June 11, 2018
by Robert (Bob) Marshall
As the architecture and design community takes a more holistic approach to occupant health, practitioners are increasingly focusing on creating acoustically sound spaces. The increased awareness of the importance of acoustics as well as a growing array of product options have made ceiling panel specification more complex, but also more significant.
For a variety of reasons, including overall acoustic performance, widespread availability, ease of installation and affordability, traditional suspended ceilings are an effective solution for many types of spaces.
Today’s ceiling panel choices include a wide range of acoustic performance levels, visuals, and price-points, from basic, budget-friendly fissured mineral fiber panels with 0.55 noise reduction coefficient (NRC) and 33 ceiling attenuation class (CAC) to smooth, white fiberglass panels with 0.95 NRC and 30 CAC, which can cost as much as 10 times more. The choice of ceiling panel can have a dramatic impact on the design of a space and the experience of its occupants.
Specifiers are rarely dealing with a single source or type of sound throughout a facility, nor does every space have the same acoustical goals. An area in which speech privacy is the most important consideration will require a different solution than one in which speech clarity is the highest priority. Is there background noise from mechanical systems to consider? Will sound come from everywhere in the space (e.g. cafeteria) or from one specific location (e.g. the front of a lecture hall)? The key is to understand the basics of sound, and to address the acoustic needs of each space individually while keeping in mind how the aesthetic of individual spaces relates to the project as a whole.
It is tempting to turn to a single-product solution in a straightforward, good-better-best hierarchy, but such general acoustical performance classifications can oversimplify the issue. While they may require less effort to specify, the odds are high either the acoustic performance, the overall aesthetic, or the budget will be compromised by limiting the spec to a single product. For example, in most open office situations, there is little reason to invest in a panel with both high NRC and high CAC. Sound absorption is key for open spaces, and unless there are unusual sources of sound (e.g. mechanical noise), sound-blocking capability is typically not necessary in these areas. A panel combining both high absorption and high sound blocking will do nothing to improve the acoustics of the space, but will potentially double the cost of the ceiling. The smarter approach is to target the right products with the right level of sound absorption and/or sound blocking to the needs of each space. Fortunately, there are product collections available that enable designers to target both performance and budget while maintaining a consistent visual throughout a facility.
Avoid the single-product compromise
Consider the challenge encountered by interior designers from Jacobs working on the Saint-Gobain North American Headquarters in Malvern, Pennsylvania. The original plan for the 11,055 m2 (119,000 sf) of open-office space called for smooth, high-NRC fiberglass panels to be installed in a floating island configuration over the workspaces, forming a dramatic contrast with the black-painted plenum and exposed ductwork in the corridors and transition areas. If the only noise source in the space had been employee speech, this plan would have been ideal. Unfortunately, there were other sound sources to consider, and while fiberglass provides excellent sound absorption for human speech, it would do little to block the low-frequency, mechanical noise produced by the HVAC system.
In a situation like this, there are several single-product options including:
Another alternative is a multiproduct solution leveraging targeted acoustic performance.
In the end, fiberglass panels with 0.95 NRC were used for most of the ceiling to absorb background speech noise in the open office. Directly below the heat pumps, a premium composite ceiling panel performed a dual acoustic function: the fiberglass face absorbs sound, while a mineral fiber backer blocks the mechanical noise from above. Most importantly from an aesthetic perspective, the difference in tiles is invisible to the occupant. Both tiles share the same clean, minimally textured surface and edge detail for a perfect match. The look also matches the mineral fiber tiles chosen for the smaller conference and meeting rooms.
Instead of an open plenum painted black, black fiberglass panels were installed around the perimeter to create the visual effect of free-floating islands while providing additional sound absorption.
|With noise reduction coefficient (NRC) ranging from 0.80 to 1 fiberglass is more effective than mineral fiber at quieting a room, as it performs well in both high and low frequencies. Mineral fiber tends to excel in high frequencies but loses absorption in lower frequencies. However, the low density of fiberglass ceiling panels, which makes them resistant to moisture and sagging, limits their ability to contain sound. Consequently, fiberglass ceiling panels typically have a very high NRC, but a relatively inconsequential CAC.
Mineral fiber ceiling panels are denser and heavier than those made from fiberglass. Their higher level of density makes them reasonably effective sound attenuators (i.e. they impede the transmission of sound from room to room). With good sound attenuation and average sound absorption, mineral fiber ceiling panels tend to have a higher CAC and lower NRC—typically 0.50 to 0.70 NRC—than their fiberglass counterparts.
These are basic characteristics, but not inviolable rules. As manufacturing methods and technologies evolve, major ceiling brands have developed mineral fiber panels with NRC as high as 0.75 and 0.80, as well as composite panels combining a highly absorbent fiberglass face with a sound-blocking mineral fiber backing for premium acoustic performance of 0.90 NRC and CAC up to 42.
ACOUSTIC PANEL RATINGS: WHAT ARE NRC AND CAC?
|The most common piece of data used to evaluate acoustic panels is the noise reduction coefficient (NRC). To appreciate the value—and limits—of NRC ratings, a little more background is needed on the subject of frequency.
Frequency is the measurement of how often a wave passes a fixed point in one second. Measured in hertz (Hz), it dictates the pitch of the sound and its audibility to the human ear. High frequency sounds are high pitched. Low-frequency sounds are low pitched. The human voice has a general range between 80 to 240 Hz for men and 140 to 500 Hz for women. For comparison, a piano produces sounds between 27.5 and 4186 Hz. The human ear can distinguish sounds between 20 and 20,000 Hz.
The NRC of a product is determined by averaging how much sound it absorbs at four frequencies: 250, 500, 1000, and 2000 Hz. This average can be represented simplistically as a decimal between 0.00 (no absorption at all) and 1.00 (complete absorption), rounded to the nearest 0.05. For general purpose interior acoustic decisions, this average is an acceptable measurement.
Another key piece of information for evaluating ceiling panels’ acoustic performance is ceiling attenuation class (CAC). This is the measurement of the ceiling’s ability to inhibit sound transmission between two rooms separated by a ceiling-height partition (per ASTM E1414, Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum). Ceilings with a 30 CAC or less do a poor job and should rarely be considered for this type of application. However, those offering a 40 CAC or more can actually be considered as a substitute for a full-height partition between rooms.
Meeting multiple needs with one consistent look
The design team from Granum A/I, the interior architects on the 49-story FMC Tower at Cira Centre South in Philadelphia, encountered a similar challenge when they helped FMC Corporation transition from a traditional, closed-office environment to an open, collaborative workspace with floor-to-ceiling windows for daylight and inspiring views.
The aesthetic goal was to maintain a smooth, consistent ceiling visual throughout the spaces.
For all open-office spaces, breakout rooms, quiet areas, and corridors, designers specified a smooth fiberglass panel with 0.95 NRC to achieve maximum sound absorption.
Enclosed huddle and conference rooms, and private offices had a different challenge: containing sound and maintaining speech clarity. For these spaces, a mineral fiber panel with 0.70 NRC and 33 CAC struck the right balance between absorption and blocking. A version of the same panel with enhanced cleanability, stain resistance, and water repellency was chosen for the kitchen areas.
Sound containment and speech privacy were the top priority for the executive offices, which used a premium composite panel with 0.90 NRC and 42 CAC.
All four products featured a consistent clean, white surface appearance to reflect the abundant daylight throughout the offices.
Challenges of educational facilities
The relative importance of acoustics, aesthetics, and budget changes when one is designing public educational facilities rather than corporate offices. When Goodwin Mills Cawood (GMC) took on the challenge of designing the Mill Creek campus as the prototype for future Williamson County, Tennessee, schools, the first priority for ceiling specification was meeting the Leadership in Energy and Environmental Design (LEED) for Schools prerequisite of 0.70 NRC or higher. Cost control was the next important project goal. And of course, they hoped to meet both these goals without sacrificing the overall visual appeal of Mill Creek Elementary and Middle Schools.
With the assistance of Acousti Engineering, the acoustical contractor on the project, GMC specified three types of acoustic panels for the buildings. To keep costs under control, they decided on an affordable but high-performing fine fissured mineral fiber panel with a 0.70 NRC and 35 CAC for classrooms, administrative offices, and conference rooms. The combination of superior sound absorption and sound blocking created the right balance for areas where speech clarity and privacy were both important.
For open areas with more ambient noise, including the media center and cafeteria, a high-performance fiberglass panel with 0.95 NRC and a smooth, bright white surface was selected. For the auditorium and music room, they invested in a premium composite panel with 0.90 NRC and 42 CAC to both contain sound and decrease reverberation for optimum sound quality. This panel shares the same smooth, clean surface visual as the panel chosen for the media center and cafeteria, keeping the more sophisticated look consistent across all the areas that would be more visible to parents, visitors and the community at large.
There is rarely a single product that meets the acoustic goals throughout a building. The smart approach for designers and specifiers is to target the right products with the right level of sound absorption and/or sound blocking to the needs of each space. This can be achieved while maintaining a consistent visual aesthetic.
Robert Marshall is the senior technical manager for CertainTeed Ceilings and a lifelong participant in the commercial ceiling industry. The product of one of the world’s first acoustic ceiling contracting businesses (a company founded by his family in 1927), he has more than 35 years of experience in contracting, distribution, and manufacturing of such assemblies. He can be reached at email@example.com.
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