The Ears Have It

Nov. 1, 2003
Meeting the new standard for classroom acoustics.

Spoken language plays an essential role in the classroom. Most classroom activities involve students listening to and participating in spoken communication with teachers and other students. The need for a standard to ensure clear communication among students and teachers is critical.

Many studies have concluded that poor classroom acoustics impede learning. The studies have shown that this problem affects young students more than older children or adults; young children's ability to process or perceive speech under less-than-perfect conditions is not as well developed. Moreover, it is not surprising that the negative effects of poor acoustics are most pronounced in those with learning disabilities and hearing impairments.

Despite these findings, many classrooms fail to provide an environment that is conducive to clear communication.

The growing awareness of the need for better acoustic conditions in the learning environment has led to a recently published standard. The American National Standards Institute (ANSI) S12.60-2002 establishes acoustic performance criteria, design requirements and guidelines for schools. The new standard recommends construction to create proper sound isolation between spaces and sets criteria for various acoustic conditions within classrooms. The goal of the standard is to optimize the environment in which teaching and learning takes place. This can affect the planning and design process significantly. Planners should consider acoustics early in the design process; acoustical guidelines can heavily influence design and budget.

Some states are embracing the new standard. In Connecticut, for instance, new school construction and classroom alterations approved after July 1, 2004, must meet the standard, according to recently approved legislation.

The standard sets forth three key recommendations: limiting interior noise levels, limiting reverberation time and isolating intrusive sound from adjacent areas.

Background noise

The standard calls for limiting interior noise levels to 35 dBA, a decibel rating that represents how the human ear perceives sound, including a correction to low-frequency noise. Yet it is common for existing classrooms to have noise levels from HVAC systems between 50 and 60 dBA. A 10-dBA increase is perceived as twice as loud, so such classrooms are roughly four times noisier than recommended.

The best way to achieve the 35-dBA criterion is proper design and noise control of the mechanical system. Many classrooms are designed with unit ventilators or fan-coil systems, which are single air-conditioning units placed at the perimeter of a room or above the ceiling to serve just that space. These types of units are less expensive than central systems, provide individual climate control and can be accessed easily for maintenance.

However, their major noise components (fan and compressor) rarely are enclosed with more than a sheet metal casing, so they generate significant noise source in a classroom. Moreover, at the air inlet and discharge openings, only a filter or grille separates these components from the room. It is difficult, if not impossible, to achieve the 35-dBA criterion with these kinds of installations. AC units need to be situated outside classrooms at a distance that allows proper sound isolation.

Rooftop air handlers situated directly above classrooms also can be problematic. Supply and return air ducts that drop directly from the underside of the unit into classroom space will not meet the 35-dBA criterion. Vibration transmission, which can be perceived as sound, also is a concern, especially when lightweight roof structures are employed.

Often a better, cost-effective HVAC alternative is a zoned system with an air handler in a dedicated mechanical room and distributed ductwork that serves multiple spaces. The location of the mechanical room can help minimize noise traveling to adjacent spaces. Proper ductwork layout and diffuser selection, along with acoustic linings and silencers, further improve acoustic quality. All these provisions have design implications (space requirements, ceiling height). This is why the issue needs to be addressed early in the planning phase.

Interior acoustics

The new standard also limits reverberation time, which is defined as the amount of time a sound lingers in a space before it diminishes to an imperceptible level. The standard limits reverberation in an average room to 0.6 seconds. The intent of the standard is to limit echoes that adversely affect speech intelligibility. In a typical classroom, reverberation time is largely a function of the sound-absorptive quality of the room's surface finishes. In order to meet the reverberation guideline, schools should avoid hard surfaces on classroom walls and ceilings — usually the largest single uninterrupted surface in the room.

Wherever it is practical on side and rear walls, schools should use suspended acoustic ceilings and acoustic wall panels instead of concrete block or drywall. A hard surface on the front wall can be helpful in projecting a teacher's voice toward the class. Tile floor surfaces are a secondary source of reverberation; carpeting can help reduce sound reflections or noise from sliding chairs and desks.

Noise transmission

The classroom acoustic standard also sets forth recommendations for the acoustic performance ratings of a room's demising construction, that is, Sound Transmission Class (STC) and Impact Insulation Class (IIC), in order to effectively isolate intrusive sound from adjacent areas. STC defines airborne noise transmission through the floor slab, walls and ceilings. IIC defines the transmission of an impact such as footsteps from a space above.

The standard recommends a minimum STC 50 rating for core learning spaces. This guideline can be met using standard building methods of full-height wall construction; that is, from the floor to the underside of the slab above, with two layers of drywall on each side of the studs and acoustic insulation provided within the stud cavity. Care should be taken to avoid large openings in the wall construction above the suspended ceiling, such as return air transfer or openings around duct or pipe penetrations. Such openings will severely undercut the STC rating and allow noise transmission from adjacent areas.

Exterior noise transmission from outdoor equipment, aircraft or traffic also can be intrusive. Window construction plays a role in controlling this. As the standard suggests, exterior noise conditions are unique for each project. Some areas may require special isolated or laminated assemblies to achieve the 35-dBA criterion. Again, this will affect the construction budget and should be considered early in the planning phase.

The standard also recommends a minimum IIC 45 rating, which dictates the use of a soft floor surface, such as carpeting, or a suspended ceiling in the space below. This will serve the dual purpose of reducing impact noise and reverberation.

The new ANSI standard appropriately outlines requirements for a high-quality acoustic environment in the classroom, but achieving the goals has widespread implications, not the least of which is cost. With decreased funding for schools, institutions may have less money available for construction. School planners and administrators must make informed decisions on where, how and why to apply the new standard in order to assure the most cost-effective approach for their schools.

Clark is an associate principal with Cerami & Associates Inc., a New York City-based acoustics, audiovisual and IT consulting firm.

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