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Tips for Better Acoustics in Classrooms | Christian School Products

Tips for Better Acoustics
by Nick Colleran

Originally Published in Christian School Products Magazine April 2012 issue.

Applications: Auditorium & Theater, Church & Worship Facilities

Material: Sound Absorbers, BlockAid®, Vib-X, Acoustical Sound Diffusers

Visual distractions are not the same as sound distractions. A person can turn eyes away and not see (although it is rumored that some teachers have developed eyes in the back of their heads). This does not work for ears.

Sound fills a room as reverberation and surrounds everything everywhere. It is inescapable. However, a student can be looking away and yet still hear everything to the point of being able to reiterate verbatim if the acoustics are correct. (Demonstrating this ability is not a wise move.)

Size vs. Quantity
If the room is too reverberant, students may begin to talk over one another, generating more noise. This problem would then be exacerbated by adding more students. This often happens in restaurants as diners talk louder in an attempt to be heard. Some noise is helpful to a restaurant where diners do not want to share their conversation with nearby tables. The opposite is true for a classroom. Teachers who have to scream to be heard waste precious time needed for education by having to repeat lesson material while it adds more stress and fatigue to their day.

Today there is more distraction, from more noise sources, more media, and  outside sounds leaking into the classroom. The students' ability to tune out noise can also tune out the lecture (similar to throwing out the baby with the bathwater).  Even if not tuned out, surrounding sonic interference can cause misunderstanding. This is especially critical in medical applications where the difference in Methyl and Ethyl (alcohol) can literally be the difference between life and death.

Old School
An older classroom may be designed to act more like an echo chamber than a place of learning and could be the poster illustration for bad acoustics with block walls, vinyl tile floors, chalkboard and windows. Windows however at least absorb some sound in the lower frequency ranges. With a Sound Absorption Coefficient (SAC) of 0.35 at 125 Hz glass is useful to acoustic control as a side benefit to its necessity for viewing. The other materials measure ridiculously close, acoustically, to the plaster used to create a natural live echo chamber for a recording studio. Painted block ranges for 0.06 – 0.10 SAC; linoleum or tile floor 0.02 – 0.03 SAC; and the chalk board, 0.01 – 0.02, winning in the category of most necessary but worst offender, acoustically.

The combination of typical classroom surface materials (tiles, windows doors, chalkboards, and plaster) produces a reverberation time of 2.5 to 3 seconds in the critical speech range. The ideal classroom should be in the range of 0.40 - 0.60 seconds RT60 (the time for sound to decay by 60 dB).

Reverberation can easily be brought down to a reasonable level with fiberglass acoustical tiles in the ceiling. Their absorption numbers range between 0.75 SAC and 0.90 SAC, many times that of the surfaces cited above. One small solution with a large effect is to keep coat closets partially open or have the garments hanging along the wall in the class room. Of course, this method has its own drawback of changing with the season as extra layers become more or less necessary with the weather. Carpet is also an option, however functional institutional thickness is not enough to produce a result similar to the high absorption of a fiberglass ceiling tile.

There are many ways to reduce apparent sound levels. One is diffusion, the scattering of sound evenly throughout the room to avoid intensity in any one spot. The author calls this the peanut butter analogy: It is hard to swallow in one lump, but easy to digest when spread over bread. A book case, where the book sizes are random, will reflect sound at different intervals. This is in essence, diffusing sound. A uniform surface will produce one big bounce (slap) back. It makes a sound case for disorganization to create a better environment for learning.

Smooth walls and hard surfaces not only add to reverberation and sound build-up, they produce annoying flutter echoes between parallel surfaces. These keep going and going like that battery bunny. These standing wave "flutters" can be reduced by diffusers or eliminated altogether by acoustical wall panels made from the same absorbing material as the ceiling tiles mentioned above.

Sound transmission into the environment is also a major cause of distraction. The source can be an open door or window where the solution is obvious: close it. However, a closed door can still allow sound from the hallway if there is a gap at the floor. A simple solution to this is a door seal that sweeps along the floor. If no light is visible at the bottom of the door, and no bottom breeze is detected, this will go a long way to reducing distraction.

A second source of neighbor noise is not as visible as it is common: flanking over the walls. Too often walls stop at the drop-tile ceiling instead of extending to the roof deck. (This is not unique to schools. The author once found short walls dividing radio studios with Spanish language on one side and Rap format on the other, making microphone placement and use of headphones to prevent distraction necessary. On another occasion a health club had their gym next to the yoga room with no thought being given to sound flanking in the plans.)

If the wall between instruction rooms is built solid, it provides a good basis for sound isolation. Unfortunately, a critical component, insulation, is often left out. While not as necessary for heating and cooling concerns on an interior wall, its absence prevents the heavy barrier of gypsum drywall from achieving its maximum sound isolation properties. Two components are necessary for sound isolation: sound barriers of heavy, dense and massive materials combined with sound absorbers that are light, porous and fluffy (often called "fuzz" by acousticians). The fuzz need only be whatever is on sale at the local building supply. It prevents the wall from acting like a drum where the vibration of one surface travels and excites the other, if the air is not trapped. It may be possible to use blown-in insulation to correct an existing structure.

Apartment neighbor noise is often reduced by adding a layer of vinyl sound barrier for mass and an acoustical wall fabric for absorption, without increasing the wall thickness more than three-eights of an inch and thereby preserving the size of the living space. This same wall fabric has found increased use in daycare facilities for both its sound properties and appearance.

All of the above approaches are aimed at reducing the problems at their source. There are sound masking approaches, but much like the buzz of fluorescent lights, once they become obvious to those easily distracted, their sound can not be ignored or "turned-off" in the head of the listener. A cover- up is almost always less effective than a correction at the source.

Bad Vibrations
One last item, not often covered, is mechanical vibrations. While not much can be done for outside mechanical noises, the hum of computer drives and other devices can be reduced by decoupling them from the flat surfaces that tend to cause amplification. One vibration pad, at less than $100 cost, can usually provide enough material to "cure" all of these devices in a classroom.

A clear understanding of what the teacher is saying can prevent an aspiring music student from hearing "take it" vs. tacit (remain silent) and launching into her own improvised saxophone solo.

Nick Colleran is a principal of Acoustics First Corporation,

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