Poor acoustics is a prevalent problem in classroom environments. A student’s ability to learn is greatly reduced in a noisy classroom. Reverberation and background noise can make it difficult to hear and understand the instructor, reducing concentration and attention span. The problem is even more serious for students with hearing problems, learning disabilities, or language barriers.
The United States is going through its largest phase of school construction and renovation in history. Although the need for good classroom acoustics has been known for decades, until fairly recently this information was not made readily available to architects and school planners.
Reverberation is the most common issue with classroom acoustics. Reverberation occurs when sound waves reflect off of the hard parallel surfaces in the room and continue to bounce back and forth until the energy is degraded. An acceptable reverberation time, (RT), for a classroom should be about 0.4 to 0.6 seconds. This can be easily tested with the hand clap method. By clapping your hands then counting how long it takes before the sound is inaudible, you can determine the approximate reverberation time. Reverberation can be solved by either decreasing the volume of the room or by increasing the amount of absorption. Though decreasing the volume of the room isn’t always an option, the acoustics in classrooms with high ceilings can be greatly improved with the installation of a dropped ceiling with acoustically rated ceiling tiles, or suspending acoustic ceiling clouds. This both reduces the room volume while adding absorption. Harsh reflections off of the walls can be eliminated by installing acoustic wall panels.
INTERIOR NOISE TRANSMISSION
Noise from adjacent rooms are also disruptive to the learning process. Over the years the need to reduce construction costs has led to the use of thinner lightweight wall materials which do little to block sound transmission. The sound blocking properties of common walls can be greatly increased by adding mass through the installation of a vinyl noise barrier, and decoupling the partition with resilient isolation clips. This reduces both airborne and structure borne noise transmission.
Unwanted noise can also flank over top of dropped ceiling tiles and into the adjacent space if the common walls are not constructed to the top of the existing ceiling or plenum. If increasing the wall height is not an option, replacing the existing ceiling tile with a barrier ceiling tile, or adding a fire rated barrier material over the existing tile, will greatly reduce noise transmission. During the planning stages, noisy areas such as gymnasiums and music rooms can be isolated from classrooms by using buffer zones such as hallways and storage rooms.
MECHANICAL EQUIPMENT NOISE
High ambient noise levels from HVAC equipment is very common in existing schools. Determining the source of HVAC noise can be complex since it may originate from many sources, and should be handled by a professional acoustical consultant with the experience and equipment to locate all of the noise sources and determine the appropriate remedies.
When designing an HVAC system in new schools there are some basic guidelines that will eliminate most mechanical noise issues:
- Locate mechanical rooftop equipment away from critical listening areas such as classrooms.
- Reduce mechanical vibrations of HVAC equipment, conduit, and ductwork through the use of vibration isolators, vibration pads, and spring isolators.
- Select air handlers with low sound level ratings.
- Install acoustic duct silencers in noise critical areas.
- Specify duct sizes large enough to prevent high air velocities, and select diffusers with Noise Criterion, (NC), ratings of 20 to 25.
- Use longer duct runs to eliminate crosstalk between rooms and lower mechanical noise.
EXTERIOR NOISE TRANSMISSION
Exterior noise sources should be addressed during site planning. Common exterior noise sources are busy roadways, aircraft, exterior mechanical equipment, playgrounds, and playing fields. Planners should consider exterior noise sources and when possible attempt to not locate classrooms near those areas. Although schools are typically constructed with brick and concrete block which are good sound barriers, classroom windows are a common weak spot. Windows should be double paned and well sealed, preferably with an acoustical rating of STC 48 or above.