The principles of
sound absorption have been around for hundreds
of years. Medieval churches used ash-filled pots
embedded in the walls to absorb sound. These
evolved into today's concept of
sound-absorbing panels, commonly called "acoustical
panels."
The basic construction of an
acoustical panel includes a sound-absorbing
substrate material and a covering of cloth,
vinyl, or other material. The first
acoustical panels used in "talking picture"
theaters employed wood fiber as the
sound-absorbing material. Today's
acoustical panels use substrates that are
more efficient at absorbing sound and meet
modern safety standards. Some panels use two or
more substrate layers in one panel to provide
special properties. The most widely-used panel
consists of a fiberglass board substrate and
cloth cover, usually referred to as a POP—a
"Plain Old Panel."
How
They Work
 Acoustical panels control sound by
selectively absorbing sound waves, thereby
reducing echoes and re-echoes. All of the
various substrates absorb sound by trapping it
in a labyrinth of fibers or open cells within
the material. As sound travels through this
maze, it loses energy due to both the travel and
the direction changes within the material.
Strictly speaking, the energy of the sound wave
is converted to heat within the substrate.
However, this heat is hardly noticeable or
measurable and will not have any effect on a
facility's heating or cooling levels. The
acoustical material is, however, essentially the
same material as used for building insulation
and, if enough of an exterior wall is covered
with panels, may contribute some insulating
value.
Echo! Echo! Echo!
To paraphrase a popular southern comedian, "If
you miss the Sunday sermon and can come in on
Wednesday and still hear it, you may need
acoustical panels." The choir director may love
a room like this but the congregation will get a
headache from "ear strain" trying to understand
the minister. Some sanctuaries prove so annoying
that the discussion of how to fix them must be
held in the lobby.
The problem can be excess
reverberation time—a measure of how rapidly a
sound decays in the room. Reverberation time can
be estimated by initiating a loud noise—a loud
clap, tipping over a folding table, or slamming
a hymnal shut—and timing its decay. With this
information and the spatial volume of the room,
the existing sound absorption in the room can be
calculated.The basic formula for
measuring sound absorption is the Sabine
formula, a simple algebraic equation named for
the man who developed it a hundred years ago.
The Sabine equation is used with the desired
reverberation time to yield the necessary
absorption for that room. The difference between
the necessary absorption and the existing
absorption indicates the degree of requirement
for
acoustical panels. So, with a loud impact
noise and a good stopwatch, absorption
requirements can be estimated with reasonable
accuracy.
There are, of course, several
factors that keep this from being too simple.
First, if there is already absorption on the
wall that will be covered or removed, the panels
will not change the acoustical characteristics
of the room by their full rated value as they
would over hard plaster. Second, if the
reverberation is not particularly high and
clarity is a problem, the answer may be
diffusion of sound, rather than
absorption. The
difference is that while absorption reduces
reverberation by controlling reflected sound,
diffusion lowers the perceived sound level by
scattering (diffusing) sound over a wide area.
Diffusion can help achieve a space that is not
too dry for the choir, nor too reverberant for
the minister's message. (A full discussion of
diffusion is reserved for another article.) A
third consideration is that absorption
characteristics depend on the sound frequency.
Some acoustical problems must be resolved by
addressing absorption or diffusion of selected
frequencies.
One common use of
acoustical
panels is to eliminate stray reflections from
stage monitors, especially where feedback is a
problem. They are also used on side walls to
avoid slap-back and side-to-side flutters.
Panels prevent coloration of the main sound
source by eliminating reflections that recombine
with the direct sound to reinforce or cancel
certain frequencies. Without reflection control,
the sound heard by the audience can be very
different than that intended.
Built-In Acoustics
Before building a new worship space, it is vital
to involve an acoustician along with the
architect. In the acoustics world, there is a
saying: "Architects have vision; but you don't
hear with your eyes." Thinking about acoustics
first rather than last will allow the installed
sound system to be effective with less signal
processing (and the attendant distortion) and at
less cost. It will certainly be more economical
than replacing or upgrading the sound system in
attempts to fix acoustical problems.
A room is acoustically
"active" or, in a sense, "alive." A dip in
frequency response is not cured with a peak
equalizer, pumping more energy into that
frequency. The room will absorb the new energy
and the electronic correction will not be a
complete cure. Eventually, extreme electronic
compensation will cause undesirable audible
effects. Whatever the acoustical needs of the
room are, it can safely be said that the budget
for acoustical control materials will be
significantly less than the cost of the second
and third failed sound systems.
Types of Panels
While well-made
standard acoustical panels from
various manufacturers may have more similarities
than differences, there are many variations on
the theme; some proprietary and some patented.
The most obvious differences are the quality of
workmanship and what is included in the pricing.
"Do you want tires with that
car?" is a phrase that translates well to the
acoustical panel business. Ask what is included.
Are edges square or shaped? Are they hardened?
Is mounting hardware included? How are they
packed: cardboard carton or wooden crate? What
does freight cost?
For
acoustical panels, the
package often weighs more than the product,
especially with
cellular Melamine (foam) panels.
This can bring about the specter of the dreaded
"dimensional weight," where freight is charged
by volume rather than by weight. By personal
experience, I've found it painful to learn that
someone has shipped a box of lightweight foam by
air at a shipping cost greater than the value of
the product.
Here is a sampling of the
panel choices available:
 POP.
The "Plain Old Panel" made of a fabric covering
over a sound-absorbing, high-density fiberglass
board. The
fabric covering is acoustically
transparent. There are other variations with at
least one manufacturer using mineral fiber
(mineral wool) in place of fiberglass. The
fiberglass board is often mistakenly called
"703," the model number for one supplier's
original acoustical fiberglass. The original
"703" fiberglass boards were of lower density
than commonly used today. Most fiberglass board
used in acoustical panels is six to seven pounds
per cubic foot.
Flat diffusion panel.
POP with a membrane below the fabric surface.
The membrane has a pattern of holes that varies
reflection and absorption over the panel
surface. The result is a scattering effect of
the sound. This is an interesting item for a
confined space but less necessary for a
sanctuary where a traditional diffuser can be
used to greater advantage for sound and, with
custom wood finishes, for appearance as well.
Barrier sandwich. Two
POPs with a layer of
heavy vinyl in between to
block sound passing through the acoustical
panels. (Absorbing panels prevent sound
reflection but do little to block penetration
through walls from room to room or inside to
outside.) This item has been used to replace
composite foams in machinery enclosures. The
barrier sandwich can be made into a removable
insert for framed glass partitions to provide
additional isolation when conflicting events are
on opposite sides of the window. For permanent
installations, standard panels with separately
supplied
barrier may be more effective for sound
and budget.
High impact surface. POP
with a 1/8-inch layer of high-density fiberglass
below the fabric. These are particularly useful
in fellowship halls or gymnasium applications to
avoid the craters created by wayward basketballs
in softer fiberglass surfaces. A side effect of
the high impact surface is its improved
absorption in the lower frequencies, more than
twice the absorption at 125 Hz for a one-inch
panel.
Tackable. Handy for the
church classroom, tackable panels allow pushpins
without destroying the underlying acoustical
material. Of course, too many hard hanging items
will block the acoustical effect or, at least
erode high frequency performance.
Cellular Melamine. The
"foam" alternative: a white foam-like product
that has become popular as a substrate for
acoustical panels. Cellular Melamine should not
be confused with
polyurethane foam, which is
restricted in use by fire codes. When selecting
fabric cover colors, keep in mind that cellular
Melamine is very white and will affect the
perceived fabric color. It also conforms easily
to curves, with the ability to bend around
curved surfaces without the need to "score" the
back surface.
Panels with images.
Custom or stock images printed on acoustically
transparent fabric. Recent improvements in
computer printing and acoustically transparent
fabrics allow hi-resolution photographs to be
used in addition to artwork and graphics printed
by conventional means.
Stretch wall. Raw
acoustical panels are mounted to the wall, then
fabric is stretched over the panels. These
custom on-site installations are useful for
covering contoured or irregular surfaces.
Full-, half- and quarter-round.
A flat panel made to conform to a curve. By
creating a trapped air cavity of varied depth
behind a one-inch panel, absorption is extended
well into the bass frequencies. One manufacturer
patented a device that is tunable by the
addition of a membrane over half of the surface.
These devices are useful where low frequency
energy is a problem and panels four-inches thick
or greater are impractical.
Substrate and Fabric Choices
The basic fiberglass board comes from three
suppliers of insulation. Board may vary in
density between six and seven pounds per cubic
foot and in color: pink, yellow or marbled.
Fiberglass density may affect stability of the
panels.
The color of the substrate is
important because it can "tint" the color of the
fabric covering. For example, a yellow substrate
can produce a desirable vintage off-white or be
totally wrong for a room that needs a white as
bright as that of a toothpaste ad. It is always
best to get a sample of the recommended
acoustical panel in advance. To avoid having the
substrate color enhance or discolor the finish,
a panel may be under-wrapped with a scrim
material to block the color below the fabric.
Scrims are usually white but are available in
black when that produces a better effect as with
an image panel.
Acoustical wall panels can
usually be made to blend with the sanctuary
interior using the 48 standard colors available
to all panel manufacturers. Custom fabrics can
be used but usually at additional cost. When
choosing custom fabrics, make sure they are
acoustically transparent or have known sound
absorbing properties of their own. Also make
sure the fabric is made of synthetic fibers so
it will not be affected by moisture and
temperature changes. Fabrics should also be
visually non-directional: color and texture
should appear the same from all angles.
Some manufacturers make
panels to custom shapes and sizes. Again, this
will usually involve extra costs. Panels have
been made in the shapes of wild animals,
spaceship interiors, corporate logos, art deco
theater shapes, and much more. Conforming an
acoustical panel to the curves of a church
window is possible. However, as with all custom
work, accurate drawings and descriptions are
essential.
Advanced planning for
acoustics (before building) can save much time,
effort, and money later.
Acoustical panels can
be part of a planned sound-control strategy for
a new building, or as solutions to sound
problems in existing rooms. With the principles
of acoustics well developed over the past 100
years, it is possible to predict acoustic
results and costs in advance. There is every
reason to expect to get it right the first time.
Nick Colleran is a member of
the Acoustical Society of America, past
president of the Society of Professional Audio
Recording Studios (SPARS), and former president
of the Virginia Production Services Association
(VPSA). Nick is currently active in acoustical
design for houses of worship, new acoustical
products and performance venues.
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