Specifying ceiling panels with a high NRC

by nithya_caleb | February 21, 2020 12:00 am

by Gary Madaras, PhD

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Standards now require high-performance sound absorption overhead inside many building types. Understanding the acoustic requirements within the building standards and translating those correctly into a project’s written specification is only the first step. One should also understand other potential impacts on the building design to ensure the occupied building sounds good overall when completed.

Section 11.5.4. of the new American National Standards Institute/Green Building Initiative (ANSI/GBI 01-2019), Green Globes Assessment Protocol for Commercial Buildings, contains sound absorption requirements for various rooms in office buildings, schools, and healthcare facilities. There are two compliance paths: one being minimum ceiling noise reduction coefficient (NRC) and the other is maximum reverberation time (RT). Patient care areas in healthcare facilities and resident care areas in senior living facilities are required to have ceilings with a minimum NRC of 0.90, or if a portion of the absorption is provided on the walls and floor, a maximum RT of 0.50 seconds. In this example, writing the project specification is straightforward. The specified ceiling panel should have an NRC of no less than 0.90. This ensures compliance with this section of the standard.

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There can be, however, other potential impacts on the building design and compliance with other sections of the standard. Section 11.5.1.2 requires the minimum sound transmission class (STC) rating of the floor/ceiling assembly to be 50. The design and specification of the floor slab, floor finishes, underlayments, and high NRC ceiling system must comply with the STC-50 requirement. Meeting the absorption requirement in 11.5.4 does not stand alone. It is related to meeting the floor-to-floor, airborne, noise isolation requirement as well, and one should not compromise the other. As project specifications are amended to comply with the high NRC requirements, there are other building design impacts that should be considered.

 Review of acoustic ceiling panel

Modular acoustic ceilings comprise of a metal suspension grid—ceiling panels made from a porous and fibrous material, such as stone wool, fiberglass, or mineral fiber—and various building systems elements, such as light fixtures, air terminals, sprinklers, and speakers.

The ceiling panels are made of lightweight materials that are typically less than 5 kg/m² (1 psf) and are fibrous and porous to permit the airborne sound energy to enter the panels and dissipate due to conversion to heat energy. This process decreases noise and reverberation, making speech more intelligible in enclosed rooms like classrooms and creating privacy, comfort, and freedom from distractions in large open spaces. In most everyday spaces, such as schools, healthcare facilities, offices, and restaurants, the higher the amount of sound absorption, the better.

A ceiling panel’s ability to absorb sound is indicated by NRC, an acoustics metric that averages the measured absorption coefficients at the 250, 500, 1000, and 2000 hertz (Hz) octave bands. NRC can be categorized as high-performance when is it 0.90 or higher. Many of the standards, guidelines, and rating systems require a minimum ceiling absorption of NRC 0.90. If the absorption is not provided by a contiguous acoustic ceiling, the alternatives should offer an equivalent amount of sound absorption.

Sound absorption in the standards, guidelines, and rating systems

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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?

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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.

 Seeing the NRC 0.90 difference

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The foundational studies conducted by NRCC 20 years ago have been corroborated more recently by research conducted by an acoustic ceiling manufacturer and presented at InterNoise 2018, an exposition on noise control engineering (Consult Look, Do You See the Noise Leaking Through that Ceiling? by Gary S. Madaras).

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 played in the space under it. A high-definition camera and analysis software tracked the location of the probe 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 (Figure 2).

Yellow and red colors in Figure 2 indicate loud noise reflecting off the acoustic ceiling while 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 light fixtures. Note 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. 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 Figure 2, the answer is NRC 0.90.

The perception of what constitutes high-performance sound absorption has slipped over time. Some have come to believe NRCs as low as 0.70 to 0.75 are acceptable, but as the sound intensity scans in Figure 2 show, at that level of performance, the ceiling is still acting more as a noise reflector than absorber. Fortunately, building standards, guidelines, and rating systems are now reinforcing what science has shown for decades.

 Improved well-being

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In 2003, a multiorganizational study conducted by academic institutions in the United States and Sweden inside a Swedish hospital investigated the effect high-performance, sound-absorptive ceiling panels had on the quality of care and physiological state of patients in an intensive coronary care unit (ICCU) (See Influence of Intensive Coronary Care Acoustics on the Quality of Care and Physiological State of Patients by Inger Hagerman et al.). The study was conducted in an actual hospital ICCU under clinical conditions and involved 94 coronary patients.

During the 20-week baseline condition, the ceiling was hard and sound-reflective. Patient physiology (heart rate and variability, blood pressure, pulse, etc.) and acoustic conditions (RT) were recorded. At the end of the baseline condition, the ceilings in the ICCU were changed to high-performing, Class A, acoustic ceiling panels. Class A in Europe is equivalent to NRC 0.90 in the United States.

Patient physiology and acoustic conditions were then monitored for another 22-week period. The study showed the effects of the high-performing, acoustic ceiling panels decreased patient heart pulse amplitude and their rehospitalization rate. In other words, the high-performance ceiling panels reduced stress in the patients’ bodies and cardiovascular systems, while they were in the hospital, resulting in fewer of them being readmitted after three months due to complications or reoccurrence.

In 2010, researchers from the Department of Psychology at the University of Kaiserslautern in Germany studied the effect of RT and noise on speech perception and listening comprehension in child and adult students (Read Effects of Noise and Reverberation on Speech Perception and Listening Comprehension of Children and Adults in a Classroom-Like Setting by Maria Klatte, Thomas Lachmann, and Markus Meis, 2010.). Acoustics conditions in a real classroom, before and after an acoustic renovation, were measured and then simulated in the laboratory of the university’s Hearing Research Center.

The acoustically unfavorable condition, representing surfaces with low sound absorption performance, had an RT of 1.10 seconds. The acoustically favorable room, representing surfaces with high-performing sound absorption, had an RT of 0.47 seconds (as mentioned, ANSI/ASA S12.60 recommends classrooms be adaptable to an RT of 0.30 seconds). More than 350 first-graders and third-graders, and adults were tested under both acoustic conditions. The results showed with the high-performance sound absorption present, speech recognition improved significantly in all test subject groups. As an example, with the high-performance absorption in place, first graders sitting in the third row recognized 22 percent more words than when the acoustic conditions were unfavorable. Similarly, adults recognized 18 percent more words under the favorable acoustic conditions.

 Ceiling alternatives

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While the standards often state the absorption requirements as ceiling NRC, it does not necessarily mean every space must have a standard, contiguous, modular, acoustic ceiling. That approach does not always agree with the preferred architectural style or aesthetic of the room or building. Other sound absorptive systems are permitted if they provide at least the same amount of absorption as an acoustic ceiling of NRC 0.90.

An equal amount of absorption can be achieved with a variety of acoustic metal or wood ceilings—suspended, horizontally oriented, clouds, or islands—or vertically oriented baffles. The first step is to determine how much absorption a ceiling with NRC 0.90 would have provided.

A sabin is the unit of sound absorption. Sound-absorptive elements that hang free in space, such as acoustic islands and baffles, have their performance specified in sabins rather than NRC. There are both metric and imperial sabins—it is important to distinguish between the two when reviewing product information.

 Metric

For every square meter of floorspace, NRC 0.90 ceiling provides 0.9 metric sabins of absorption. Another way to look at it is to multiply the total area of the floor in square meters by 0.9 metric sabins per square meter to get the total number of metric sabins required inside the room by any absorption system.

Example

A 100-m² (1076-sf) classroom should have 90 metric sabins of absorption over it (100 m² x NRC 0.90 = 90 metric sabins). If a baffle provides 1 metric sabin of absorption each, then 90 baffles would be needed over the classroom.

Imperial

For every square foot of floorspace, an NRC 0.90 ceiling provides 0.9 imperial sabins of absorption. Therefore, multiply the area of the floor in square feet by 0.9 sabins per square foot to get the total number of imperial sabins required inside the room by any absorption system.

Example

A 1000-sf (93-m²) classroom should have 900 imperial sabins of absorption over it (1000 sf x NRC 0.90 = 900 imperial sabins). If a baffle provides 10 imperial sabins of absorption each, then 90 baffles would be needed.

Conclusion

Building standards, guidelines, and rating systems are now requiring high-performance acoustic absorption overhead with a minimum NRC of 0.90. This requirement is based on both quantitative research and studies showing the positive impact on occupant well-being in offices, schools, and healthcare facilities. Specifiers should remember complying with the high NRC requirement may impact other acoustical requirements, such as minimum airborne sound isolation between vertically oriented rooms. Other sound-absorptive systems, such as acoustic islands and baffles, can be used in lieu of ceilings if they provide equivalent absorption.

[14]Gary Madaras, PhD, is an acoustics specialist at Rockfon. He helps designers and specifiers learn the optimized acoustics design approach and apply it correctly to their projects. He is a member of the Acoustical Society of America (ASA), the Canadian Acoustical Association (CAA), and the Institute of Noise Control Engineering (INCE). He authors technical articles and speaks publicly on the topic of optimizing acoustic experiences. Madaras can be reached at gary.madaras@rockfon.com[15].

Source URL: https://www.constructionspecifier.com/specifying-ceiling-panels-with-a-high-nrc/

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