{"id":104820,"date":"2020-02-21T00:00:12","date_gmt":"2020-02-21T05:00:12","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=104820"},"modified":"2020-01-28T15:27:36","modified_gmt":"2020-01-28T20:27:36","slug":"specifying-ceiling-panels-with-a-high-nrc","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/specifying-ceiling-panels-with-a-high-nrc\/","title":{"rendered":"Specifying ceiling panels with a high NRC"},"content":{"rendered":"

by Gary Madaras, PhD<\/strong><\/p>\n

\"Photo<\/a>
Photo \u00a9 Robert Pepple, Images courtesy Rockfon<\/span><\/em><\/figcaption><\/figure>\n

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\u2019s 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.<\/p>\n

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<\/em>, 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.<\/p>\n

\"Building<\/a>
Building standards and guidelines require good acoustics and are evolving with more stringent requirements. Many now require the use of high noise reduction coefficient (NRC) ceiling panels.<\/span>
Photo \u00a9 Joe Ciarlante<\/span><\/em><\/figcaption><\/figure>\n

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.<\/p>\n

\u00a0Review of acoustic ceiling panel<\/strong><\/p>\n

Modular acoustic ceilings comprise of a metal suspension grid\u2014ceiling panels made from a porous and fibrous material, such as stone wool, fiberglass, or mineral fiber\u2014and various building systems elements, such as light fixtures, air terminals, sprinklers, and speakers.<\/p>\n

The ceiling panels are made of lightweight materials that are typically less than 5 kg\/m2<\/sup> (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.<\/p>\n

A ceiling panel\u2019s 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.<\/p>\n

<\/p>\n

Sound absorption in the standards, guidelines, and rating systems<\/strong><\/p>\n

\"Modular<\/a>
Modular acoustic ceilings comprise a metal suspension grid, ceiling panels, and various building system elements, such as light fixtures, air terminals, sprinklers, and speakers. Even when the ceiling panels offer high sound absorption, these components may result in unwanted sound transfer.<\/span><\/figcaption><\/figure>\n

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<\/em>, 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.).\u00a0According to Sound Matters: How to achieve acoustic comfort in the contemporary office<\/em>, a related GSA document, published in December 2011:<\/p>\n

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.<\/span><\/p>\n

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).\u00a0Complying with this criterion improves the functioning of the cardiovascular, endocrine, and nervous systems of the building\u2019s occupants.<\/p>\n

Section 5.3 of ANSI\/Acoustical Society of America (ASA) S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools<\/em>, 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.<\/p>\n

Why NRC 0.90?<\/strong><\/p>\n

\"Figure<\/a>
Figure 1<\/strong>: In this figure, speech privacy (vertical axis) improves with lower speech intelligibility index (SII) values. Once ceiling sound absorption average (SAA)\/NRC is 0.90 and higher, acceptable speech privacy can be achieved in open offices (gray box in lower right). Each incremental increase in SAA\/NRC above 0.90 results in an appreciable improvement to speech privacy.<\/span><\/figcaption><\/figure>\n

While it is known good acoustic design requires high-performing sound absorption of NRC 0.90 or higher, the question of \u2018why\u2019 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.<\/p>\n

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 \u201cAcoustic Design Guide for Open Offices,\u201d \u201cMeasurements of Sound Propagation between Mock-up Workstations,\u201d \u201cAcoustical Design of Conventional Open Plan Offices,\u201d \u201cA Renewed Look at Open Office Acoustical Design,\u201d and \u201cAcoustical Design for Open-plan Offices\u201d for more information.).\u00a0Those 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.<\/p>\n

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.<\/p>\n

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).<\/p>\n

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.<\/p>\n

<\/p>\n

\u00a0Seeing the NRC 0.90 difference<\/strong><\/p>\n

\"In<\/a>
In modular ceiling systems, panels are made of lightweight materials such as stone wool, fiberglass, or mineral fiber that 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 and creating privacy, comfort, and freedom from distractions in large open spaces.<\/span><\/figcaption><\/figure>\n

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?<\/em> by Gary S. Madaras).<\/p>\n

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).<\/p>\n

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\u2014plaque-style\u2014air 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.<\/p>\n

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.<\/p>\n