Can You Hear Me? Optimizing Learning through Sustainable Acoustic Design

Understanding acoustic design, surface materials and services will provide optimal educational environments.
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Sponsored by Ceilings Plus, Meyer Sound Laboratories, Inc. and Serious Materials
Celeste Allen Novak, AIA, LEED AP

Perforated panels and acoustical performance

One of the leaders in setting performance values for school construction is the Collaborative for High Performance Schools (CHPS).2 This organization has developed a report card that parents and professionals can use to rate their schools. How satisfied are parents with the acoustics in their child's classroom? How disruptive are the potential noise sources in the room? Is the mechanical equipment too loud? Can you hear sounds from neighboring classrooms? Does noise penetrate from the outdoors? CHPS sets minimum performance values for good classroom acoustics so teachers can speak without straining their voices and students can hear and effectively communicate to enhance their learning experience.

The CHPS Report Card provides a table of acoustical standards that includes capping ambient noise levels at 45 dBA and limiting the maximum unoccupied Reverberation Time to 0.6 seconds.3 The USGBC LEED® for Schools, Environmental Quality Prerequisite 3: Minimum Acoustical Performance requires that the professional design classrooms meet the reverberation time requirements of ANSI S12.60-2002. According to the American National Standard Institute (ANSI) Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools (ANSI S12.60-2002) a background level less than 35 dBA is preferred for superior acoustics, particularly for young children and those with hearing impairments.

Long reverberation times reduce speech intelligibility in a classroom and it is here that perforated panels are most useful to the designer. Reverberation Time (RT60) is a measure of how quickly, in seconds, sound reflections in a room decay 60 dB or become inaudible. Rooms designed for good speech intelligibility should generally have shorter reverberation times. Reverberation time is typically controlled by the judicial application of ceiling and wall finishes that have higher noise reduction coefficients (NRC).

Noise reduction coefficient (NRC) is a measure of how well a product absorbs sound. Simply put, higher NRC values mean better sound absorption. By selecting ceiling and wall finishes with higher NRC values, the architect and acoustical consultant can control the reverberation time in spaces that require good speech intelligibility. Even without acoustic absorptive materials added, wood and metal perforated panels can have an NRC of 0.40. Add a paper-thin, non-woven acoustical fabric and the NRC can reach as high as 0.75 with as little as six inches of airspace above the ceiling panel. With reconstituted cotton batts that can have up to 85 percent pre-consumer recycled content, the NRC can be 0.95 - an almost perfectly absorptive material.

The artful combination of wood and metal panels vertical and slope surfaces was used throughout the corridors and public areas of this athletic facility to visually unify the building. The warmth of the wood softens the effect of the hard concrete masonry walls. Where required, panels were perforated to yield a high acoustical value of NRC=0.90.

Photo courtesy of Ceilings Plus
Tompkins Cortland Community College, Dryden, New York
Architect: JMZ Architects and Planners, P.C.

NRC values this high are difficult to obtain with conventional wood panels that are made with ½ to ¾ inch thick particleboard cores. Creating perforations in such panels requires drilling - a slow and expensive process that allows only a limited range of hole sizes. More, the thickness of the cores make it physically difficult for sound waves to pass through small holes to reach the noise absorbing materials on the back side. The new types of wood panels, however, made with aluminum cores, are easy to perforate and are so thin that noise readily passes through even tiny perforations.

In school auditoriums and theaters, metal and wood ceilings can be shaped to create good sound diffusion with curved and angled panels. Some panels can be perforated to absorb sound and others can be non-perforated to reflect sound. Perforated wall and ceiling panels can be specified to "tune" the acoustics in a room. By adjusting the hole size, hole spacing, and cavity depth, panels can be designed to absorb more sound at a particular group of frequencies, much like adjusting the bass, mid, and treble controls on your home stereo.

The artful combination of wood and metal panels on vertical and sloped surfaces, was used throughout the corridors and public areas of this athletic facility to visually unify the building. The warmth of the wood softens the effect of the hard concrete masonry walls. Where required, panels were perforated to yield a high acoustical value of NRC=0.90.

Perforated panels can also be used to create acoustically transparent surfaces that allow speakers and electro-acoustic systems to be installed above the ceiling or behind wall panels, reducing visual clutter. In many instances there are already noise-reducing materials above the ceiling in the fireproofing, thermal insulation, or even the air space that will allow noise to be dissipated above the ceiling. More, acoustically transparent surfaces can be used to create longer reverberation time, a requirement in auditoria designed for symphonic music, by adding the space above a ceiling to the acoustical volume of a room.

Materials matter - recycling and FSC-certified wood

Aluminum panels can be specified to contain up to 85 percent recycled aluminum, including as much as 75 percent post-consumer recycled content - usually from beverage cans. Aluminum is one of the most readily recyclable materials and can be recycled repeatedly without loss of strength or metallurgic value. In addition, the steel suspension systems used to support panels can have between 25 percent and 30 percent recycled material content. Professionals can also specify acoustical insulation that is made from recycled cotton; the batts are factory-installed to reduce time and labor costs on the job site.

These panels are light weight systems, are easy to handle, and can reduce a building's deadload. Lighter panels also reduce the shipping impact on a construction budget. For large projects, panel manufacturing can be set up near the project site to increase the regional material content and further reduce the environmental impact of shipping.

 

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Originally published in Schools of the 21st Century
Originally published in January 2010

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