Reducing Building System Noise in Modern Classrooms

As the focus on quality of education and the condition of schools increases, the acoustics of classrooms has taken a front seat in the evaluation of existing and future facilities. Excessive background noise within classrooms can distract or fatigue students and make it difficult to hear instructors. Replacing existing systems and designing future buildings with acoustics in mind will help improve student comprehension as well as overall operation costs. Unit ventilators are a thing of the past, but what about some of the other conventional systems? The good news is that we are seeing a move towards centralized air distribution systems that are more energy efficient and quieter.
It is important to provide distance between classrooms and noise-generating mechanical equipment. Acoustically speaking, fan coil units in ceiling and window air conditioning units are difficult, if not impossible, to mitigate adequately. If the unit is old then a fan coil unit replacement could help the noise as it will be more efficient but addressing the low frequency noise generated by air moving equipment requires either mass to contain it or distance to permit low frequency noise to dissipate into unoccupied or non-critical areas. By allowing low frequency noise to radiate into non-critical areas, less substantial structure is required, which reduces construction costs in addition to the noise within the classroom. Because of this, a system using a central plant and air handling units is often the most economical way of providing air conditioning to the classroom, even though other methods appear at first glance to be more cost effective.
Air moving equipment should be located outside of the classrooms. Designs should include noise buffer areas to help isolate sensitive spaces from noise-generating rooms. Typically, locations used as noise buffer spaces include storage areas, corridors, copy/print rooms and telecommunication closets.
Noise generators to consider isolating from classrooms include air moving equipment, condensers, elevator shafts and projectors. These adjacencies should be evaluated in both the vertical and horizontal aspects. The required distance applies to both vertical and horizontal adjacencies. In plain view, an air handling unit directly adjacent to a noise sensitive space stands out as obvious poor space planning. At the same time, locating down-shot package units on the roof directly above the classroom can be considered a “cost-saving option” for reasons of waterproofing. In the long run, the required mitigation to sufficiently attenuate the noise exposure caused by this latter condition typically causes this option to be more expensive, not less.
If it is not possible to relocate air moving equipment outside of classrooms, then small mechanical enclosures can be created within a ceiling plenum or in a classroom closet. Shoehorning an acoustical enclosure around the fan coil unit, ensuring there is a fire-rated access hatch, and providing a low pressure drop sound trap on the inlet and outlet of the unit can all help to reduce the existing building system’s noise level. Having a high Ceiling Attenuation Class (CAC) ceiling panel will also help reduce casing radiated noise from the fan coil unit. If possible, a shaft wall consisting of two layers of 5/8-inch drywall, a CH stud with batt insulation and a 1-inch coreboard should be used. It is important the shaft wall enclosure be airtight, with all penetrations including ductwork sealed tightly with batt insulation and acoustical caulk.
Both the vertically adjacent package unit and the fan coil unit in the ceiling plenum above will need to be mounted using a combination neoprene and spring isolators. Rated deflections should not be used to select the springs; rather, the actual anticipated load of the spring should be used to determine the actual spring deflection. The entire package air handling unit must be mounted on a spring curb isolator. Internal isolation of the fan is not sufficient because the condensing section would still be rigidly connected to the structure.
Ductwork should be designed for low velocities to promote laminar flow. Laminar flow is important for two reasons. Turbulent air requires more force to move, demanding larger noisier fans capable of overcoming higher-pressure drops. And, turbulent air itself also generates noise. Abrupt changes to the airflow, such as pressure reductions, turns and take offs, should be avoided. For critical spaces, wide ducts and smooth transitions are required.
Installing quality air moving equipment that operates more efficiently usually results in lower sound levels. Rethinking the traditional HVAC system approach can also help reduce operational costs while improving ambient noise levels. Approaches other than traditional HVAC design, such as chilled beams, can help reduce the airflow requirements while providing the same result in temperature control.
While a package unit may present a lower construction cost, these savings may be offset by additional structural and noise mitigation measures required due to the higher sound levels generated by package units.
Variable air volume and constant air volume boxes, as well as grilles and diffusers should be selected so the manufacturer’s noise rating is no more than NC-25. All ducts downstream of terminal boxes should be sized at low friction rates (0.06-inch per 100 feet at the main duct and 0.04-inch per 100 feet or lower for the connections to the diffusers). Ideally the layout would be self-balancing, avoiding the need for dampers. If dampers are necessary, they should be separated from the closest diffuser by no less than 5 feet of acoustical flexible ductwork, or 7 feet of sheet metal ductwork with 1-inch thick internal acoustical lining.
If there are concerns using fiberglass in the air stream, there are alternatives to the commonly used fiberglass acoustical lining. With coated polyimide foam duct lining, fibers are not exposed to the airflow. Coated polyimide foam duct lining is half the weight of fiberglass and can meet or exceed the requirements outlined in ASTM C 1071. Not all foam duct liners are equal. There are some foam duct liners on the market that do not have similar acoustical absorption characteristics as fiberglass. Recycled cotton acoustical lining provides comparable thermal and acoustical properties to the traditional fiberglass internal lining.
To avoid short-circuiting, the acoustical separation penetrations through partitions that extend full height should be avoided. If penetration of a full-height partition cannot be avoided, the penetrations should occur through partitions that include entryways, preferably above the door itself. Any penetration through a full height wall should be separated from the drywall by ½ inch, which should be filled with fiberglass and sealed with a silicone caulk on both sides of the partition.
Including all these acoustical considerations early in space planning and HVAC design can significantly improve the quality of the educational space and reduce operational costs.
Aaron Bétit is a senior consultant in architectural acoustics and mechanical systems at Acentech Inc., a multi-disciplinary acoustics, audiovisual systems design and vibration-consulting firm. He is based in Acentech’s Los Angeles office. For more information, please visit www.acentech.com.