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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.
Distraction
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.
Timing
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).
Taming
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.
Spreading
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.
Flutters
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.
Flanking
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.)
Blocking
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.
Cover-Ups
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, www.acousticsfirst.com. |
