The walls of this early childhood classroom at Central Institute for the Deaf in St. Louis, Mo., have tackable sound-absorbing panels.
In May the U.S. House passed legislation that allows federal funds to be used for acoustic improvements in schools, bringing us one step closer to quieter classrooms. ASHA also hosted a Capitol Hill briefing on "Classroom Noise and Acoustics: The Unseen Barriers to Learning" on May 15 to highlight the impact of reduced noise and good acoustics on academic performance, underscore the cost-effectiveness of improvements, and showcase quiet classrooms around the nation.
Architectural design strategies for quiet classrooms and practical ideas for improving speech intelligibility in new or existing buildings were presented at the Capitol Hill briefing.
Audibility Alone Is Never Enough
Whether designing large classrooms or small treatment rooms, the architect's highest acoustic priority is always speech intelligibility, and achieving this requires more than an overall increase in the audibility of the target signal or a decrease in noise. For younger learners, attention should be focused on the audibility of critical speech frequencies—those between 500 Hz and 6,000 Hz—because background noise in this range can mask the consonant and blend sounds that allow the brain to distinguish between similar-sounding words. Older learners, who have acquired more spoken language vocabulary, are better able to "fill in" these dropped consonants and words from context.
Because one classroom's signal is always the next classroom's noise, the most productive design strategies always begin with identifying and reducing noise before considering any amplification of signal. Some sources of noise are more detrimental to speech intelligibility than others. Prioritizing these sources in four categories helps to spend construction or renovation dollars efficiently:
- Mechanical system noise from air ducts, pipes, and related machinery
- Outside noise from traffic, playgrounds, and other sources outside the building
- Building noise from corridors, other rooms, and other floors inside the building
- Room noise generated by each classroom's occupants
Mechanical System Noise
Mechanical noise is usually the primary noise culprit because of its volume and vast frequency range. The low-frequency rumble of big mechanical units can be reduced by placing them on base isolation pads and locating them as far as practical from classrooms. If they are mounted on the roof, they are best located over restrooms, teacher work areas, or supply and file rooms.
Acoustical linings can prevent sheet metal ductwork from transmitting mid-range fan noise over long distances. Oversized ducts allow the volume of air to be delivered at lower velocity, eliminating the high-frequency hiss when it enters the room through a diffuser. Above all, mechanical devices with moving parts should be kept outside the walls of all learning spaces, including the space above the ceiling.
Noise from existing in-room mechanical units can be reduced by building an acoustical housing over each unit. The cost of this kind of custom enclosure, which must restrict sound transmission without reducing air flow, is usually beyond the range of practicability. The only other option is for the teacher to pre-cool the classroom to a level that allows the unit to be turned off long enough to deliver a lesson.
Outside and Building Noise
Protecting an entire school from outside noise is similar to the task of protecting an individual classroom from building noise—the key is to wrap the quiet space with wall, floor, roof, and ceiling assemblies that carefully layer materials with different sound-absorbing properties. Much like the way layered clothing resists the transmission of heat, layered building assemblies are much more efficient than uniform assemblies at resisting sound transmission. (See illustration for a typical exterior wall construction assembly.) The effectiveness of the most high-performing assemblies, however, can be erased by very small holes or cracks, so the architect's responsibilities extend well into construction to ensure that contractors are thoroughly connecting walls to the floor and roof and painstakingly sealing every pipe and duct penetration.
In a well-sealed building or room, the next area of attention is doors and windows. In the presence of sustained ambient noise levels of 60 dBA or higher, specially designed window and door units may become necessary. Locating the building far from traffic or other noise sources may avoid the additional cost of these windows and doors. Similarly, the interior of the building can be zoned to place classrooms as far as possible from cafeterias, gyms, or music rooms.
It is impossible to prevent noise in the classroom. To maximize speech intelligibility, the ceiling and walls of noisy classrooms can be covered with sound-absorbing materials that counteract the tendency for room noise to build up by reverberation. Effectiveness is measured as reverberation time (RT)—the number of seconds it takes for this "built-up" sound to diminish by 60 dB once the source is turned off. Because most classrooms are quieter when instruction is taking place, an appropriately low RT of 0.4 seconds or less likely can be achieved with a high-performing acoustical ceiling (with a noise reduction coefficient of 0.8 or higher).
In early childhood classrooms, which are generally larger and filled with a wide variety of simultaneous activities, it may be necessary to extend the absorptive materials onto walls high above the floor on all sides of the room. These panels are generally tackable and useful for displaying artwork, but the same sound-absorbing properties also make them vulnerable to damage and dirt from young fingers.
Building systems and assemblies can be used to remediate virtually any level of noise, but it is always more economical to prevent as much noise as possible in the first place. In much the same way, many of the energy-saving and cost-saving features employed in sustainable or "green" buildings are equally effective at reducing classroom noise.
Smaller, more efficient mechanical systems generate less sound, and tighter building envelopes prevent the transmission of sound as well as heat. From this perspective, background noise is appropriately exposed not just as a kind of harmful pollution, but also as an expensive and unnecessary one.