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Acoustical Considerations for Classrooms
By: Cameron Girard

Poor classroom acoustics has long been the invisible problem that has the farthest reaching implications for learning. Excessive noise and reverberation degrade speech intelligibility, resulting in reduced understanding and therefore reduced learning. In many classrooms in the United States, the speech intelligibility rating is 75% or less. That means, in speech intelligibility tests, listeners with normal hearing can only understand 75% of the words read from a list.

Imagine reading a textbook with every fourth word missing. Wouldn’t that make comprehension near impossible? Fortunately, poor classroom acoustics can usually be remedied with some basic knowledge and commercially available treatment. But before getting into specific treatment, let’s go over some basic acoustic principles.

Noise
Obviously, it’s difficult to understand what the instructor is saying when there is a lot of naturally occurring noise in the room. A glut of factors can be considered noise sources, including HVAC “rumble,” traffic outside the building and students moving in their chairs. These sources contribute to a “noise floor” that makes understanding speech very difficult. Since there is no one “cure-all” for an excessive noise floor, it is often best to seek the assistance of a professional acoustical consultant to properly diagnose and find a solution to these issues.

Reverberation: Undesirable vs. Useful Reflections
When not attributed to a noise issue, the culprit of poor classroom acoustics is often excessive reverberation. In simple terms, reverberation is the sound energy that remains in the listening environment as a result of lingering reflections. As mentioned before, these reflections can easily interfere with speech intelligibility. As you may have experienced at some point, it can be difficult to understand what is being said when reflections from old information cover up what is newly spoken.

The reverberation time (RT or RT60) is used to determine how quickly sound decays. The RT is dependent upon the volume and surface materials of a given room. Large spaces with hard materials (tile, drywall, etc.) have longer reverberation times, while small rooms built with “softer” materials sound more “dead”.  Ideally, classrooms should have relatively short RT’s, somewhere in the .6-.8 second range.

A long reverberation time is not the only factor that should be considered when treating a classroom with poor acoustics. Flutter echo is a particularly significant problem when it occurs between the side walls at the front of the classroom where the teacher is speaking. This condition can be heard as a “ringing” sound (when one claps) as the sound rapidly bounces back and forth between two parallel walls. Flutter and other discrete echoes are considered “undesirable” reflections and should be controlled with absorptive or diffusive materials.

Not all reflections are bad though. There are “useful” reflections that reinforce spoken word, rather than cover it up. The teacher’s voice can be propagated throughout the room by shaping a sound reflecting gypsum board ceiling over the front of the room or by making the center of the ceiling a hard, reflecting surface (see figure 1). This will help project the speaker, so they don’t have to strain their voice to be heard over the students.

Reducing Reverberation
Often reducing the dimensions of a classroom to attain a more suitable reverberation time is not feasible, but one can improve the acoustics by introducing sound absorptive materials.  Typical classrooms usually have a dropped “acoustical” ceiling that has some absorptive qualities. In classrooms that don’t have this ceiling, reverberation can be reduced by installing an acoustical ceiling or a number of fabric faced fiberglass panels. Likewise, if there isn’t carpeting in the room, you can marginally reduce the reverberation time by installing sound absorptive flooring.

Wall Treatment: Acoustic Panels
If the ceiling and floor are at least rudimentarily treated, then hard walls are usually at fault for poor speech intelligibility. Absorptive wall panels are a common treatment to control lateral reflections and reverberation.

These panels are popular because they can be customized with a variety of colors, edge designs and fabric facings.  They also can come with a high-density fiberglass adder that improves durability. In classrooms, these “Hi-impact” panels are particularly useful because the adder allows for the panels to be used as tack boards. This brings an extra level of functionality to the panels outside of their absorptive properties.

Though wall panels are a perfectly suitable treatment, uncovered areas between the panels can sometimes allow a few hard reflections and/or flutter echo to still occur (although full treatment of the walls would likely result in a room sounding too “dead”). For these situations, Acoustics First often recommends Sound Channels acoustic wall fabric.

Acoustical Wall Fabric
In many instances, acoustic wall fabric is actually a viable alternative to traditional wall panels. Unlike a typical “wall carpet,” Sound Channels is made of 100% recycled content and has ridges to increase surface area and absorption. Recently, an improved design has increased the effectiveness by 25% (NRC of .25). Also, perhaps most importantly, the uniform coverage you get by treating the walls with acoustic wall fabric eliminates the flutter/slap from reflective parallel walls (without making the space too “dead”). Acoustic wall fabrics are generally light weight and most can be put up just like any other wallcovering.

Also of note are the additional benefits when using Sound Channels in early education classrooms. The effective range that this wall fabric controls is the higher speech frequencies, which is the ideal range for classrooms with younger children (there are not many bass/baritone kindergarteners). 

Another advantage is in keeping the treatment clean. Wall panels may suck up sound, but they can also absorb fluids (like the occasional juice box). Sound Channels, on the other hand, is resistant to moisture, mildew and rot. It is also is non-allergenic and is highly resilient to common wear.

Acoustical Considerations for Classrooms
Although this knowledge has been around for decades, classrooms across the country continue to be plagued by a lack of acoustical forethought. Perhaps as this information becomes more readily available to architects, contractors, administrators and teachers we will begin to see (and hear) better sounding classrooms. School is challenging enough on students and teachers as it is, so let’s not compound their daily obstacles by continuing to overlook classroom acoustics. 

Cameron Girard is a member of Acoustics First’s technical team, www.acousticsfirst.com. He uses his experience as an acoustic consultant to help clients improve classrooms, auditoriums, churches and many other critical listening spaces. 

Resources:
B. Seep, R. Glosemeyer, E. Hulce and M. Linn. Classroom Acoustics – A resource for creating learning environments with desirable listening conditions. Acoustical Society of America. Mellville, NY, 2000

W. J. Cavanaugh and J. A. Wilkes. Architectural Acoustics: Principles and Practice. John Wiley & Sons, New York, 1991







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