savvy manufacturers will report Sabins of absorption by frequency for their products. When dealing in Sabins, be careful with the units of area. There are Sabins and metric Sabins, depending on how the area was calculated. Note that ASTM has phased out the use of the word Sabin, but product manufacturers still rely on and report it.
• Ceiling attenuation class: CAC is a metric that applies to acoustic ceiling panels, but is a measure of the panel’s capacity to prevent sound traveling from one enclosed room over an interior partition and through an open ceiling plenum into another room. CAC does not apply to open offices. It should not be included in specifications for ceiling panels in large open spaces without walls.
Standards, Guidelines, Rating Systems
The General Services Administration (GSA), Washington, Facilities Standards for The Public Buildings Service (P-100) requires ceilings over open offices areas to be NRC 0.90 or higher for 100% of the space. This information can be found in table 3.1 on p. 101 of the 2018 version.
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 (Comfort section 80, p. 130, v. 1) requires that the ceiling over open office spaces be NRC 0.90 or higher for the entire surface area, exclusive of light fixtures and air devices. Complying with this criterion improves the functioning of the cardiovascular, endocrine, and nervous systems of building occupants.
Why NRC 0.90?
While it is known that good acoustic design of open offices 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 it to be necessary.
In the early 2000s, the National Research Council Canada (see references at the end of this article) methodically isolated and tested a dozen physical features of open offices relative to speech privacy between workstations. Those features included ceiling absorption and height, screen-wall absorption and height, light fixtures, workstation size, and furnishings. Some studies used mockups of actual cubicles in open spaces. Other studies used sophisticated acoustical-analysis software, based on the image sources technique.
Of the office-design features studied, it was found that ceiling absorption, screen-wall height, and workstation plan size have the largest effects on speech privacy in open offices. The most significant noise paths are those that reflect sound from the ceiling and diffract sound over the separating screen-wall. However, only a very limited range of these parameters will lead to acceptable speech privacy.
The ceiling is a critical element in any open office. There are no obstacles to prevent sound from reaching the ceiling and being reflected down into adjacent cubicles. The absorptive properties of the ceiling can have a large effect, but speech-privacy values are only substantially reduced for quite 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 not achievable and not much influenced by the ceiling absorption because too much sound is still reflected off these low-performing panels.
The main conclusion about ceiling absorption after years of intense investigation is that a minimum ceiling absorption for acceptable speech privacy is NRC/SAA 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.
Seeing NRC 0.90 Difference
The foundational studies conducted by the National Research Council of Canada 18 years ago have been corroborated more recently by research presented at InterNoise 2018 (“Look, Do You See the Noise Leaking Through that Ceiling?” Madaras, G.S., InterNoise 2018, Chicago).
A sound-intensity probe was used to scan an acoustic-ceiling system with panels of different absorption performance levels of NRC 0.60 to 0.95 while loud, broadband noise was being played in the space under it. A high-definition camera and analysis software tracked the probe location and the sound intensity levels it measured. These location-specific sound-intensity data were then processed into color sound maps, which were overlaid onto the digital image of the ceiling.
Yellow and red colors in Fig. 2 indicate loud noise reflecting off the acoustic ceiling. Blue indicates noise being absorbed by the acoustic ceiling. Red areas are mostly caused by noise reflecting off the hard, painted-metal, plaque-style air diffuser and from light fixtures. Note that the open return-air grille on the right side of the images (blue) acts as an effective sound absorber because the noise passes through the opening into the plenum and is not reflected back. The base question is: At what NRC rating does an acoustic ceiling stop behaving like a reflector (red and yellow) and behave more like an effective absorber (blue)? Based on the series of images in Fig. 2, the answer is NRC 0.90.
Implementing the required level of sound absorption overhead does not mean that every open office must have a standard acoustic ceiling from wall to wall. There are numerous materials, systems, and combinations thereof that can provide the necessary amount of sound absorption.
While a standard acoustic wall-to-wall ceiling might not be desired as the aesthetic for all open office spaces, when appropriate, the solution should be obvious at this point. Select a ceiling panel with an NRC of 0.90 or higher. Stone wool and glass-fiber ceiling panels can easily achieve this level of performance, and higher, without a large incremental cost increase, compared with lower-performing panels. Wet-felted mineral-fiber panels are not able to achieve NRC 0.90.
If a contiguous ceiling is acceptable, but a different or higher-grade finish is desired, consider perforated or slotted ceiling systems made from metal or wood. Most of these systems offer a variety of finishes and perforation patterns. To achieve an NRC rating of 0.90 or higher, a more open perforation pattern of 5% to 10% or more, and the use of a fibrous batt on the topside will most likely be required. If fiber erosion is a concern, some manufacturers offer encapsulated fibrous batts.
Should a ceiling not fit with a project’s desired aesthetic, know that an equal amount of absorption can be achieved with a variety of acoustic metal decks, spray-on deck treatments, suspended horizontally oriented clouds or islands, or vertically oriented baffles. The first step is to determine how many Sabins of absorption a ceiling with NRC 0.90 would have provided if it would have been used. For every 10 sq. ft. of open office space, an NRC 0.90 ceiling would provide 9 Sabins of absorption. Another way to look at is to multiply the area of the open office by 0.9 Sabins/sq. ft. to determine the total number of Sabins required by any other solution.
Example: A 2,500-sq.-ft. open office area should have 2,250 Sabins of absorption over it. (2,500 sq. ft. x NRC 0.90 = 2,250 Sabins)
The amount of sound absorption provided by acoustic products and materials varies by frequency. Therefore, the number of Sabins varies by frequency as well. Ideally, the acoustic solution used should provide the calculated total number of Sabins at all frequencies.
In reality, it becomes increasingly difficult to meet this ideal absorption goal for frequencies below 500 Hz. Try to reach this ideal goal at 500 Hz and above. It might be necessary to accept a lower amount of absorption below 500 Hz unless commercially available, specialty low-frequency absorbers are incorporated into the design. This is seldom done because the most common and distracting noises inside open offices areas occur at 500 Hz and higher.
Consider combining systems if no single approach provides all of the necessary absorption. For example, perhaps an array of vertically suspended acoustic baffles is desired visually, but too many baffles in too tight of an array are required. Instead, consider using an aesthetically pleasing baffle array and accomplish the additional absorption by using a lower-performing acoustic-metal deck above them or apply a treatment to the underside of the deck. Often, the best acoustic experiences are a result of using a variety of materials, each with their own acoustic advantages, arranged at different heights and orientations. There is no need to become overly concerned with heights and spacing. The most critical design aspect is the total amount of absorption. Try to distribute it somewhat uniformly over the entire open-office area.
Architects and specifiers should not feel limited by the acoustic requirements of the spaces they are designing. While it is has been known for decades that treatments of NRC 0.90 or higher are required over open offices to achieve an acceptable level of speech privacy, there are numerous design options ranging in aesthetics and budget. With so many choices, the acoustics can, in fact, be the creative inspiration for the space. Also remember that an NRC 0.90 ceiling over an open office space is only one of at least five requirements needed to make a space successful. No single aspect alone guarantees acoustic success.
Gary Madaras, PhD, ASA, INCE, Assoc. AIA, is the acoustics specialist at Rockfon North America, Chicago, where he helps designers and specifiers learn the Optimized Acoustics design approach. Madaras is a member of the Acoustical Society of America (ASA), the Canadian Acoustical Association (CAA), and the Institute of Noise Control Engineering (INCE). He can be reached at email@example.com.
Here are some additional acoustical-design resources:
• “Acoustic Design Guide for Open Offices,” Warnock, A.C.C., National Research Council Canada report IRC-RR-163, March 2004.
• “Measurements of Sound Propagation between Mock-Up Workstations,” Bradely, J.S., Wang, C., National Research Council Canada report IRC-RR-145, January 2001.
• “Acoustical Design of Conventional Open Plan Offices,” Bradley, J.S., National Research Council Canada report NRCC-46399, June, 2003.
• “A Renewed Look at Open Office Acoustical Design,” Bradley, J.S. National Research Council Canada report NRCC-46399, August, 2003.
• “Acoustical Design for Open-Plan Offices,” Bradley, J.S., National Research Council Canada construction update no. 63, October, 2004.